Readers/Writer Lock Probes
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Readers/Writer Lock Probes

Readers/writer locks enforce a policy of allowing multiple readers or a single writer — but not both — to be in a critical section. These locks are typically used for structures that are searched more frequently than they are modified and for which there is substantial time in the critical section. If critical section times are short, readers/writer locks will implicitly serialize over the shared memory used to implement the lock, giving them no advantage over adaptive locks. See rwlock ( 9F ) for more details on readers/writer locks.

The probes pertaining to readers/writer locks are in Table 18–4. For each probe, arg0 contains a pointer to the krwlock_t structure that represents the adaptive lock.

Table 18.4. Readers/Writer Lock Probes

rw-acquire

Hold-event probe that fires immediately after a readers/writer lock is acquired. arg1 contains the constant RW_READER if the lock was acquired as a reader, and RW_WRITER if the lock was acquired as a writer.

rw-block

Contention-event probe that fires after a thread that has blocked on a held readers/writer lock has reawakened and has acquired the lock. arg1 contains the length of time (in nanoseconds) that the current thread had to sleep to acquire the lock. arg2 contains the constant RW_READER if the lock was acquired as a reader, and RW_WRITER if the lock was acquired as a writer. arg3 and arg4 contain more information on the reason for blocking. arg3 is non-zero if and only if the lock was held as a writer when the current thread blocked. arg4 contains the readers count when the current thread blocked. If both the rw-block and rw-acquire probes are enabled, rw-block fires before rw-acquire.

rw-upgrade

Hold-event probe that fires after a thread has successfully upgraded a readers/writer lock from a reader to a writer. Upgrades do not have an associated contention event because they are only possible through a non-blocking interface, rw_tryupgrade ( TRYUPGRADE.9F ) .

rw-downgrade

Hold-event probe that fires after a thread had downgraded its ownership of a readers/writer lock from writer to reader. Downgrades do not have an associated contention event because they always succeed without contention.

rw-release

Hold-event probe that fires immediately after a readers/writer lock is released. arg1 contains the constant RW_READER if the released lock was held as a reader, and RW_WRITER if the released lock was held as a writer. Due to upgrades and downgrades, the lock may not have been released as it was acquired.

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