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From: Sunil Pandey <skpgkp2@sourceware.org> To: glibc-cvs@sourceware.org Subject: [glibc/release/2.34/master] nptl: Effectively skip CAS in spinlock loop Date: Wed, 28 Sep 2022 16:06:14 +0000 (GMT) [thread overview] Message-ID: <20220928160614.95682385782C@sourceware.org> (raw) https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=43760d33d78f9ea8c8af942b570112ee801b99df commit 43760d33d78f9ea8c8af942b570112ee801b99df Author: Jangwoong Kim <6812skiii@gmail.com> Date: Tue Dec 14 21:30:51 2021 +0900 nptl: Effectively skip CAS in spinlock loop The commit: "Add LLL_MUTEX_READ_LOCK [BZ #28537]" SHA1: d672a98a1af106bd68deb15576710cd61363f7a6 introduced LLL_MUTEX_READ_LOCK, to skip CAS in spinlock loop if atomic load fails. But, "continue" inside of do-while loop does not skip the evaluation of escape expression, thus CAS is not skipped. Replace do-while with while and skip LLL_MUTEX_TRYLOCK if LLL_MUTEX_READ_LOCK fails. Reviewed-by: H.J. Lu <hjl.tools@gmail.com> (cherry picked from commit 6b8dbbd03ac88f169b65b5c7d7278576a11d2e44) Diff: --- nptl/pthread_mutex_lock.c | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-) diff --git a/nptl/pthread_mutex_lock.c b/nptl/pthread_mutex_lock.c index 49901ffa0a..bbe754a272 100644 --- a/nptl/pthread_mutex_lock.c +++ b/nptl/pthread_mutex_lock.c @@ -147,10 +147,9 @@ PTHREAD_MUTEX_LOCK (pthread_mutex_t *mutex) break; } atomic_spin_nop (); - if (LLL_MUTEX_READ_LOCK (mutex) != 0) - continue; } - while (LLL_MUTEX_TRYLOCK (mutex) != 0); + while (LLL_MUTEX_READ_LOCK (mutex) != 0 + || LLL_MUTEX_TRYLOCK (mutex) != 0); mutex->__data.__spins += (cnt - mutex->__data.__spins) / 8; }
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