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Re: [PATCH 1/3] Add a fast path for C rd/wrlock v2
- From: "Carlos O'Donell" <carlos at redhat dot com>
- To: Andi Kleen <andi at firstfloor dot org>, libc-alpha at sourceware dot org
- Cc: Andi Kleen <ak at linux dot intel dot com>
- Date: Mon, 09 Jun 2014 15:13:05 -0400
- Subject: Re: [PATCH 1/3] Add a fast path for C rd/wrlock v2
- Authentication-results: sourceware.org; auth=none
- References: <1399412209-28245-1-git-send-email-andi at firstfloor dot org> <1399412209-28245-2-git-send-email-andi at firstfloor dot org>
On 05/06/2014 05:36 PM, Andi Kleen wrote:
> From: Andi Kleen <ak@linux.intel.com>
>
> 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.
Sounds like the best of both worlds. Thanks for the cleanup.
> nptl/:
> 2014-05-06 Andi Kleen <ak@linux.intel.com>
>
> * pthread_rwlock_rdlock (__pthread_rwlock_rdlock):
> Split into __do_pthread_rwlock_rdlock and __pthread_rwlock_rdlock.
> * pthread_rwlock_wrlock (__pthread_rwlock_wrlock):
> Split into __do_pthread_rwlock_wrlock and __pthread_wrlock_rdlock.
Looks good to me.
> v2: Rename some functions and add space.
> ---
> nptl/pthread_rwlock_rdlock.c | 88 ++++++++++++++++++++++++++++++--------------
> nptl/pthread_rwlock_wrlock.c | 59 ++++++++++++++++++++---------
> 2 files changed, 103 insertions(+), 44 deletions(-)
>
> diff --git a/nptl/pthread_rwlock_rdlock.c b/nptl/pthread_rwlock_rdlock.c
> index 3773f7d..1df0327 100644
> --- a/nptl/pthread_rwlock_rdlock.c
> +++ b/nptl/pthread_rwlock_rdlock.c
> @@ -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)
OK.
> {
> 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. */
OK.
>
> 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;
> + }
OK, shuffle around the check because we do it once already
in the fast path, and we need to do it here *after* all the
other checks but before we loop.
> }
>
> /* 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);
> +}
OK.
> +
> weak_alias (__pthread_rwlock_rdlock, pthread_rwlock_rdlock)
> hidden_def (__pthread_rwlock_rdlock)
> diff --git a/nptl/pthread_rwlock_wrlock.c b/nptl/pthread_rwlock_wrlock.c
> index 1613d45..de54e51 100644
> --- a/nptl/pthread_rwlock_wrlock.c
> +++ b/nptl/pthread_rwlock_wrlock.c
> @@ -25,29 +25,15 @@
>
>
> /* Acquire write lock for RWLOCK. */
> -int
> -__pthread_rwlock_wrlock (rwlock)
> - pthread_rwlock_t *rwlock;
> +static int __attribute__((noinline))
> +__pthread_rwlock_wrlock_slow (pthread_rwlock_t *rwlock)
OK.
> {
> int result = 0;
>
> - LIBC_PROBE (wrlock_entry, 1, rwlock);
> -
> - /* Make sure we are alone. */
> - lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
> + /* Caller has taken the lock. */
OK.
>
> while (1)
> {
> - /* Get the rwlock if there is no writer and no reader. */
> - if (rwlock->__data.__writer == 0 && rwlock->__data.__nr_readers == 0)
> - {
> - /* Mark self as writer. */
> - rwlock->__data.__writer = THREAD_GETMEM (THREAD_SELF, tid);
> -
> - LIBC_PROBE (wrlock_acquire_write, 1, rwlock);
> - break;
> - }
> -
OK.
> /* 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
> @@ -80,6 +66,16 @@ __pthread_rwlock_wrlock (rwlock)
>
> /* To start over again, remove the thread from the writer list. */
> --rwlock->__data.__nr_writers_queued;
> +
> + /* Get the rwlock if there is no writer and no reader. */
> + if (rwlock->__data.__writer == 0 && rwlock->__data.__nr_readers == 0)
> + {
> + /* Mark self as writer. */
> + rwlock->__data.__writer = THREAD_GETMEM (THREAD_SELF, tid);
> +
> + LIBC_PROBE (wrlock_acquire_write, 1, rwlock);
> + break;
OK.
> + }
> }
>
> /* We are done, free the lock. */
> @@ -88,5 +84,34 @@ __pthread_rwlock_wrlock (rwlock)
> return result;
> }
>
> +/* Fast path of acquiring write lock for RWLOCK. */
> +
> +int
> +__pthread_rwlock_wrlock (pthread_rwlock_t *rwlock)
> +{
> + LIBC_PROBE (wrlock_entry, 1, rwlock);
> +
> + /* Make sure we are alone. */
> + lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
> +
> + /* Get the rwlock if there is no writer and no reader. */
> + if (__glibc_likely((rwlock->__data.__writer |
> + rwlock->__data.__nr_readers) == 0))
> + {
> + /* Mark self as writer. */
> + rwlock->__data.__writer = THREAD_GETMEM (THREAD_SELF, tid);
> +
> + LIBC_PROBE (wrlock_acquire_write, 1, rwlock);
> +
> + /* We are done, free the lock. */
> + lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
> +
> + return 0;
> + }
> +
> + return __pthread_rwlock_wrlock_slow (rwlock);
> +}
> +
> +
OK.
> weak_alias (__pthread_rwlock_wrlock, pthread_rwlock_wrlock)
> hidden_def (__pthread_rwlock_wrlock)
>
Looks good to me, and should go in right away.
Cheers,
Carlos.