From: abush wang <abushwangs@gmail.com>
To: triegel@redhat.com,
abushwang via Libc-alpha <libc-alpha@sourceware.org>,
adhemerval.zanella@linaro.org
Subject: pthread_rwlock_rdlock return in low priority
Date: Tue, 7 Mar 2023 20:23:44 +0800 [thread overview]
Message-ID: <CAMLoAPab2zP4=t2V10f=E5KNNLqY4ZjjrPtH8-hUj64LvbU4-g@mail.gmail.com> (raw)
[-- Attachment #1: Type: text/plain, Size: 2233 bytes --]
hi, Riegel
I have noticed reader will return directly on fast-path in
pthread_rwlock_common.c
>* /* We have registered as a reader, so if we are in a read phase, we have
*>* acquired a read lock. This is also the reader--reader fast-path.
*>* Even if there is a primary writer, we just return. If writers are to
*>* be preferred and we are the only active reader, we could try to enter a
*>* write phase to let the writer proceed. This would be okay because we
*>* cannot have acquired the lock previously as a reader (which could result
*>* in deadlock if we would wait for the primary writer to run). However,
*>* this seems to be a corner case and handling it specially not
be worth the
*>* complexity. */
*>* if (__glibc_likely ((r & PTHREAD_RWLOCK_WRPHASE) == 0))
*>* return 0;
*
However, there is a situation:
main, thread_wr, thread_rd.
SCHED_FIFO priority:
main > thread_wr > thread_rd
main first acquires read lock, then create thread_wr which will
block on the lock.
Next, main creates thread_rd. this thread will acquires read lock
on fast-path even
though it has a lower priority compared to thread_wr.
You can get demo from the following
repository:https://github.com/emscripten-core/posixtestsuite.git
./conformance/interfaces/pthread_rwlock_rdlock/2-1.c
According to "man -M man-pages-posix-2017/ 3p pthread_rwlock_rdlock"
>* DESCRIPTION
*>* The pthread_rwlock_rdlock() function shall apply a read lock to the
*>* read-write lock referenced by rwlock. The calling thread acquires the
*>* read lock if a writer does not hold the lock and there are no
*>* writers blocked on the lock.
*>>* If the Thread Execution Scheduling option is supported, and the
*>* threads involved in the lock are executing with the scheduling
*>* policies SCHED_FIFO or SCHED_RR, the calling thread shall not acquire
*>* the lock if a writer holds the lock or if writers of higher or equal
*>* priority are blocked on the lock; other‐ wise, the calling thread
*>* shall acquire the lock.
*
I was wondering that whether this
, and whether
this posix standard should be enforced.
Thanks
abushwang
next reply other threads:[~2023-03-07 12:23 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-07 12:23 abush wang [this message]
2023-03-07 17:09 ` Xi Ruoyao
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