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

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

[yeatesrichardr at johndeere dot com]

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.

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

[yeatesrichardr at johndeere dot com]

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

--- Comment #1 from Ric Yeates <yeatesrichardr at johndeere dot com> ---
Forgot to include example output.

$ futex_bug
FAILURE: ETIMEDOUT should have been returned after 10 or more seconds, returned
after 0 seconds.

$ uname -a
Linux PCMA4MA000002.local.domain 5.4.47-2.2.0 #1 SMP PREEMPT Mon Oct 26
23:52:12 UTC 2020 aarch64 GNU/Linux

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[Bug nptl/26801] pthread_mutex_clocklock with CLOCK_MONOTONIC can fail on PI mutexes

[yeatesrichardr at johndeere dot com]

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

Ric Yeates <yeatesrichardr at johndeere dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|pthread_mutex_clocklock     |pthread_mutex_clocklock
                   |with CLOCK_MONOTONIC can    |with CLOCK_MONOTONIC can
                   |fail on PI futexs           |fail on PI mutexes

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[Bug nptl/26801] pthread_mutex_clocklock with CLOCK_MONOTONIC can fail on PI mutexes

[yeatesrichardr at johndeere dot com]

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

Ric Yeates <yeatesrichardr at johndeere dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |yeatesrichardr at johndeere dot co
                   |                            |m

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[Bug nptl/26801] pthread_mutex_clocklock with CLOCK_MONOTONIC can fail on PI mutexes

[adhemerval.zanella at linaro dot org]

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

Adhemerval Zanella <adhemerval.zanella at linaro dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |adhemerval.zanella at linaro dot o
                   |                            |rg

--- Comment #2 from Adhemerval Zanella <adhemerval.zanella at linaro dot org> ---
We need kernel support for FUTEX_LOCK_PI with CLOCK_REALTIME due possible non
continuous clock, but currently Linux does not supporting set the clock for the
PI operations (it always assumes CLOCK_REALTIME).

I think it would be better to just return EINVAL is a clock different than
CLOCK_REALTIME is used with pthread_mutex_clocklock and PTHREAD_PRIO_INHERIT.

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[Bug nptl/26801] pthread_mutex_clocklock with CLOCK_MONOTONIC can fail on PI mutexes

[adhemerval.zanella at linaro dot org]

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

Adhemerval Zanella <adhemerval.zanella at linaro dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
           Assignee|unassigned at sourceware dot org   |adhemerval.zanella at linaro dot o
                   |                            |rg
   Target Milestone|---                         |2.33
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #3 from Adhemerval Zanella <adhemerval.zanella at linaro dot org> ---
Fixed on 2.33.

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