[Bug nptl/24161] __run_fork_handlers self-deadlocks in malloc/tst-mallocfork2

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https://sourceware.org/bugzilla/show_bug.cgi?id=24161

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=60f80624257ef84eacfd9b400bda1b5a5e8e7816

commit 60f80624257ef84eacfd9b400bda1b5a5e8e7816
Author: Florian Weimer <fweimer@redhat.com>
Date:   Fri Feb 8 12:55:21 2019 +0100

    nptl: Avoid fork handler lock for async-signal-safe fork [BZ #24161]

    Commit 27761a1042daf01987e7d79636d0c41511c6df3c ("Refactor atfork
    handlers") introduced a lock, atfork_lock, around fork handler list
    accesses.  It turns out that this lock occasionally results in
    self-deadlocks in malloc/tst-mallocfork2:

    (gdb) bt
    #0  __lll_lock_wait_private ()
        at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:63
    #1  0x00007f160c6f927a in __run_fork_handlers (who=(unknown: 209394016),
        who@entry=atfork_run_prepare) at register-atfork.c:116
    #2  0x00007f160c6b7897 in __libc_fork () at ../sysdeps/nptl/fork.c:58
    #3  0x00000000004027d6 in sigusr1_handler (signo=<optimized out>)
        at tst-mallocfork2.c:80
    #4  sigusr1_handler (signo=<optimized out>) at tst-mallocfork2.c:64
    #5  <signal handler called>
    #6  0x00007f160c6f92e4 in __run_fork_handlers
(who=who@entry=atfork_run_parent)
        at register-atfork.c:136
    #7  0x00007f160c6b79a2 in __libc_fork () at ../sysdeps/nptl/fork.c:152
    #8  0x0000000000402567 in do_test () at tst-mallocfork2.c:156
    #9  0x0000000000402dd2 in support_test_main (argc=1, argv=0x7ffc81ef1ab0,
        config=config@entry=0x7ffc81ef1970) at support_test_main.c:350
    #10 0x0000000000402362 in main (argc=<optimized out>, argv=<optimized out>)
        at ../support/test-driver.c:168

    If no locking happens in the single-threaded case (where fork is
    expected to be async-signal-safe), this deadlock is avoided.
    (pthread_atfork is not required to be async-signal-safe, so a fork
    call from a signal handler interrupting pthread_atfork is not
    a problem.)

    (cherry picked from commit 669ff911e2571f74a2668493e326ac9a505776bd)

-----------------------------------------------------------------------

Summary of changes:
 ChangeLog              |   10 ++++++++++
 NEWS                   |    1 +
 nptl/register-atfork.c |    8 +++++---
 sysdeps/nptl/fork.c    |    6 +++---
 sysdeps/nptl/fork.h    |    8 +++++---
 5 files changed, 24 insertions(+), 9 deletions(-)

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