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Add a fast path for C rd/wrlock v2

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One difference of the C versions to the assembler wr/rdlock
is that the C compiler saves some registers which are unnecessary
for the fast path in the prologue of the functions. Split the
uncontended fast path out into a separate function. Only when contention is
detected is the full featured function called. This makes
the fast path code (nearly) identical to the assembler version,
and gives uncontended performance within a few cycles.

v2: Rename some functions and add space.
This commit is contained in:
Andi Kleen
2014-03-17 05:48:28 -07:00
parent 995a46bbfb
commit a832bdd362
3 changed files with 110 additions and 44 deletions
Download Patch File Download Diff File Expand all files Collapse all files

88
nptl/pthread_rwlock_rdlock.c
View File

@ -24,39 +24,16 @@
#include <stap-probe.h>
/* Acquire read lock for RWLOCK. */
int
__pthread_rwlock_rdlock (rwlock)
pthread_rwlock_t *rwlock;
/* Acquire read lock for RWLOCK. Slow path. */
static int __attribute__((noinline))
__pthread_rwlock_rdlock_slow (pthread_rwlock_t *rwlock)
{
int result = 0;
LIBC_PROBE (rdlock_entry, 1, rwlock);
/* Make sure we are alone. */
lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
/* Lock is taken in caller. */
while (1)
{
/* Get the rwlock if there is no writer... */
if (rwlock->__data.__writer == 0
/* ...and if either no writer is waiting or we prefer readers. */
&& (!rwlock->__data.__nr_writers_queued
|| PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
{
/* Increment the reader counter. Avoid overflow. */
if (__glibc_unlikely (++rwlock->__data.__nr_readers == 0))
{
/* Overflow on number of readers. */
--rwlock->__data.__nr_readers;
result = EAGAIN;
}
else
LIBC_PROBE (rdlock_acquire_read, 1, rwlock);
break;
}
/* Make sure we are not holding the rwlock as a writer. This is
a deadlock situation we recognize and report. */
if (__builtin_expect (rwlock->__data.__writer
@ -88,6 +65,25 @@ __pthread_rwlock_rdlock (rwlock)
lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
--rwlock->__data.__nr_readers_queued;
/* Get the rwlock if there is no writer... */
if (rwlock->__data.__writer == 0
/* ...and if either no writer is waiting or we prefer readers. */
&& (!rwlock->__data.__nr_writers_queued
|| PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
{
/* Increment the reader counter. Avoid overflow. */
if (__glibc_unlikely (++rwlock->__data.__nr_readers == 0))
{
/* Overflow on number of readers. */
--rwlock->__data.__nr_readers;
result = EAGAIN;
}
else
LIBC_PROBE (rdlock_acquire_read, 1, rwlock);
break;
}
}
/* We are done, free the lock. */
@ -96,5 +92,43 @@ __pthread_rwlock_rdlock (rwlock)
return result;
}
/* Fast path of acquiring read lock on RWLOCK. */
int
__pthread_rwlock_rdlock (pthread_rwlock_t *rwlock)
{
int result = 0;
LIBC_PROBE (rdlock_entry, 1, rwlock);
/* Make sure we are alone. */
lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
/* Get the rwlock if there is no writer... */
if (rwlock->__data.__writer == 0
/* ...and if either no writer is waiting or we prefer readers. */
&& (!rwlock->__data.__nr_writers_queued
|| PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
{
/* Increment the reader counter. Avoid overflow. */
if (__glibc_unlikely (++rwlock->__data.__nr_readers == 0))
{
/* Overflow on number of readers. */
--rwlock->__data.__nr_readers;
result = EAGAIN;
}
else
LIBC_PROBE (rdlock_acquire_read, 1, rwlock);
/* We are done, free the lock. */
lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
return result;
}
return __pthread_rwlock_rdlock_slow (rwlock);
}
weak_alias (__pthread_rwlock_rdlock, pthread_rwlock_rdlock)
hidden_def (__pthread_rwlock_rdlock)