linux-kernel - Re: [PATCH] Use ERESTARTNOHAND if poll() is interrupted by a signal

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Message-ID: <20070731210817.GA97@tv-sign.ru>
Date:	Wed, 1 Aug 2007 01:08:17 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Manfred Spraul <manfred@...orfullife.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Roland McGrath <roland@...hat.com>
Subject: Re: [PATCH] Use ERESTARTNOHAND if poll() is interrupted by a signal

On 07/31, Manfred Spraul wrote:
>
> Mainline has the same problem: poll() returns to user space if it's 
> interrupted by SIGSTOP/SIGCONT.
> >I guess the consequences of the thing-which-this-fixes aren't huge, s I ca
> >queue this up for 2.6.24, after Oleg's
> >do_poll-return-eintr-when-signalled.patch?
> >
> >  
> Yes, please queue it: most/all linux versions show this behavior. 
> Additionally, poll() is usually called in a loop and a spurious wakeup 
> has no consequences.

... and so the current behaviour is more or less correct, even if not
optimal.

But this patch in fact adds a bug. We don't even need the "special"
signals like SIGSTOP/SIGCONT or freezer to hit this bug.

Suppose that sys_poll() was interrupted by a "normal" signal which
has a handler. It is quite possible that another thread can steal
this signal before us. Now, ERESTARTNOHAND means we restart sys_poll()
with the same (old) timeout, this means that sys_poll() does _not_
return when timeout expired (if no fds ready), and this is bug.

Also, the false signal_wake_up() is possible, and again, the spurious
-EINTR is better than restart with the same timeout.

What we need is ERESTART_RESTARTBLOCK, and restart_block.arg2 should
have the new timeout value, which takes the time we already slept
into account.

Oleg.

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