linux-kernel - Re: [PATCH] coredump: wait on the core pattern umh at least once

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Date:	Sat, 29 Oct 2011 16:13:23 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Scott James Remnant <scott@...split.com>
Cc:	linux-kernel@...r.kernel.org,
	Mandeep Singh Baines <msb@...omium.org>,
	Neil Horman <nhorman@...driver.com>
Subject: Re: [PATCH] coredump: wait on the core pattern umh at least once

On 10/28, Scott James Remnant wrote:
>
> If a thread crashes as a result of a signal on the thread group leader
> that signal can still be pending,

No. do_coredump() clears TIF_SIGPENDING.

The problem is, this is obviously not enough and should be fixed.

> While the helper obviously has to deal with that, tweaking this loop
> so it runs at least one iteration even in that case helps a lot.

I don't understand this patch. It doesn't look right at all.

> @@ -2030,11 +2030,11 @@ static void wait_for_dump_helpers(struct file *file)
>  	pipe->readers++;
>  	pipe->writers--;
>
> -	while ((pipe->readers > 1) && (!signal_pending(current))) {
> +	do {
>  		wake_up_interruptible_sync(&pipe->wait);
>  		kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
>  		pipe_wait(pipe);

This can hang forever. We didn't check pipe->readers, it it is zero
nobody can wakeup us.

> +	} while ((pipe->readers > 1) && (!signal_pending(current)));

And, it doesn't make any sense to call pipe_wait() with signal_pending(),
it won't block. Note that pipe_wait() schedules in TASK_INTERRUPTIBLE.

I already tried to explain why this signal_pending() was added, but
apparently I was not clear. I'll try again in the previous thread.

Oleg.

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