[PATCH]: Fix blocking pthread_join.

[Rich Felker]

On Wed, Apr 25, 2018 at 01:27:07PM +0200, Stefan Liebler wrote:
> Hi,
> 
> On s390 (31bit) if glibc is build with -Os, pthread_join sometimes
> blocks indefinitely. This is e.g. observable with
> testcase intl/tst-gettext6.
> 
> pthread_join is calling lll_wait_tid(tid), which performs the futex-wait
> syscall in a loop as long as tid != 0 (thread is alive).
> 
> On s390 (and build with -Os), tid is loaded from memory before
> comparing against zero and then the tid is loaded a second time
> in order to pass it to the futex-wait-syscall.
> If the thread exits in between, then the futex-wait-syscall is
> called with the value zero and it waits until a futex-wake occurs.
> As the thread is already exited, there won't be a futex-wake.
> 
> In lll_wait_tid, the tid is stored to the local variable __tid,
> which is then used as argument for the futex-wait-syscall.
> But unfortunately the compiler is allowed to reload the value
> from memory.
> 
> With this patch, the tid is loaded by dereferencing a volatile pointer.
> Then the compiler is not allowed to reload the value for __tid from memory.
> 
> Okay to commit?

There should probably be a bugzilla issue for this, no?

Rich