[Bug nptl/26801] New: pthread_mutex_clocklock with CLOCK_MONOTONIC can fail on PI futexs

["yeatesrichardr at johndeere dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=26801

            Bug ID: 26801
           Summary: pthread_mutex_clocklock with CLOCK_MONOTONIC can fail
                    on PI futexs
           Product: glibc
           Version: 2.30
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: nptl
          Assignee: unassigned at sourceware dot org
          Reporter: yeatesrichardr at johndeere dot com
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

Created attachment 12925
  --> https://sourceware.org/bugzilla/attachment.cgi?id=12925&action=edit
Test program showing early return from pthread_mutex_clocklock()

When a pthread mutex is ROBUST_NP, ERRORCHECK, and PRIO_INHERIT (other
combination may fall into this same category) and pthread_mutex_clocklock() is
used with CLOCK_MONOTONIC, the call will return immediately with ETIMEDOUT if
the mutex is already taken at the time of the call.

This was tested with glibc 2.30, but still appears to be present in the latest
source code. The root cause is at line 381 of pthread_mutex_timedlock.c:
   int e = futex_lock_pi64 (&mutex->__data.__lock, abstime, private);
Here, the CLOCK_MONOTONIC abstime is being passed to futex_lock_pi64 which
requires a CLOCK_REALTIME abstime. Of course, a CLOCK_MONOTONIC looks a lot
"earlier" than a CLOCK_REALTIME, so it times out immediately.

It's not sufficient to convert from CLOCK_MONOTONIC to CLOCK_REALTIME since
that would violate the assumption one is using, that changes to the wall clock
time won't interfere with CLOCK_MONOTONIC timed waits.

This was tested with kernel 5.4.47.

Example program is attached.

-- 
You are receiving this mail because:
You are on the CC list for the bug.