Re: [PATCH v2] nptl: Handle spurious EINTR when thread cancellation is disabled (BZ#29029)

[Szabolcs Nagy <szabolcs.nagy@arm.com>]

The 04/14/2022 12:49, Adhemerval Zanella via Libc-alpha wrote:
> --- a/nptl/pthread_cancel.c
> +++ b/nptl/pthread_cancel.c
...
> +  /* Some syscalls are never restarted after being interrupted by a signal
> +     handler, regardless of the use of SA_RESTART (they always fail with
> +     EINTR).  So pthread_cancel cannot send SIGCANCEL unless the cancellation
> +     is enabled and set as asynchronous (in this case the cancellation will
> +     be acted in the cancellation handler instead by the syscall wrapper).
> +     Otherwise the target thread is set as 'cancelling' (CANCELING_BITMASK)
> +     by atomically setting 'cancelhandling' and the cancelation will be acted
> +     upon on next cancellation entrypoing in the target thread.
> +
> +     It also requires to atomically check if cancellation is enabled and
> +     asynchronous, so both cancellation state and type are tracked on
> +     'cancelhandling'.  */
> +
> +  int result = 0;
> +  int oldval = atomic_load_relaxed (&pd->cancelhandling);
> +  int newval;
> +  do
>      {
> -      /* A single-threaded process should be able to kill itself, since there
> -	 is nothing in the POSIX specification that says that it cannot.  So
> -	 we set multiple_threads to true so that cancellation points get
> -	 executed.  */
> -      THREAD_SETMEM (THREAD_SELF, header.multiple_threads, 1);
> +      newval = oldval | CANCELING_BITMASK | CANCELED_BITMASK;
> +      if (oldval == newval)
> +	break;
> +
> +      /* If the cancellation is handled asynchronously just send a
> +	 signal.  We avoid this if possible since it's more
> +	 expensive.  */
> +      if (cancel_enabled_and_canceled_and_async (newval))
> +	{
> +	  /* Mark the cancellation as "in progress".  */
> +	  int newval2 = oldval | CANCELING_BITMASK;
> +	  if (!atomic_compare_exchange_weak_acquire (&pd->cancelhandling,
> +						     &oldval, newval2))
> +	    continue;

this continue looks wrong, the cas can fail spuriously
(cancelhandling == oldval) and then continue jumps to...

> +
> +	  if (pd == THREAD_SELF)
> +	    /* This is not merely an optimization: An application may
> +	       call pthread_cancel (pthread_self ()) without calling
> +	       pthread_create, so the signal handler may not have been
> +	       set up for a self-cancel.  */
> +	    {
> +	      pd->result = PTHREAD_CANCELED;
> +	      if ((newval & CANCELTYPE_BITMASK) != 0)
> +		__do_cancel ();
> +	    }
> +	  else
> +	    /* The cancellation handler will take care of marking the
> +	       thread as canceled.  */
> +	    result = __pthread_kill_internal (th, SIGCANCEL);
> +
> +	  break;
> +	}
> +
> +	/* A single-threaded process should be able to kill itself, since
> +	   there is nothing in the POSIX specification that says that it
> +	   cannot.  So we set multiple_threads to true so that cancellation
> +	   points get executed.  */
> +	THREAD_SETMEM (THREAD_SELF, header.multiple_threads, 1);
>  #ifndef TLS_MULTIPLE_THREADS_IN_TCB
>        __libc_multiple_threads = 1;
>  #endif
> -
> -      THREAD_SETMEM (pd, result, PTHREAD_CANCELED);
> -      if (pd->cancelstate == PTHREAD_CANCEL_ENABLE
> -	  && pd->canceltype == PTHREAD_CANCEL_ASYNCHRONOUS)
> -	__do_cancel ();
> -      return 0;
>      }
> +  while (!atomic_compare_exchange_weak_acquire (&pd->cancelhandling, &oldval,
> +						newval));

...here and this cas updates cancelhandling to newval without
actually doing any cancellation.

>  
> -  return __pthread_kill_internal (th, SIGCANCEL);
> +  return result;
>  }