[Bug nptl/21291] New: pthread cancellation fails when contending lock

["dimitri.staessens at ugent dot be" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=21291

            Bug ID: 21291
           Summary: pthread cancellation fails when contending lock
           Product: glibc
           Version: 2.25
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: nptl
          Assignee: unassigned at sourceware dot org
          Reporter: dimitri.staessens at ugent dot be
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

Created attachment 9930
  --> https://sourceware.org/bugzilla/attachment.cgi?id=9930&action=edit
code example that reproduces the problem

We found that with the 2.25 release of glibc, sometimes pthread_cancel fails to
cancel threads that are contending a robust mutex. At least, that's what we
think is happening. It's not that easy to explain, so find attached a code
example that reproduces the problem.

In the code it has a TIMEOUT defined, increasing the value will reduce the
chances of the cancel failing as the threads are more likely to be in the
timedwait. On glibc 2.24 it works even with a timeout set to 0 (it is set to 0
in the attached source file).

We run the following commands:

gcc robust_test.c -lpthread -lrt -o robust_test
./robust_test

expected output is:

Initializing...
Starting threads...
Sleeping for one second...
Thread started...
Thread started...
Cancelling threads...
Threads finished.
Bye.

Which works fine on glibc 2.24-2 (Arch Linux). However, on glibc 2.25-1, it
hangs at "Cancelling threads...":
Initializing...
Starting threads...
Sleeping for one second...
Thread started...
Thread started...
Cancelling threads...

I hope this report is useful to you.

Sincerely,

Dimitri Staessens.

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