spinlock.c

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/*
 * Split spinlock implementation out into its own file, so it can be
 * compiled in a FTRACE-compatible way.
 */
#include <linux/kernel_stat.h>
#include <linux/spinlock.h>
#include <linux/debugfs.h>
#include <linux/log2.h>
#include <linux/gfp.h>

#include <asm/paravirt.h>

#include <xen/interface/xen.h>
#include <xen/events.h>

#include "xen-ops.h"
#include "debugfs.h"

#ifdef CONFIG_XEN_DEBUG_FS
static struct xen_spinlock_stats
{
    u64 taken;
    u32 taken_slow;
    u32 taken_slow_nested;
    u32 taken_slow_pickup;
    u32 taken_slow_spurious;
    u32 taken_slow_irqenable;

    u64 released;
    u32 released_slow;
    u32 released_slow_kicked;

#define HISTO_BUCKETS   30
    u32 histo_spin_total[HISTO_BUCKETS+1];
    u32 histo_spin_spinning[HISTO_BUCKETS+1];
    u32 histo_spin_blocked[HISTO_BUCKETS+1];

    u64 time_total;
    u64 time_spinning;
    u64 time_blocked;
} spinlock_stats;

static u8 zero_stats;

static unsigned lock_timeout = 1 << 10;
#define TIMEOUT lock_timeout

static inline void check_zero(void)
{
    if (unlikely(zero_stats)) {
        memset(&spinlock_stats, 0, sizeof(spinlock_stats));
        zero_stats = 0;
    }
}

#define ADD_STATS(elem, val)            \
    do { check_zero(); spinlock_stats.elem += (val); } while(0)

static inline u64 spin_time_start(void)
{
    return xen_clocksource_read();
}

static void __spin_time_accum(u64 delta, u32 *array)
{
    unsigned index = ilog2(delta);

    check_zero();

    if (index < HISTO_BUCKETS)
        array[index]++;
    else
        array[HISTO_BUCKETS]++;
}

static inline void spin_time_accum_spinning(u64 start)
{
    u32 delta = xen_clocksource_read() - start;

    __spin_time_accum(delta, spinlock_stats.histo_spin_spinning);
    spinlock_stats.time_spinning += delta;
}

static inline void spin_time_accum_total(u64 start)
{
    u32 delta = xen_clocksource_read() - start;

    __spin_time_accum(delta, spinlock_stats.histo_spin_total);
    spinlock_stats.time_total += delta;
}

static inline void spin_time_accum_blocked(u64 start)
{
    u32 delta = xen_clocksource_read() - start;

    __spin_time_accum(delta, spinlock_stats.histo_spin_blocked);
    spinlock_stats.time_blocked += delta;
}
#else  /* !CONFIG_XEN_DEBUG_FS */
#define TIMEOUT         (1 << 10)
#define ADD_STATS(elem, val)    do { (void)(val); } while(0)

static inline u64 spin_time_start(void)
{
    return 0;
}

static inline void spin_time_accum_total(u64 start)
{
}
static inline void spin_time_accum_spinning(u64 start)
{
}
static inline void spin_time_accum_blocked(u64 start)
{
}
#endif  /* CONFIG_XEN_DEBUG_FS */

/*
 * Size struct xen_spinlock so it's the same as arch_spinlock_t.
 */
#if NR_CPUS < 256
typedef u8 xen_spinners_t;
# define inc_spinners(xl) \
    asm(LOCK_PREFIX " incb %0" : "+m" ((xl)->spinners) : : "memory");
# define dec_spinners(xl) \
    asm(LOCK_PREFIX " decb %0" : "+m" ((xl)->spinners) : : "memory");
#else
typedef u16 xen_spinners_t;
# define inc_spinners(xl) \
    asm(LOCK_PREFIX " incw %0" : "+m" ((xl)->spinners) : : "memory");
# define dec_spinners(xl) \
    asm(LOCK_PREFIX " decw %0" : "+m" ((xl)->spinners) : : "memory");
#endif

struct xen_spinlock {
    unsigned char lock;     /* 0 -> free; 1 -> locked */
    xen_spinners_t spinners;    /* count of waiting cpus */
};

static int xen_spin_is_locked(struct arch_spinlock *lock)
{
    struct xen_spinlock *xl = (struct xen_spinlock *)lock;

    return xl->lock != 0;
}

static int xen_spin_is_contended(struct arch_spinlock *lock)
{
    struct xen_spinlock *xl = (struct xen_spinlock *)lock;

    /* Not strictly true; this is only the count of contended
       lock-takers entering the slow path. */
    return xl->spinners != 0;
}

static int xen_spin_trylock(struct arch_spinlock *lock)
{
    struct xen_spinlock *xl = (struct xen_spinlock *)lock;
    u8 old = 1;

    asm("xchgb %b0,%1"
        : "+q" (old), "+m" (xl->lock) : : "memory");

    return old == 0;
}

static DEFINE_PER_CPU(int, lock_kicker_irq) = -1;
static DEFINE_PER_CPU(struct xen_spinlock *, lock_spinners);

/*
 * Mark a cpu as interested in a lock.  Returns the CPU's previous
 * lock of interest, in case we got preempted by an interrupt.
 */
static inline struct xen_spinlock *spinning_lock(struct xen_spinlock *xl)
{
    struct xen_spinlock *prev;

    prev = __this_cpu_read(lock_spinners);
    __this_cpu_write(lock_spinners, xl);

    wmb();          /* set lock of interest before count */

    inc_spinners(xl);

    return prev;
}

/*
 * Mark a cpu as no longer interested in a lock.  Restores previous
 * lock of interest (NULL for none).
 */
static inline void unspinning_lock(struct xen_spinlock *xl, struct xen_spinlock *prev)
{
    dec_spinners(xl);
    wmb();          /* decrement count before restoring lock */
    __this_cpu_write(lock_spinners, prev);
}

static noinline int xen_spin_lock_slow(struct arch_spinlock *lock, bool irq_enable)
{
    struct xen_spinlock *xl = (struct xen_spinlock *)lock;
    struct xen_spinlock *prev;
    int irq = __this_cpu_read(lock_kicker_irq);
    int ret;
    u64 start;

    /* If kicker interrupts not initialized yet, just spin */
    if (irq == -1)
        return 0;

    start = spin_time_start();

    /* announce we're spinning */
    prev = spinning_lock(xl);

    ADD_STATS(taken_slow, 1);
    ADD_STATS(taken_slow_nested, prev != NULL);

    do {
        unsigned long flags;

        /* clear pending */
        xen_clear_irq_pending(irq);

        /* check again make sure it didn't become free while
           we weren't looking  */
        ret = xen_spin_trylock(lock);
        if (ret) {
            ADD_STATS(taken_slow_pickup, 1);

            /*
             * If we interrupted another spinlock while it
             * was blocking, make sure it doesn't block
             * without rechecking the lock.
             */
            if (prev != NULL)
                xen_set_irq_pending(irq);
            goto out;
        }

        flags = arch_local_save_flags();
        if (irq_enable) {
            ADD_STATS(taken_slow_irqenable, 1);
            raw_local_irq_enable();
        }

        /*
         * Block until irq becomes pending.  If we're
         * interrupted at this point (after the trylock but
         * before entering the block), then the nested lock
         * handler guarantees that the irq will be left
         * pending if there's any chance the lock became free;
         * xen_poll_irq() returns immediately if the irq is
         * pending.
         */
        xen_poll_irq(irq);

        raw_local_irq_restore(flags);

        ADD_STATS(taken_slow_spurious, !xen_test_irq_pending(irq));
    } while (!xen_test_irq_pending(irq)); /* check for spurious wakeups */

    kstat_incr_irqs_this_cpu(irq, irq_to_desc(irq));

out:
    unspinning_lock(xl, prev);
    spin_time_accum_blocked(start);

    return ret;
}

static inline void __xen_spin_lock(struct arch_spinlock *lock, bool irq_enable)
{
    struct xen_spinlock *xl = (struct xen_spinlock *)lock;
    unsigned timeout;
    u8 oldval;
    u64 start_spin;

    ADD_STATS(taken, 1);

    start_spin = spin_time_start();

    do {
        u64 start_spin_fast = spin_time_start();

        timeout = TIMEOUT;

        asm("1: xchgb %1,%0\n"
            "   testb %1,%1\n"
            "   jz 3f\n"
            "2: rep;nop\n"
            "   cmpb $0,%0\n"
            "   je 1b\n"
            "   dec %2\n"
            "   jnz 2b\n"
            "3:\n"
            : "+m" (xl->lock), "=q" (oldval), "+r" (timeout)
            : "1" (1)
            : "memory");

        spin_time_accum_spinning(start_spin_fast);

    } while (unlikely(oldval != 0 &&
              (TIMEOUT == ~0 || !xen_spin_lock_slow(lock, irq_enable))));

    spin_time_accum_total(start_spin);
}

static void xen_spin_lock(struct arch_spinlock *lock)
{
    __xen_spin_lock(lock, false);
}

static void xen_spin_lock_flags(struct arch_spinlock *lock, unsigned long flags)
{
    __xen_spin_lock(lock, !raw_irqs_disabled_flags(flags));
}

static noinline void xen_spin_unlock_slow(struct xen_spinlock *xl)
{
    int cpu;

    ADD_STATS(released_slow, 1);

    for_each_online_cpu(cpu) {
        /* XXX should mix up next cpu selection */
        if (per_cpu(lock_spinners, cpu) == xl) {
            ADD_STATS(released_slow_kicked, 1);
            xen_send_IPI_one(cpu, XEN_SPIN_UNLOCK_VECTOR);
            break;
        }
    }
}

static void xen_spin_unlock(struct arch_spinlock *lock)
{
    struct xen_spinlock *xl = (struct xen_spinlock *)lock;

    ADD_STATS(released, 1);

    smp_wmb();      /* make sure no writes get moved after unlock */
    xl->lock = 0;       /* release lock */

    /*
     * Make sure unlock happens before checking for waiting
     * spinners.  We need a strong barrier to enforce the
     * write-read ordering to different memory locations, as the
     * CPU makes no implied guarantees about their ordering.
     */
    mb();

    if (unlikely(xl->spinners))
        xen_spin_unlock_slow(xl);
}

static irqreturn_t dummy_handler(int irq, void *dev_id)
{
    BUG();
    return IRQ_HANDLED;
}

void __cpuinit xen_init_lock_cpu(int cpu)
{
    int irq;
    const char *name;

    name = kasprintf(GFP_KERNEL, "spinlock%d", cpu);
    irq = bind_ipi_to_irqhandler(XEN_SPIN_UNLOCK_VECTOR,
                     cpu,
                     dummy_handler,
                     IRQF_DISABLED|IRQF_PERCPU|IRQF_NOBALANCING,
                     name,
                     NULL);

    if (irq >= 0) {
        disable_irq(irq); /* make sure it's never delivered */
        per_cpu(lock_kicker_irq, cpu) = irq;
    }

    printk("cpu %d spinlock event irq %d\n", cpu, irq);
}

void xen_uninit_lock_cpu(int cpu)
{
    unbind_from_irqhandler(per_cpu(lock_kicker_irq, cpu), NULL);
}

void __init xen_init_spinlocks(void)
{
    BUILD_BUG_ON(sizeof(struct xen_spinlock) > sizeof(arch_spinlock_t));

    pv_lock_ops.spin_is_locked = xen_spin_is_locked;
    pv_lock_ops.spin_is_contended = xen_spin_is_contended;
    pv_lock_ops.spin_lock = xen_spin_lock;
    pv_lock_ops.spin_lock_flags = xen_spin_lock_flags;
    pv_lock_ops.spin_trylock = xen_spin_trylock;
    pv_lock_ops.spin_unlock = xen_spin_unlock;
}

#ifdef CONFIG_XEN_DEBUG_FS

static struct dentry *d_spin_debug;

static int __init xen_spinlock_debugfs(void)
{
    struct dentry *d_xen = xen_init_debugfs();

    if (d_xen == NULL)
        return -ENOMEM;

    d_spin_debug = debugfs_create_dir("spinlocks", d_xen);

    debugfs_create_u8("zero_stats", 0644, d_spin_debug, &zero_stats);

    debugfs_create_u32("timeout", 0644, d_spin_debug, &lock_timeout);

    debugfs_create_u64("taken", 0444, d_spin_debug, &spinlock_stats.taken);
    debugfs_create_u32("taken_slow", 0444, d_spin_debug,
               &spinlock_stats.taken_slow);
    debugfs_create_u32("taken_slow_nested", 0444, d_spin_debug,
               &spinlock_stats.taken_slow_nested);
    debugfs_create_u32("taken_slow_pickup", 0444, d_spin_debug,
               &spinlock_stats.taken_slow_pickup);
    debugfs_create_u32("taken_slow_spurious", 0444, d_spin_debug,
               &spinlock_stats.taken_slow_spurious);
    debugfs_create_u32("taken_slow_irqenable", 0444, d_spin_debug,
               &spinlock_stats.taken_slow_irqenable);

    debugfs_create_u64("released", 0444, d_spin_debug, &spinlock_stats.released);
    debugfs_create_u32("released_slow", 0444, d_spin_debug,
               &spinlock_stats.released_slow);
    debugfs_create_u32("released_slow_kicked", 0444, d_spin_debug,
               &spinlock_stats.released_slow_kicked);

    debugfs_create_u64("time_spinning", 0444, d_spin_debug,
               &spinlock_stats.time_spinning);
    debugfs_create_u64("time_blocked", 0444, d_spin_debug,
               &spinlock_stats.time_blocked);
    debugfs_create_u64("time_total", 0444, d_spin_debug,
               &spinlock_stats.time_total);

    debugfs_create_u32_array("histo_total", 0444, d_spin_debug,
                spinlock_stats.histo_spin_total, HISTO_BUCKETS + 1);
    debugfs_create_u32_array("histo_spinning", 0444, d_spin_debug,
                spinlock_stats.histo_spin_spinning, HISTO_BUCKETS + 1);
    debugfs_create_u32_array("histo_blocked", 0444, d_spin_debug,
                spinlock_stats.histo_spin_blocked, HISTO_BUCKETS + 1);

    return 0;
}
fs_initcall(xen_spinlock_debugfs);

#endif  /* CONFIG_XEN_DEBUG_FS */