From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <sourceware-bugzilla@sourceware.org>
Received: by sourceware.org (Postfix, from userid 48)
 id 1FD6A386F41D; Wed, 28 Oct 2020 13:54:31 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 1FD6A386F41D
From: "yeatesrichardr at johndeere dot com"
 <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug nptl/26801] New: pthread_mutex_clocklock with CLOCK_MONOTONIC
 can fail on PI futexs
Date: Wed, 28 Oct 2020 13:54:30 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: glibc
X-Bugzilla-Component: nptl
X-Bugzilla-Version: 2.30
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: yeatesrichardr at johndeere dot com
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P2
X-Bugzilla-Assigned-To: unassigned at sourceware dot org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: bug_id short_desc product version bug_status
 bug_severity priority component assigned_to reporter cc target_milestone
 attachments.created
Message-ID: <bug-26801-131@http.sourceware.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Bugzilla-URL: http://sourceware.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
X-BeenThere: glibc-bugs@sourceware.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Glibc-bugs mailing list <glibc-bugs.sourceware.org>
List-Unsubscribe: <https://sourceware.org/mailman/options/glibc-bugs>,
 <mailto:glibc-bugs-request@sourceware.org?subject=unsubscribe>
List-Archive: <https://sourceware.org/pipermail/glibc-bugs/>
List-Help: <mailto:glibc-bugs-request@sourceware.org?subject=help>
List-Subscribe: <https://sourceware.org/mailman/listinfo/glibc-bugs>,
 <mailto:glibc-bugs-request@sourceware.org?subject=subscribe>
X-List-Received-Date: Wed, 28 Oct 2020 13:54:31 -0000

https://sourceware.org/bugzilla/show_bug.cgi?id=3D26801

            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=3D12925&action=3Ded=
it
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 lat=
est
source code. The root cause is at line 381 of pthread_mutex_timedlock.c:
   int e =3D 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 cl=
ock
time won't interfere with CLOCK_MONOTONIC timed waits.

This was tested with kernel 5.4.47.

Example program is attached.

--=20
You are receiving this mail because:
You are on the CC list for the bug.=