Re: [PATCH][RFC] Pollable /proc/<pid>/ - avoid SIGCHLD/poll() races

[Mattias Engdegård]

Paul Menage <[e-mail]> wrote:
>The only real user-space solution to this is to have the SIGCHLD handler
>somehow cause the select() to return immediately by e.g. writing a byte
>to a looped pipe which is included in the select() readfd set, but this
>seems a little contrived.

I don't think it's contrived --- writing not a byte, but the pid and
return status of the dead child to a pipe is an old but useful trick.
It gives a natural serialisation of child deaths, and also eliminates
the common race where a child dies before its parent has recorded its
pid in a data structure. See it as a safe way of converting an
asynchronous signal to a queued event.

Using pipes to wake up blocking select()s is a useful thing in general,
and often a lot cleaner than using signals when dealing with threads.