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Internal SIGTIMER use (was: Re: [PATCH v3 02/21] nptl: Fix Race conditions in pthread cancellation (BZ#12683))

* Adhemerval Zanella:

> -  sa.sa_sigaction = sigcancel_handler;
> -  sa.sa_flags = SA_SIGINFO;
> -  (void) __libc_sigaction (SIGCANCEL, &sa, NULL);
> +  {
> +    struct sigaction sa;
> +    sa.sa_sigaction = sigcancel_handler;
> +    /* The signal handle should be non-interruptible to avoid the risk of
> +       spurious EINTR caused by SIGCANCEL sent to process or if pthread_cancel
> +       is called while cancellation is disabled in the target thread.  */
> +    sa.sa_flags = SA_SIGINFO | SA_RESTART;
> +    sa.sa_mask = SIGALL_SET;
> +    __libc_sigaction (SIGCANCEL, &sa, NULL);
> +  }

Since SIGCANCEL and SIGTIMER are the same, I wondered whether this
change impacts timer_create.

Here is what i found: timer_create arranges for SIGCANCEL/SIGTIMER to be
sent to the internal helper thread, which does this:

      /* sigwaitinfo cannot be used here, since it deletes
         SIGCANCEL == SIGTIMER from the set.  */

      /* XXX The size argument hopefully will have to be changed to the
         real size of the user-level sigset_t.  */
      int result = SYSCALL_CANCEL (rt_sigtimedwait, &ss, &si, NULL, _NSIG / 8);

      if (result > 0)
          if (si.si_code == SI_TIMER)
              struct timer *tk = (struct timer *) si.si_ptr;
          else if (si.si_code == SI_TKILL)
            /* The thread is canceled.  */
            pthread_exit (NULL);

This suggests to me that the helper thread does NOT depend on EINTR
being generated for SIGCANCEL/SIGTIMER, and it should be fine to use
SA_RESTART for that signal as far as timer_create is concerned.

If you agree, it probably makes sense to add this bit of information to
the commit message.


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