[Bug dynamic-link/18457] pthread_join deadlock in library destructor

["cvs-commit at gcc dot gnu.org" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=18457

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=e400f3ccd36fe91d432cc7d45b4ccc799dece763

commit e400f3ccd36fe91d432cc7d45b4ccc799dece763
Author: Siddhesh Poyarekar <siddhesh@redhat.com>
Date:   Fri Jul 24 19:13:38 2015 +0530

    Use IE model for static variables in libc.so, libpthread.so and rtld

    The recently introduced TLS variables in the thread-local destructor
    implementation (__cxa_thread_atexit_impl) used the default GD access
    model, resulting in a call to __tls_get_addr.  This causes a deadlock
    with recent changes to the way TLS is initialized because DTV
    allocations are delayed and hence despite knowing the offset to the
    variable inside its TLS block, the thread has to take the global rtld
    lock to safely update the TLS offset.

    This causes deadlocks when a thread is instantiated and joined inside
    a destructor of a dlopen'd DSO.  The correct long term fix is to
    somehow not take the lock, but that will need a lot deeper change set
    to alter the way in which the big rtld lock is used.

    Instead, this patch just eliminates the call to __tls_get_addr for the
    thread-local variables inside libc.so, libpthread.so and rtld by
    building all of their units with -mtls-model=initial-exec.

    There were concerns that the static storage for TLS is limited and
    hence we should not be using it.  Additionally, dynamically loaded
    modules may result in libc.so looking for this static storage pretty
    late in static binaries.  Both concerns are valid when using TLSDESC
    since that is where one may attempt to allocate a TLS block from
    static storage for even those variables that are not IE.  They're not
    very strong arguments for the traditional TLS model though, since it
    assumes that the static storage would be used sparingly and definitely
    not by default.  Hence, for now this would only theoretically affect
    ARM architectures.

    The impact is hence limited to statically linked binaries that dlopen
    modules that in turn load libc.so, all that on arm hardware.  It seems
    like a small enough impact to justify fixing the larger problem that
    currently affects everything everywhere.

    This still does not solve the original problem completely.  That is,
    it is still possible to deadlock on the big rtld lock with a small
    tweak to the test case attached to this patch.  That problem is
    however not a regression in 2.22 and hence could be tackled as a
    separate project.  The test case is picked up as is from Alex's patch.

    This change has been tested to verify that it does not cause any
    issues on x86_64.

    ChangeLog:

        [BZ #18457]
        * nptl/Makefile (tests): New test case tst-join7.
        (modules-names): New test case module tst-join7mod.
        * nptl/tst-join7.c: New file.
        * nptl/tst-join7mod.c: New file.
        * Makeconfig (tls-model): Pass -ftls-model=initial-exec for
        all translation units in libc.so, libpthread.so and rtld.

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

Summary of changes:
 ChangeLog                              |   10 +++++++
 Makeconfig                             |    6 +++-
 NEWS                                   |   12 ++++----
 nptl/Makefile                          |   10 +++++-
 nptl/tst-join7.c                       |   46 ++++++++++++++++++++++++++++++++
 nptl/{tst-detach1.c => tst-join7mod.c} |   46 ++++++++++++++++++--------------
 6 files changed, 101 insertions(+), 29 deletions(-)
 create mode 100644 nptl/tst-join7.c
 copy nptl/{tst-detach1.c => tst-join7mod.c} (52%)

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