[PATCH v2] nptl: pthread_rwlock_rdlock return in low priority

[abush wang]

(*This is just a mail for advisory*)

hi, 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 fast-path is reasonable, and whether
this posix standard should be enforced.

Thanks