xprt.c

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/*
 *  linux/net/sunrpc/xprt.c
 *
 *  This is a generic RPC call interface supporting congestion avoidance,
 *  and asynchronous calls.
 *
 *  The interface works like this:
 *
 *  -   When a process places a call, it allocates a request slot if
 *  one is available. Otherwise, it sleeps on the backlog queue
 *  (xprt_reserve).
 *  -   Next, the caller puts together the RPC message, stuffs it into
 *  the request struct, and calls xprt_transmit().
 *  -   xprt_transmit sends the message and installs the caller on the
 *  transport's wait list. At the same time, if a reply is expected,
 *  it installs a timer that is run after the packet's timeout has
 *  expired.
 *  -   When a packet arrives, the data_ready handler walks the list of
 *  pending requests for that transport. If a matching XID is found, the
 *  caller is woken up, and the timer removed.
 *  -   When no reply arrives within the timeout interval, the timer is
 *  fired by the kernel and runs xprt_timer(). It either adjusts the
 *  timeout values (minor timeout) or wakes up the caller with a status
 *  of -ETIMEDOUT.
 *  -   When the caller receives a notification from RPC that a reply arrived,
 *  it should release the RPC slot, and process the reply.
 *  If the call timed out, it may choose to retry the operation by
 *  adjusting the initial timeout value, and simply calling rpc_call
 *  again.
 *
 *  Support for async RPC is done through a set of RPC-specific scheduling
 *  primitives that `transparently' work for processes as well as async
 *  tasks that rely on callbacks.
 *
 *  Copyright (C) 1995-1997, Olaf Kirch <[email protected]>
 *
 *  Transport switch API copyright (C) 2005, Chuck Lever <[email protected]>
 */

#include <linux/module.h>

#include <linux/types.h>
#include <linux/interrupt.h>
#include <linux/workqueue.h>
#include <linux/net.h>
#include <linux/ktime.h>

#include <linux/sunrpc/clnt.h>
#include <linux/sunrpc/metrics.h>
#include <linux/sunrpc/bc_xprt.h>

#include "sunrpc.h"

/*
 * Local variables
 */

#ifdef RPC_DEBUG
# define RPCDBG_FACILITY    RPCDBG_XPRT
#endif

/*
 * Local functions
 */
static void  xprt_init(struct rpc_xprt *xprt, struct net *net);
static void xprt_request_init(struct rpc_task *, struct rpc_xprt *);
static void xprt_connect_status(struct rpc_task *task);
static int      __xprt_get_cong(struct rpc_xprt *, struct rpc_task *);
static void  xprt_destroy(struct rpc_xprt *xprt);

static DEFINE_SPINLOCK(xprt_list_lock);
static LIST_HEAD(xprt_list);

/*
 * The transport code maintains an estimate on the maximum number of out-
 * standing RPC requests, using a smoothed version of the congestion
 * avoidance implemented in 44BSD. This is basically the Van Jacobson
 * congestion algorithm: If a retransmit occurs, the congestion window is
 * halved; otherwise, it is incremented by 1/cwnd when
 *
 *  -   a reply is received and
 *  -   a full number of requests are outstanding and
 *  -   the congestion window hasn't been updated recently.
 */
#define RPC_CWNDSHIFT       (8U)
#define RPC_CWNDSCALE       (1U << RPC_CWNDSHIFT)
#define RPC_INITCWND        RPC_CWNDSCALE
#define RPC_MAXCWND(xprt)   ((xprt)->max_reqs << RPC_CWNDSHIFT)

#define RPCXPRT_CONGESTED(xprt) ((xprt)->cong >= (xprt)->cwnd)

int xprt_register_transport(struct xprt_class *transport)
{
    struct xprt_class *t;
    int result;

    result = -EEXIST;
    spin_lock(&xprt_list_lock);
    list_for_each_entry(t, &xprt_list, list) {
        /* don't register the same transport class twice */
        if (t->ident == transport->ident)
            goto out;
    }

    list_add_tail(&transport->list, &xprt_list);
    printk(KERN_INFO "RPC: Registered %s transport module.\n",
           transport->name);
    result = 0;

out:
    spin_unlock(&xprt_list_lock);
    return result;
}
EXPORT_SYMBOL_GPL(xprt_register_transport);

int xprt_unregister_transport(struct xprt_class *transport)
{
    struct xprt_class *t;
    int result;

    result = 0;
    spin_lock(&xprt_list_lock);
    list_for_each_entry(t, &xprt_list, list) {
        if (t == transport) {
            printk(KERN_INFO
                "RPC: Unregistered %s transport module.\n",
                transport->name);
            list_del_init(&transport->list);
            goto out;
        }
    }
    result = -ENOENT;

out:
    spin_unlock(&xprt_list_lock);
    return result;
}
EXPORT_SYMBOL_GPL(xprt_unregister_transport);

int xprt_load_transport(const char *transport_name)
{
    struct xprt_class *t;
    int result;

    result = 0;
    spin_lock(&xprt_list_lock);
    list_for_each_entry(t, &xprt_list, list) {
        if (strcmp(t->name, transport_name) == 0) {
            spin_unlock(&xprt_list_lock);
            goto out;
        }
    }
    spin_unlock(&xprt_list_lock);
    result = request_module("xprt%s", transport_name);
out:
    return result;
}
EXPORT_SYMBOL_GPL(xprt_load_transport);

int xprt_reserve_xprt(struct rpc_xprt *xprt, struct rpc_task *task)
{
    struct rpc_rqst *req = task->tk_rqstp;
    int priority;

    if (test_and_set_bit(XPRT_LOCKED, &xprt->state)) {
        if (task == xprt->snd_task)
            return 1;
        goto out_sleep;
    }
    xprt->snd_task = task;
    if (req != NULL) {
        req->rq_bytes_sent = 0;
        req->rq_ntrans++;
    }

    return 1;

out_sleep:
    dprintk("RPC: %5u failed to lock transport %p\n",
            task->tk_pid, xprt);
    task->tk_timeout = 0;
    task->tk_status = -EAGAIN;
    if (req == NULL)
        priority = RPC_PRIORITY_LOW;
    else if (!req->rq_ntrans)
        priority = RPC_PRIORITY_NORMAL;
    else
        priority = RPC_PRIORITY_HIGH;
    rpc_sleep_on_priority(&xprt->sending, task, NULL, priority);
    return 0;
}
EXPORT_SYMBOL_GPL(xprt_reserve_xprt);

static void xprt_clear_locked(struct rpc_xprt *xprt)
{
    xprt->snd_task = NULL;
    if (!test_bit(XPRT_CLOSE_WAIT, &xprt->state)) {
        smp_mb__before_clear_bit();
        clear_bit(XPRT_LOCKED, &xprt->state);
        smp_mb__after_clear_bit();
    } else
        queue_work(rpciod_workqueue, &xprt->task_cleanup);
}

/*
 * xprt_reserve_xprt_cong - serialize write access to transports
 * @task: task that is requesting access to the transport
 *
 * Same as xprt_reserve_xprt, but Van Jacobson congestion control is
 * integrated into the decision of whether a request is allowed to be
 * woken up and given access to the transport.
 */
int xprt_reserve_xprt_cong(struct rpc_xprt *xprt, struct rpc_task *task)
{
    struct rpc_rqst *req = task->tk_rqstp;
    int priority;

    if (test_and_set_bit(XPRT_LOCKED, &xprt->state)) {
        if (task == xprt->snd_task)
            return 1;
        goto out_sleep;
    }
    if (req == NULL) {
        xprt->snd_task = task;
        return 1;
    }
    if (__xprt_get_cong(xprt, task)) {
        xprt->snd_task = task;
        req->rq_bytes_sent = 0;
        req->rq_ntrans++;
        return 1;
    }
    xprt_clear_locked(xprt);
out_sleep:
    dprintk("RPC: %5u failed to lock transport %p\n", task->tk_pid, xprt);
    task->tk_timeout = 0;
    task->tk_status = -EAGAIN;
    if (req == NULL)
        priority = RPC_PRIORITY_LOW;
    else if (!req->rq_ntrans)
        priority = RPC_PRIORITY_NORMAL;
    else
        priority = RPC_PRIORITY_HIGH;
    rpc_sleep_on_priority(&xprt->sending, task, NULL, priority);
    return 0;
}
EXPORT_SYMBOL_GPL(xprt_reserve_xprt_cong);

static inline int xprt_lock_write(struct rpc_xprt *xprt, struct rpc_task *task)
{
    int retval;

    spin_lock_bh(&xprt->transport_lock);
    retval = xprt->ops->reserve_xprt(xprt, task);
    spin_unlock_bh(&xprt->transport_lock);
    return retval;
}

static bool __xprt_lock_write_func(struct rpc_task *task, void *data)
{
    struct rpc_xprt *xprt = data;
    struct rpc_rqst *req;

    req = task->tk_rqstp;
    xprt->snd_task = task;
    if (req) {
        req->rq_bytes_sent = 0;
        req->rq_ntrans++;
    }
    return true;
}

static void __xprt_lock_write_next(struct rpc_xprt *xprt)
{
    if (test_and_set_bit(XPRT_LOCKED, &xprt->state))
        return;

    if (rpc_wake_up_first(&xprt->sending, __xprt_lock_write_func, xprt))
        return;
    xprt_clear_locked(xprt);
}

static bool __xprt_lock_write_cong_func(struct rpc_task *task, void *data)
{
    struct rpc_xprt *xprt = data;
    struct rpc_rqst *req;

    req = task->tk_rqstp;
    if (req == NULL) {
        xprt->snd_task = task;
        return true;
    }
    if (__xprt_get_cong(xprt, task)) {
        xprt->snd_task = task;
        req->rq_bytes_sent = 0;
        req->rq_ntrans++;
        return true;
    }
    return false;
}

static void __xprt_lock_write_next_cong(struct rpc_xprt *xprt)
{
    if (test_and_set_bit(XPRT_LOCKED, &xprt->state))
        return;
    if (RPCXPRT_CONGESTED(xprt))
        goto out_unlock;
    if (rpc_wake_up_first(&xprt->sending, __xprt_lock_write_cong_func, xprt))
        return;
out_unlock:
    xprt_clear_locked(xprt);
}

void xprt_release_xprt(struct rpc_xprt *xprt, struct rpc_task *task)
{
    if (xprt->snd_task == task) {
        xprt_clear_locked(xprt);
        __xprt_lock_write_next(xprt);
    }
}
EXPORT_SYMBOL_GPL(xprt_release_xprt);

void xprt_release_xprt_cong(struct rpc_xprt *xprt, struct rpc_task *task)
{
    if (xprt->snd_task == task) {
        xprt_clear_locked(xprt);
        __xprt_lock_write_next_cong(xprt);
    }
}
EXPORT_SYMBOL_GPL(xprt_release_xprt_cong);

static inline void xprt_release_write(struct rpc_xprt *xprt, struct rpc_task *task)
{
    spin_lock_bh(&xprt->transport_lock);
    xprt->ops->release_xprt(xprt, task);
    spin_unlock_bh(&xprt->transport_lock);
}

/*
 * Van Jacobson congestion avoidance. Check if the congestion window
 * overflowed. Put the task to sleep if this is the case.
 */
static int
__xprt_get_cong(struct rpc_xprt *xprt, struct rpc_task *task)
{
    struct rpc_rqst *req = task->tk_rqstp;

    if (req->rq_cong)
        return 1;
    dprintk("RPC: %5u xprt_cwnd_limited cong = %lu cwnd = %lu\n",
            task->tk_pid, xprt->cong, xprt->cwnd);
    if (RPCXPRT_CONGESTED(xprt))
        return 0;
    req->rq_cong = 1;
    xprt->cong += RPC_CWNDSCALE;
    return 1;
}

/*
 * Adjust the congestion window, and wake up the next task
 * that has been sleeping due to congestion
 */
static void
__xprt_put_cong(struct rpc_xprt *xprt, struct rpc_rqst *req)
{
    if (!req->rq_cong)
        return;
    req->rq_cong = 0;
    xprt->cong -= RPC_CWNDSCALE;
    __xprt_lock_write_next_cong(xprt);
}

void xprt_release_rqst_cong(struct rpc_task *task)
{
    __xprt_put_cong(task->tk_xprt, task->tk_rqstp);
}
EXPORT_SYMBOL_GPL(xprt_release_rqst_cong);

void xprt_adjust_cwnd(struct rpc_task *task, int result)
{
    struct rpc_rqst *req = task->tk_rqstp;
    struct rpc_xprt *xprt = task->tk_xprt;
    unsigned long cwnd = xprt->cwnd;

    if (result >= 0 && cwnd <= xprt->cong) {
        /* The (cwnd >> 1) term makes sure
         * the result gets rounded properly. */
        cwnd += (RPC_CWNDSCALE * RPC_CWNDSCALE + (cwnd >> 1)) / cwnd;
        if (cwnd > RPC_MAXCWND(xprt))
            cwnd = RPC_MAXCWND(xprt);
        __xprt_lock_write_next_cong(xprt);
    } else if (result == -ETIMEDOUT) {
        cwnd >>= 1;
        if (cwnd < RPC_CWNDSCALE)
            cwnd = RPC_CWNDSCALE;
    }
    dprintk("RPC:       cong %ld, cwnd was %ld, now %ld\n",
            xprt->cong, xprt->cwnd, cwnd);
    xprt->cwnd = cwnd;
    __xprt_put_cong(xprt, req);
}
EXPORT_SYMBOL_GPL(xprt_adjust_cwnd);

void xprt_wake_pending_tasks(struct rpc_xprt *xprt, int status)
{
    if (status < 0)
        rpc_wake_up_status(&xprt->pending, status);
    else
        rpc_wake_up(&xprt->pending);
}
EXPORT_SYMBOL_GPL(xprt_wake_pending_tasks);

void xprt_wait_for_buffer_space(struct rpc_task *task, rpc_action action)
{
    struct rpc_rqst *req = task->tk_rqstp;
    struct rpc_xprt *xprt = req->rq_xprt;

    task->tk_timeout = req->rq_timeout;
    rpc_sleep_on(&xprt->pending, task, action);
}
EXPORT_SYMBOL_GPL(xprt_wait_for_buffer_space);

void xprt_write_space(struct rpc_xprt *xprt)
{
    spin_lock_bh(&xprt->transport_lock);
    if (xprt->snd_task) {
        dprintk("RPC:       write space: waking waiting task on "
                "xprt %p\n", xprt);
        rpc_wake_up_queued_task(&xprt->pending, xprt->snd_task);
    }
    spin_unlock_bh(&xprt->transport_lock);
}
EXPORT_SYMBOL_GPL(xprt_write_space);

void xprt_set_retrans_timeout_def(struct rpc_task *task)
{
    task->tk_timeout = task->tk_rqstp->rq_timeout;
}
EXPORT_SYMBOL_GPL(xprt_set_retrans_timeout_def);

void xprt_set_retrans_timeout_rtt(struct rpc_task *task)
{
    int timer = task->tk_msg.rpc_proc->p_timer;
    struct rpc_clnt *clnt = task->tk_client;
    struct rpc_rtt *rtt = clnt->cl_rtt;
    struct rpc_rqst *req = task->tk_rqstp;
    unsigned long max_timeout = clnt->cl_timeout->to_maxval;

    task->tk_timeout = rpc_calc_rto(rtt, timer);
    task->tk_timeout <<= rpc_ntimeo(rtt, timer) + req->rq_retries;
    if (task->tk_timeout > max_timeout || task->tk_timeout == 0)
        task->tk_timeout = max_timeout;
}
EXPORT_SYMBOL_GPL(xprt_set_retrans_timeout_rtt);

static void xprt_reset_majortimeo(struct rpc_rqst *req)
{
    const struct rpc_timeout *to = req->rq_task->tk_client->cl_timeout;

    req->rq_majortimeo = req->rq_timeout;
    if (to->to_exponential)
        req->rq_majortimeo <<= to->to_retries;
    else
        req->rq_majortimeo += to->to_increment * to->to_retries;
    if (req->rq_majortimeo > to->to_maxval || req->rq_majortimeo == 0)
        req->rq_majortimeo = to->to_maxval;
    req->rq_majortimeo += jiffies;
}

int xprt_adjust_timeout(struct rpc_rqst *req)
{
    struct rpc_xprt *xprt = req->rq_xprt;
    const struct rpc_timeout *to = req->rq_task->tk_client->cl_timeout;
    int status = 0;

    if (time_before(jiffies, req->rq_majortimeo)) {
        if (to->to_exponential)
            req->rq_timeout <<= 1;
        else
            req->rq_timeout += to->to_increment;
        if (to->to_maxval && req->rq_timeout >= to->to_maxval)
            req->rq_timeout = to->to_maxval;
        req->rq_retries++;
    } else {
        req->rq_timeout = to->to_initval;
        req->rq_retries = 0;
        xprt_reset_majortimeo(req);
        /* Reset the RTT counters == "slow start" */
        spin_lock_bh(&xprt->transport_lock);
        rpc_init_rtt(req->rq_task->tk_client->cl_rtt, to->to_initval);
        spin_unlock_bh(&xprt->transport_lock);
        status = -ETIMEDOUT;
    }

    if (req->rq_timeout == 0) {
        printk(KERN_WARNING "xprt_adjust_timeout: rq_timeout = 0!\n");
        req->rq_timeout = 5 * HZ;
    }
    return status;
}

static void xprt_autoclose(struct work_struct *work)
{
    struct rpc_xprt *xprt =
        container_of(work, struct rpc_xprt, task_cleanup);

    xprt->ops->close(xprt);
    clear_bit(XPRT_CLOSE_WAIT, &xprt->state);
    xprt_release_write(xprt, NULL);
}

void xprt_disconnect_done(struct rpc_xprt *xprt)
{
    dprintk("RPC:       disconnected transport %p\n", xprt);
    spin_lock_bh(&xprt->transport_lock);
    xprt_clear_connected(xprt);
    xprt_wake_pending_tasks(xprt, -EAGAIN);
    spin_unlock_bh(&xprt->transport_lock);
}
EXPORT_SYMBOL_GPL(xprt_disconnect_done);

void xprt_force_disconnect(struct rpc_xprt *xprt)
{
    /* Don't race with the test_bit() in xprt_clear_locked() */
    spin_lock_bh(&xprt->transport_lock);
    set_bit(XPRT_CLOSE_WAIT, &xprt->state);
    /* Try to schedule an autoclose RPC call */
    if (test_and_set_bit(XPRT_LOCKED, &xprt->state) == 0)
        queue_work(rpciod_workqueue, &xprt->task_cleanup);
    xprt_wake_pending_tasks(xprt, -EAGAIN);
    spin_unlock_bh(&xprt->transport_lock);
}

void xprt_conditional_disconnect(struct rpc_xprt *xprt, unsigned int cookie)
{
    /* Don't race with the test_bit() in xprt_clear_locked() */
    spin_lock_bh(&xprt->transport_lock);
    if (cookie != xprt->connect_cookie)
        goto out;
    if (test_bit(XPRT_CLOSING, &xprt->state) || !xprt_connected(xprt))
        goto out;
    set_bit(XPRT_CLOSE_WAIT, &xprt->state);
    /* Try to schedule an autoclose RPC call */
    if (test_and_set_bit(XPRT_LOCKED, &xprt->state) == 0)
        queue_work(rpciod_workqueue, &xprt->task_cleanup);
    xprt_wake_pending_tasks(xprt, -EAGAIN);
out:
    spin_unlock_bh(&xprt->transport_lock);
}

static void
xprt_init_autodisconnect(unsigned long data)
{
    struct rpc_xprt *xprt = (struct rpc_xprt *)data;

    spin_lock(&xprt->transport_lock);
    if (!list_empty(&xprt->recv))
        goto out_abort;
    if (test_and_set_bit(XPRT_LOCKED, &xprt->state))
        goto out_abort;
    spin_unlock(&xprt->transport_lock);
    set_bit(XPRT_CONNECTION_CLOSE, &xprt->state);
    queue_work(rpciod_workqueue, &xprt->task_cleanup);
    return;
out_abort:
    spin_unlock(&xprt->transport_lock);
}

void xprt_connect(struct rpc_task *task)
{
    struct rpc_xprt *xprt = task->tk_xprt;

    dprintk("RPC: %5u xprt_connect xprt %p %s connected\n", task->tk_pid,
            xprt, (xprt_connected(xprt) ? "is" : "is not"));

    if (!xprt_bound(xprt)) {
        task->tk_status = -EAGAIN;
        return;
    }
    if (!xprt_lock_write(xprt, task))
        return;

    if (test_and_clear_bit(XPRT_CLOSE_WAIT, &xprt->state))
        xprt->ops->close(xprt);

    if (xprt_connected(xprt))
        xprt_release_write(xprt, task);
    else {
        task->tk_rqstp->rq_bytes_sent = 0;
        task->tk_timeout = task->tk_rqstp->rq_timeout;
        rpc_sleep_on(&xprt->pending, task, xprt_connect_status);

        if (test_bit(XPRT_CLOSING, &xprt->state))
            return;
        if (xprt_test_and_set_connecting(xprt))
            return;
        xprt->stat.connect_start = jiffies;
        xprt->ops->connect(task);
    }
}

static void xprt_connect_status(struct rpc_task *task)
{
    struct rpc_xprt *xprt = task->tk_xprt;

    if (task->tk_status == 0) {
        xprt->stat.connect_count++;
        xprt->stat.connect_time += (long)jiffies - xprt->stat.connect_start;
        dprintk("RPC: %5u xprt_connect_status: connection established\n",
                task->tk_pid);
        return;
    }

    switch (task->tk_status) {
    case -EAGAIN:
        dprintk("RPC: %5u xprt_connect_status: retrying\n", task->tk_pid);
        break;
    case -ETIMEDOUT:
        dprintk("RPC: %5u xprt_connect_status: connect attempt timed "
                "out\n", task->tk_pid);
        break;
    default:
        dprintk("RPC: %5u xprt_connect_status: error %d connecting to "
                "server %s\n", task->tk_pid, -task->tk_status,
                xprt->servername);
        xprt_release_write(xprt, task);
        task->tk_status = -EIO;
    }
}

struct rpc_rqst *xprt_lookup_rqst(struct rpc_xprt *xprt, __be32 xid)
{
    struct rpc_rqst *entry;

    list_for_each_entry(entry, &xprt->recv, rq_list)
        if (entry->rq_xid == xid)
            return entry;

    dprintk("RPC:       xprt_lookup_rqst did not find xid %08x\n",
            ntohl(xid));
    xprt->stat.bad_xids++;
    return NULL;
}
EXPORT_SYMBOL_GPL(xprt_lookup_rqst);

static void xprt_update_rtt(struct rpc_task *task)
{
    struct rpc_rqst *req = task->tk_rqstp;
    struct rpc_rtt *rtt = task->tk_client->cl_rtt;
    unsigned int timer = task->tk_msg.rpc_proc->p_timer;
    long m = usecs_to_jiffies(ktime_to_us(req->rq_rtt));

    if (timer) {
        if (req->rq_ntrans == 1)
            rpc_update_rtt(rtt, timer, m);
        rpc_set_timeo(rtt, timer, req->rq_ntrans - 1);
    }
}

void xprt_complete_rqst(struct rpc_task *task, int copied)
{
    struct rpc_rqst *req = task->tk_rqstp;
    struct rpc_xprt *xprt = req->rq_xprt;

    dprintk("RPC: %5u xid %08x complete (%d bytes received)\n",
            task->tk_pid, ntohl(req->rq_xid), copied);

    xprt->stat.recvs++;
    req->rq_rtt = ktime_sub(ktime_get(), req->rq_xtime);
    if (xprt->ops->timer != NULL)
        xprt_update_rtt(task);

    list_del_init(&req->rq_list);
    req->rq_private_buf.len = copied;
    /* Ensure all writes are done before we update */
    /* req->rq_reply_bytes_recvd */
    smp_wmb();
    req->rq_reply_bytes_recvd = copied;
    rpc_wake_up_queued_task(&xprt->pending, task);
}
EXPORT_SYMBOL_GPL(xprt_complete_rqst);

static void xprt_timer(struct rpc_task *task)
{
    struct rpc_rqst *req = task->tk_rqstp;
    struct rpc_xprt *xprt = req->rq_xprt;

    if (task->tk_status != -ETIMEDOUT)
        return;
    dprintk("RPC: %5u xprt_timer\n", task->tk_pid);

    spin_lock_bh(&xprt->transport_lock);
    if (!req->rq_reply_bytes_recvd) {
        if (xprt->ops->timer)
            xprt->ops->timer(task);
    } else
        task->tk_status = 0;
    spin_unlock_bh(&xprt->transport_lock);
}

static inline int xprt_has_timer(struct rpc_xprt *xprt)
{
    return xprt->idle_timeout != 0;
}

int xprt_prepare_transmit(struct rpc_task *task)
{
    struct rpc_rqst *req = task->tk_rqstp;
    struct rpc_xprt *xprt = req->rq_xprt;
    int err = 0;

    dprintk("RPC: %5u xprt_prepare_transmit\n", task->tk_pid);

    spin_lock_bh(&xprt->transport_lock);
    if (req->rq_reply_bytes_recvd && !req->rq_bytes_sent) {
        err = req->rq_reply_bytes_recvd;
        goto out_unlock;
    }
    if (!xprt->ops->reserve_xprt(xprt, task))
        err = -EAGAIN;
out_unlock:
    spin_unlock_bh(&xprt->transport_lock);
    return err;
}

void xprt_end_transmit(struct rpc_task *task)
{
    xprt_release_write(task->tk_rqstp->rq_xprt, task);
}

void xprt_transmit(struct rpc_task *task)
{
    struct rpc_rqst *req = task->tk_rqstp;
    struct rpc_xprt *xprt = req->rq_xprt;
    int status, numreqs;

    dprintk("RPC: %5u xprt_transmit(%u)\n", task->tk_pid, req->rq_slen);

    if (!req->rq_reply_bytes_recvd) {
        if (list_empty(&req->rq_list) && rpc_reply_expected(task)) {
            /*
             * Add to the list only if we're expecting a reply
             */
            spin_lock_bh(&xprt->transport_lock);
            /* Update the softirq receive buffer */
            memcpy(&req->rq_private_buf, &req->rq_rcv_buf,
                    sizeof(req->rq_private_buf));
            /* Add request to the receive list */
            list_add_tail(&req->rq_list, &xprt->recv);
            spin_unlock_bh(&xprt->transport_lock);
            xprt_reset_majortimeo(req);
            /* Turn off autodisconnect */
            del_singleshot_timer_sync(&xprt->timer);
        }
    } else if (!req->rq_bytes_sent)
        return;

    req->rq_connect_cookie = xprt->connect_cookie;
    req->rq_xtime = ktime_get();
    status = xprt->ops->send_request(task);
    if (status != 0) {
        task->tk_status = status;
        return;
    }

    dprintk("RPC: %5u xmit complete\n", task->tk_pid);
    task->tk_flags |= RPC_TASK_SENT;
    spin_lock_bh(&xprt->transport_lock);

    xprt->ops->set_retrans_timeout(task);

    numreqs = atomic_read(&xprt->num_reqs);
    if (numreqs > xprt->stat.max_slots)
        xprt->stat.max_slots = numreqs;
    xprt->stat.sends++;
    xprt->stat.req_u += xprt->stat.sends - xprt->stat.recvs;
    xprt->stat.bklog_u += xprt->backlog.qlen;
    xprt->stat.sending_u += xprt->sending.qlen;
    xprt->stat.pending_u += xprt->pending.qlen;

    /* Don't race with disconnect */
    if (!xprt_connected(xprt))
        task->tk_status = -ENOTCONN;
    else if (!req->rq_reply_bytes_recvd && rpc_reply_expected(task)) {
        /*
         * Sleep on the pending queue since
         * we're expecting a reply.
         */
        rpc_sleep_on(&xprt->pending, task, xprt_timer);
    }
    spin_unlock_bh(&xprt->transport_lock);
}

static struct rpc_rqst *xprt_dynamic_alloc_slot(struct rpc_xprt *xprt, gfp_t gfp_flags)
{
    struct rpc_rqst *req = ERR_PTR(-EAGAIN);

    if (!atomic_add_unless(&xprt->num_reqs, 1, xprt->max_reqs))
        goto out;
    req = kzalloc(sizeof(struct rpc_rqst), gfp_flags);
    if (req != NULL)
        goto out;
    atomic_dec(&xprt->num_reqs);
    req = ERR_PTR(-ENOMEM);
out:
    return req;
}

static bool xprt_dynamic_free_slot(struct rpc_xprt *xprt, struct rpc_rqst *req)
{
    if (atomic_add_unless(&xprt->num_reqs, -1, xprt->min_reqs)) {
        kfree(req);
        return true;
    }
    return false;
}

void xprt_alloc_slot(struct rpc_xprt *xprt, struct rpc_task *task)
{
    struct rpc_rqst *req;

    spin_lock(&xprt->reserve_lock);
    if (!list_empty(&xprt->free)) {
        req = list_entry(xprt->free.next, struct rpc_rqst, rq_list);
        list_del(&req->rq_list);
        goto out_init_req;
    }
    req = xprt_dynamic_alloc_slot(xprt, GFP_NOWAIT|__GFP_NOWARN);
    if (!IS_ERR(req))
        goto out_init_req;
    switch (PTR_ERR(req)) {
    case -ENOMEM:
        dprintk("RPC:       dynamic allocation of request slot "
                "failed! Retrying\n");
        task->tk_status = -ENOMEM;
        break;
    case -EAGAIN:
        rpc_sleep_on(&xprt->backlog, task, NULL);
        dprintk("RPC:       waiting for request slot\n");
    default:
        task->tk_status = -EAGAIN;
    }
    spin_unlock(&xprt->reserve_lock);
    return;
out_init_req:
    task->tk_status = 0;
    task->tk_rqstp = req;
    xprt_request_init(task, xprt);
    spin_unlock(&xprt->reserve_lock);
}
EXPORT_SYMBOL_GPL(xprt_alloc_slot);

void xprt_lock_and_alloc_slot(struct rpc_xprt *xprt, struct rpc_task *task)
{
    /* Note: grabbing the xprt_lock_write() ensures that we throttle
     * new slot allocation if the transport is congested (i.e. when
     * reconnecting a stream transport or when out of socket write
     * buffer space).
     */
    if (xprt_lock_write(xprt, task)) {
        xprt_alloc_slot(xprt, task);
        xprt_release_write(xprt, task);
    }
}
EXPORT_SYMBOL_GPL(xprt_lock_and_alloc_slot);

static void xprt_free_slot(struct rpc_xprt *xprt, struct rpc_rqst *req)
{
    spin_lock(&xprt->reserve_lock);
    if (!xprt_dynamic_free_slot(xprt, req)) {
        memset(req, 0, sizeof(*req));   /* mark unused */
        list_add(&req->rq_list, &xprt->free);
    }
    rpc_wake_up_next(&xprt->backlog);
    spin_unlock(&xprt->reserve_lock);
}

static void xprt_free_all_slots(struct rpc_xprt *xprt)
{
    struct rpc_rqst *req;
    while (!list_empty(&xprt->free)) {
        req = list_first_entry(&xprt->free, struct rpc_rqst, rq_list);
        list_del(&req->rq_list);
        kfree(req);
    }
}

struct rpc_xprt *xprt_alloc(struct net *net, size_t size,
        unsigned int num_prealloc,
        unsigned int max_alloc)
{
    struct rpc_xprt *xprt;
    struct rpc_rqst *req;
    int i;

    xprt = kzalloc(size, GFP_KERNEL);
    if (xprt == NULL)
        goto out;

    xprt_init(xprt, net);

    for (i = 0; i < num_prealloc; i++) {
        req = kzalloc(sizeof(struct rpc_rqst), GFP_KERNEL);
        if (!req)
            break;
        list_add(&req->rq_list, &xprt->free);
    }
    if (i < num_prealloc)
        goto out_free;
    if (max_alloc > num_prealloc)
        xprt->max_reqs = max_alloc;
    else
        xprt->max_reqs = num_prealloc;
    xprt->min_reqs = num_prealloc;
    atomic_set(&xprt->num_reqs, num_prealloc);

    return xprt;

out_free:
    xprt_free(xprt);
out:
    return NULL;
}
EXPORT_SYMBOL_GPL(xprt_alloc);

void xprt_free(struct rpc_xprt *xprt)
{
    put_net(xprt->xprt_net);
    xprt_free_all_slots(xprt);
    kfree(xprt);
}
EXPORT_SYMBOL_GPL(xprt_free);

void xprt_reserve(struct rpc_task *task)
{
    struct rpc_xprt *xprt = task->tk_xprt;

    task->tk_status = 0;
    if (task->tk_rqstp != NULL)
        return;

    task->tk_timeout = 0;
    task->tk_status = -EAGAIN;
    xprt->ops->alloc_slot(xprt, task);
}

static inline __be32 xprt_alloc_xid(struct rpc_xprt *xprt)
{
    return (__force __be32)xprt->xid++;
}

static inline void xprt_init_xid(struct rpc_xprt *xprt)
{
    xprt->xid = net_random();
}

static void xprt_request_init(struct rpc_task *task, struct rpc_xprt *xprt)
{
    struct rpc_rqst *req = task->tk_rqstp;

    INIT_LIST_HEAD(&req->rq_list);
    req->rq_timeout = task->tk_client->cl_timeout->to_initval;
    req->rq_task    = task;
    req->rq_xprt    = xprt;
    req->rq_buffer  = NULL;
    req->rq_xid     = xprt_alloc_xid(xprt);
    req->rq_release_snd_buf = NULL;
    xprt_reset_majortimeo(req);
    dprintk("RPC: %5u reserved req %p xid %08x\n", task->tk_pid,
            req, ntohl(req->rq_xid));
}

void xprt_release(struct rpc_task *task)
{
    struct rpc_xprt *xprt;
    struct rpc_rqst *req;

    if (!(req = task->tk_rqstp))
        return;

    xprt = req->rq_xprt;
    if (task->tk_ops->rpc_count_stats != NULL)
        task->tk_ops->rpc_count_stats(task, task->tk_calldata);
    else if (task->tk_client)
        rpc_count_iostats(task, task->tk_client->cl_metrics);
    spin_lock_bh(&xprt->transport_lock);
    xprt->ops->release_xprt(xprt, task);
    if (xprt->ops->release_request)
        xprt->ops->release_request(task);
    if (!list_empty(&req->rq_list))
        list_del(&req->rq_list);
    xprt->last_used = jiffies;
    if (list_empty(&xprt->recv) && xprt_has_timer(xprt))
        mod_timer(&xprt->timer,
                xprt->last_used + xprt->idle_timeout);
    spin_unlock_bh(&xprt->transport_lock);
    if (req->rq_buffer)
        xprt->ops->buf_free(req->rq_buffer);
    if (req->rq_cred != NULL)
        put_rpccred(req->rq_cred);
    task->tk_rqstp = NULL;
    if (req->rq_release_snd_buf)
        req->rq_release_snd_buf(req);

    dprintk("RPC: %5u release request %p\n", task->tk_pid, req);
    if (likely(!bc_prealloc(req)))
        xprt_free_slot(xprt, req);
    else
        xprt_free_bc_request(req);
}

static void xprt_init(struct rpc_xprt *xprt, struct net *net)
{
    atomic_set(&xprt->count, 1);

    spin_lock_init(&xprt->transport_lock);
    spin_lock_init(&xprt->reserve_lock);

    INIT_LIST_HEAD(&xprt->free);
    INIT_LIST_HEAD(&xprt->recv);
#if defined(CONFIG_SUNRPC_BACKCHANNEL)
    spin_lock_init(&xprt->bc_pa_lock);
    INIT_LIST_HEAD(&xprt->bc_pa_list);
#endif /* CONFIG_SUNRPC_BACKCHANNEL */

    xprt->last_used = jiffies;
    xprt->cwnd = RPC_INITCWND;
    xprt->bind_index = 0;

    rpc_init_wait_queue(&xprt->binding, "xprt_binding");
    rpc_init_wait_queue(&xprt->pending, "xprt_pending");
    rpc_init_priority_wait_queue(&xprt->sending, "xprt_sending");
    rpc_init_priority_wait_queue(&xprt->backlog, "xprt_backlog");

    xprt_init_xid(xprt);

    xprt->xprt_net = get_net(net);
}

struct rpc_xprt *xprt_create_transport(struct xprt_create *args)
{
    struct rpc_xprt *xprt;
    struct xprt_class *t;

    spin_lock(&xprt_list_lock);
    list_for_each_entry(t, &xprt_list, list) {
        if (t->ident == args->ident) {
            spin_unlock(&xprt_list_lock);
            goto found;
        }
    }
    spin_unlock(&xprt_list_lock);
    printk(KERN_ERR "RPC: transport (%d) not supported\n", args->ident);
    return ERR_PTR(-EIO);

found:
    xprt = t->setup(args);
    if (IS_ERR(xprt)) {
        dprintk("RPC:       xprt_create_transport: failed, %ld\n",
                -PTR_ERR(xprt));
        goto out;
    }
    INIT_WORK(&xprt->task_cleanup, xprt_autoclose);
    if (xprt_has_timer(xprt))
        setup_timer(&xprt->timer, xprt_init_autodisconnect,
                (unsigned long)xprt);
    else
        init_timer(&xprt->timer);

    if (strlen(args->servername) > RPC_MAXNETNAMELEN) {
        xprt_destroy(xprt);
        return ERR_PTR(-EINVAL);
    }
    xprt->servername = kstrdup(args->servername, GFP_KERNEL);
    if (xprt->servername == NULL) {
        xprt_destroy(xprt);
        return ERR_PTR(-ENOMEM);
    }

    dprintk("RPC:       created transport %p with %u slots\n", xprt,
            xprt->max_reqs);
out:
    return xprt;
}

static void xprt_destroy(struct rpc_xprt *xprt)
{
    dprintk("RPC:       destroying transport %p\n", xprt);
    del_timer_sync(&xprt->timer);

    rpc_destroy_wait_queue(&xprt->binding);
    rpc_destroy_wait_queue(&xprt->pending);
    rpc_destroy_wait_queue(&xprt->sending);
    rpc_destroy_wait_queue(&xprt->backlog);
    cancel_work_sync(&xprt->task_cleanup);
    kfree(xprt->servername);
    /*
     * Tear down transport state and free the rpc_xprt
     */
    xprt->ops->destroy(xprt);
}

void xprt_put(struct rpc_xprt *xprt)
{
    if (atomic_dec_and_test(&xprt->count))
        xprt_destroy(xprt);
}

struct rpc_xprt *xprt_get(struct rpc_xprt *xprt)
{
    if (atomic_inc_not_zero(&xprt->count))
        return xprt;
    return NULL;
}