--- /dev/null
+From 16b9af737c77b153fca4f36cbdbe94f7416c0b42 Mon Sep 17 00:00:00 2001
+From: Malte Skarupke <malteskarupke@fastmail.fm>
+Date: Mon, 16 Jun 2025 23:38:40 -0700
+Subject: [PATCH] nptl: Remove unnecessary quadruple check in pthread_cond_wait
+
+pthread_cond_wait was checking whether it was in a closed group no less than
+four times. Checking once is enough. Here are the four checks:
+
+1. While spin-waiting. This was dead code: maxspin is set to 0 and has been
+ for years.
+2. Before deciding to go to sleep, and before incrementing grefs: I kept this
+3. After incrementing grefs. There is no reason to think that the group would
+ close while we do an atomic increment. Obviously it could close at any
+ point, but that doesn't mean we have to recheck after every step. This
+ check was equally good as check 2, except it has to do more work.
+4. When we find ourselves in a group that has a signal. We only get here after
+ we check that we're not in a closed group. There is no need to check again.
+ The check would only have helped in cases where the compare_exchange in the
+ next line would also have failed. Relying on the compare_exchange is fine.
+
+Removing the duplicate checks clarifies the code.
+
+The following commits have been cherry-picked from Glibc master branch:
+Bug : https://sourceware.org/bugzilla/show_bug.cgi?id=25847
+
+Upstream-Status: Backport
+[https://sourceware.org/git/?p=glibc.git;a=commit;h=4f7b051f8ee3feff1b53b27a906f245afaa9cee1]
+
+Signed-off-by: Sunil Dora <sunilkumar.dora@windriver.com>
+---
+ nptl/pthread_cond_wait.c | 49 ----------------------------------------
+ 1 file changed, 49 deletions(-)
+
+diff --git a/nptl/pthread_cond_wait.c b/nptl/pthread_cond_wait.c
+index cee1968756..47e834cade 100644
+--- a/nptl/pthread_cond_wait.c
++++ b/nptl/pthread_cond_wait.c
+@@ -366,7 +366,6 @@ static __always_inline int
+ __pthread_cond_wait_common (pthread_cond_t *cond, pthread_mutex_t *mutex,
+ clockid_t clockid, const struct __timespec64 *abstime)
+ {
+- const int maxspin = 0;
+ int err;
+ int result = 0;
+
+@@ -425,33 +424,6 @@ __pthread_cond_wait_common (pthread_cond_t *cond, pthread_mutex_t *mutex,
+ uint64_t g1_start = __condvar_load_g1_start_relaxed (cond);
+ unsigned int lowseq = (g1_start & 1) == g ? signals : g1_start & ~1U;
+
+- /* Spin-wait first.
+- Note that spinning first without checking whether a timeout
+- passed might lead to what looks like a spurious wake-up even
+- though we should return ETIMEDOUT (e.g., if the caller provides
+- an absolute timeout that is clearly in the past). However,
+- (1) spurious wake-ups are allowed, (2) it seems unlikely that a
+- user will (ab)use pthread_cond_wait as a check for whether a
+- point in time is in the past, and (3) spinning first without
+- having to compare against the current time seems to be the right
+- choice from a performance perspective for most use cases. */
+- unsigned int spin = maxspin;
+- while (spin > 0 && ((int)(signals - lowseq) < 2))
+- {
+- /* Check that we are not spinning on a group that's already
+- closed. */
+- if (seq < (g1_start >> 1))
+- break;
+-
+- /* TODO Back off. */
+-
+- /* Reload signals. See above for MO. */
+- signals = atomic_load_acquire (cond->__data.__g_signals + g);
+- g1_start = __condvar_load_g1_start_relaxed (cond);
+- lowseq = (g1_start & 1) == g ? signals : g1_start & ~1U;
+- spin--;
+- }
+-
+ if (seq < (g1_start >> 1))
+ {
+ /* If the group is closed already,
+@@ -482,24 +454,6 @@ __pthread_cond_wait_common (pthread_cond_t *cond, pthread_mutex_t *mutex,
+ an atomic read-modify-write operation and thus extend the release
+ sequence. */
+ atomic_fetch_add_acquire (cond->__data.__g_refs + g, 2);
+- signals = atomic_load_acquire (cond->__data.__g_signals + g);
+- g1_start = __condvar_load_g1_start_relaxed (cond);
+- lowseq = (g1_start & 1) == g ? signals : g1_start & ~1U;
+-
+- if (seq < (g1_start >> 1))
+- {
+- /* group is closed already, so don't block */
+- __condvar_dec_grefs (cond, g, private);
+- goto done;
+- }
+-
+- if ((int)(signals - lowseq) >= 2)
+- {
+- /* a signal showed up or G1/G2 switched after we grabbed the
+- refcount */
+- __condvar_dec_grefs (cond, g, private);
+- break;
+- }
+
+ // Now block.
+ struct _pthread_cleanup_buffer buffer;
+@@ -533,9 +487,6 @@ __pthread_cond_wait_common (pthread_cond_t *cond, pthread_mutex_t *mutex,
+ /* Reload signals. See above for MO. */
+ signals = atomic_load_acquire (cond->__data.__g_signals + g);
+ }
+-
+- if (seq < (__condvar_load_g1_start_relaxed (cond) >> 1))
+- goto done;
+ }
+ /* Try to grab a signal. See above for MO. (if we do another loop
+ iteration we need to see the correct value of g1_start) */
+--
+2.49.0
+
projects / thirdparty / openembedded / openembedded-core-contrib.git / commitdiff
