[Bug nptl/20973] New: robust mutexes: Lost wake-ups

["triegel at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=20973

            Bug ID: 20973
           Summary: robust mutexes: Lost wake-ups
           Product: glibc
           Version: 2.25
            Status: NEW
          Severity: normal
          Priority: P2
         Component: nptl
          Assignee: unassigned at sourceware dot org
          Reporter: triegel at redhat dot com
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

Assume that Thread 1 waits to acquire a robust mutex using futexes to block
(and thus sets the FUTEX_WAITERS flag), and is unblocked when this mutex is
released.  If Thread 2 concurrently acquires the lock and is killed, Thread 1
can recover from the died owner but fail to restore the FUTEX_WAITERS flag. 
This can lead to a Thread 3 that also blocked using futexes at the same time as
Thread 1 to not get woken up because FUTEX_WAITERS is not set anymore.

This has been previously reported here:
https://bugzilla.redhat.com/show_bug.cgi?id=1401665
That BZ also contains a testcase.

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