wait() on a PID that is not your child

Posted Oct 9, 2011 7:25 UTC (Sun) by ebiederm (subscriber, #35028)
In reply to: wait() on a PID that is not your child by neilbrown
Parent article: A Plumber's Wish List for Linux

/proc/$PID/ is linked to the process and it very much becomes empty when a process dies.

If a new process gets the same pid a different directory is created.

The tricky bit is actually is the way process death updates are implemented internally. The data structures are backwards and need to be turnedd around so poll on the file descriptor could be implemented.

There is still the race of changing into the directory at the top of this thread but pid reuse is typically slow enough that race should be hard to hit.

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wait() on a PID that is not your child

Posted Oct 9, 2011 10:54 UTC (Sun) by neilbrown (subscriber, #359) [Link]

hmm... I must have missed an important piece in the code. I just tested and the old empty directory definitely stays empty as you said. Thanks for the correction.

I don't think the race at the top is real. Whatever mechanism was used to determine which pid to wait for can be repeated after the "cd /proc/$pid" to see if it is still the same. If it is the same, then it is perfectly safe to wait for files to disappear (if/when there is a mechanism to do that).