CODEWITHSUNDEEP
linuxmultithreadingsignalsposixaio
bottaio
bottaio

Reputation: 5093

pthread_sigmask not working properly with aio callback threads

My application is sometimes terminating from SIGIO or SIGUSR1 signals even though I have blocked these signals.

My main thread starts off with blocking SIGIO and SIGUSR1, then makes 2 AIO read operations. These operations use threads to get notification about operation status. The notify functions (invoked as detached threads) start another AIO operation (they manipulate the data that has been read and start writing it back to the file) and notification is handled by sending signal (one operation uses SIGIO, the other uses SIGUSR1) to this process. I am receiving these signals synchronously by calling sigwait in the main thread. Unfortunately, sometimes my program crashes, being stopped by SIGUSR1 or SIGIO signal (which should be blocked by a sigmask).

One possible solution is to set SIG_IGN handlers for them but this doesn't solve the problem. Their handlers shouldn't be invoked, rather should they be retrieved from pending signals by sigwait in the next iteration of the main program loop.

I have no idea which thread handles this signal in this manner. Maybe it's the init who receives this signal? Or some shell thread? I have no idea.

Upvotes: 1

Views: 408

Answers (1)

pilcrow
pilcrow

Reputation: 58651

I'd hazard a guess that the signal is being received by one of your AIO callback threads, or by the very thread which generates the signal. (Prove me wrong and I'll delete this answer.)

Unfortunately per the standard, "[t]he signal mask of [a SIGEV_THREAD] thread is implementation-defined." For example, on Linux (glibc 2.12), if I block SIGUSR1 in main, then contrive to run a SIGEV_THREAD handler from an aio_read call, the handler runs with SIGUSR1 unblocked.

This makes SIGEV_THREAD handlers unsuitable for an application that must reliably and portably handle signals.

Upvotes: 1

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