[lttng-dev] [rp] [RFC] Userspace RCU library internal error handling

[Josh Triplett]

On Thu, Jun 21, 2012 at 12:41:13PM -0400, Mathieu Desnoyers wrote:
> Currently, liburcu calls "exit(-1)" upon internal consistency error.
> This is not pretty, and usually frowned upon in libraries.

Agreed.

> One example of failure path where we use this is if pthread_mutex_lock()
> would happen to fail within synchronize_rcu(). Clearly, this should
> _never_ happen: it would typically be triggered only by memory
> corruption (or other terrible things like that). That being said, we
> clearly don't want to make "synchronize_rcu()" return errors like that
> to the application, because it would complexify the application error
> handling needlessly.

I think you can safely ignore any error conditions you know you can't
trigger.  pthread_mutex_lock can only return an error under two
conditions: an uninitialized mutex, or an error-checking mutex already
locked by the current thread.  Neither of those can happen in this case.
Given that, I'd suggest either calling pthread_mutex_lock and ignoring
any possibility of error, or adding an assert.

> So instead of calling exit(-1), one possibility would be to do something
> like this:
> 
> #include <signal.h>
> #include <pthread.h>
> #include <stdio.h>
> 
> #define urcu_die(fmt, ...)                      \
>         do {    \
>                 fprintf(stderr, fmt, ##__VA_ARGS__);    \
>                 (void) pthread_kill(pthread_self(), SIGBUS);    \
>         } while (0)
> 
> and call urcu_die(); in those "unrecoverable error" cases, instead of
> calling exit(-1). Therefore, if an application chooses to trap those
> signals, it can, which is otherwise not possible with a direct call to
> exit().

It looks like you want to use signals as a kind of exception mechanism,
to allow the application to clean up (though not to recover).  assert
seems much clearer to me for "this can't happen" cases, and assert also
generates a signal that the application can catch and clean up.

- Josh Triplett