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Re: scratch/sigchld-fd 8f0ce42 1/2: Fix deadlock when receiving SIGCHLD
From: |
Eli Zaretskii |
Subject: |
Re: scratch/sigchld-fd 8f0ce42 1/2: Fix deadlock when receiving SIGCHLD during 'pselect'. |
Date: |
Wed, 20 Jan 2021 17:30:53 +0200 |
> From: Andreas Schwab <schwab@linux-m68k.org>
> Cc: emacs-devel@gnu.org, phst@google.com, p.stephani2@gmail.com
> Date: Wed, 20 Jan 2021 11:17:24 +0100
>
> On Jan 20 2021, Eli Zaretskii wrote:
>
> > If/when the signal arrives, the handler removes the file descriptor of the
> > process from the set on which pselect will wait. Isn't that sufficient to
> > prevent us from waiting for a dead process?
>
> Who removes it?
The SIGCHLD signal handler. Or I thought it did, here:
/* If process has terminated, stop waiting for its output. */
if (WIFSIGNALED (status) || WIFEXITED (status))
{
bool clear_desc_flag = 0;
p->alive = 0;
if (p->infd >= 0)
clear_desc_flag = 1;
/* clear_desc_flag avoids a compiler bug in Microsoft C. */
if (clear_desc_flag)
delete_read_fd (p->infd); <<<<<<<<<<<<<<<<<<<<<<<<<
}
But it looks like it only removes the process's file descriptor from
the set to be used for setting up arguments to pselect the _next_
iteration through the loop.
So maybe this is the problem we should solve: make the SIGCHLD handler
reset the bit of the process's output from the fd_set passed to
pselect. Then, if SIGCHLD hits at any time before pselect is called,
it will not wait for output from that process.
> > If not, why not?
>
> What happens when the signal occurs between setting up the set and
> calling pselect?
Yes, see above.
But it sounds like the situation shown in Philipp's log is different.
- Re: scratch/sigchld-fd 8f0ce42 1/2: Fix deadlock when receiving SIGCHLD during 'pselect'., (continued)
- Re: scratch/sigchld-fd 8f0ce42 1/2: Fix deadlock when receiving SIGCHLD during 'pselect'., Eli Zaretskii, 2021/01/19
- Re: scratch/sigchld-fd 8f0ce42 1/2: Fix deadlock when receiving SIGCHLD during 'pselect'., Andreas Schwab, 2021/01/20
- Re: scratch/sigchld-fd 8f0ce42 1/2: Fix deadlock when receiving SIGCHLD during 'pselect'., Eli Zaretskii, 2021/01/20
- Re: scratch/sigchld-fd 8f0ce42 1/2: Fix deadlock when receiving SIGCHLD during 'pselect'., Andreas Schwab, 2021/01/20
- Re: scratch/sigchld-fd 8f0ce42 1/2: Fix deadlock when receiving SIGCHLD during 'pselect'., Eli Zaretskii, 2021/01/20
- Re: scratch/sigchld-fd 8f0ce42 1/2: Fix deadlock when receiving SIGCHLD during 'pselect'., Andreas Schwab, 2021/01/20
- Re: scratch/sigchld-fd 8f0ce42 1/2: Fix deadlock when receiving SIGCHLD during 'pselect'.,
Eli Zaretskii <=
- Re: scratch/sigchld-fd 8f0ce42 1/2: Fix deadlock when receiving SIGCHLD during 'pselect'., Andreas Schwab, 2021/01/20
- Re: scratch/sigchld-fd 8f0ce42 1/2: Fix deadlock when receiving SIGCHLD during 'pselect'., Eli Zaretskii, 2021/01/20
- Re: scratch/sigchld-fd 8f0ce42 1/2: Fix deadlock when receiving SIGCHLD during 'pselect'., Andreas Schwab, 2021/01/20
- Re: scratch/sigchld-fd 8f0ce42 1/2: Fix deadlock when receiving SIGCHLD during 'pselect'., Eli Zaretskii, 2021/01/20
- Re: scratch/sigchld-fd 8f0ce42 1/2: Fix deadlock when receiving SIGCHLD during 'pselect'., Andreas Schwab, 2021/01/20