Re: [RFC PATCH glibc] pthread_setspecific: Provide signal-safety across keys

[Florian Weimer <fw at deneb dot enyo dot de>]

* Mathieu Desnoyers:

>  	  if (value == NULL)
>  	    /* We don't have to do anything.  The value would in any case
>  	       be NULL.  We can save the memory allocation.  */
>  	    return 0;
>  
> +	  /* Ensure that pthread_setspecific and pthread_getspecific are
> +             signal-safe when used on different keys.  */
> +          sigfillset (&ss);
> +          pthread_sigmask (SIG_BLOCK, &ss, &old_ss);
> +          /* Read level2 again with signals disabled.  */
> +          level2 = THREAD_GETMEM_NC (self, specific, idx1st);
> +          if (level2 != NULL)
> +            goto skip_resize;
> +
>  	  level2
>  	    = (struct pthread_key_data *) calloc (PTHREAD_KEY_2NDLEVEL_SIZE,
>  						  sizeof (*level2));

I don't see how this adds signal safety.  If the function is called
from a signal handler, interrupting the malloc code, then this will
still result in undefined behavior (heap corruption or deadlocks).

Does lttng-ust call these functions even if the application does not?
Then you really need to switch to real thread-local variables with the
intial-exec model, possibly with indirection through a thread-local
pointer variable and on-demand allocation using mmap in case you do
not want to have these allocations in every thread.