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SubjectRe: [RESEND][RFC PATCH v2] waitfd
On Wed, 2009-01-07 at 12:53 -0800, Roland McGrath wrote:

> New syscall should have gone to linux-api, I think.
> Do we really need another one for this? How about using signalfd plus
> setting the child's exit_signal to a queuing (SIGRTMIN+n) signal instead of
> SIGCHLD? It's slightly more magical for the userland process to know to do
> that (fork -> clone SIGRTMIN). But compared to adding a syscall we don't
> really have to add, maybe better.
This wouldn't help the init daemon case:

- the exit_signal is set on the child, not on the parent.

While the init daemon could clone() every new process and set
exit_signal, this would not be set for processes reparented to init.

Even if we had a new syscall to change the exit_signal of a given
process, *and* had the init reparent notification patch, this still
wouldn't be sufficient; you'd have a race condition between the time
you were notified of the reparent, and the time you set exit_signal,
in which the child could die.

Since exit_signal is always reset to SIGCHLD before reparenting, this
could be done by resetting it to a different signal; but at this point
we're getting into a rather twisty method full of traps.

- exit_signal is reset to SIGCHLD on exec().

Pretty much a plan-killer ;)

Scott James Remnant
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