linux-kernel - Re: [PATCH 1/2] signals: collect_signal: remove the unneeded sigismember() check

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Message-ID: <20080521122032.GA179@tv-sign.ru>
Date:	Wed, 21 May 2008 16:20:32 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Roland McGrath <roland@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] signals: collect_signal: remove the unneeded sigismember() check

On 05/20, Roland McGrath wrote:
>
> > It's basically the equivalent of a "spin_lock_irq()" for DRM, where 
> > signals are the "interrupts" that need to be blocked while holding the 
> > lock.
> 
> Well, sure, that makes sense.  But that's not what it does.  I can see what
> it does, I just can't tell why it really makes any sense.

As for me, I can't even see what it does,

> Despite the name, block_all_signals() in fact blocks no signals.  What
> it does is install notifier/notifier_mask, which makes the hook get
> called for those particular signals and it can decide to delay the
> signal (in a kooky fashion that's not really reliable).

Suppose that the task has the pending SIG which is "blocked" by DRM.

dequeue_signal() calls ->notifier(), it nacks the signal, we clear
TIF_SIGPENDING. Then dequeue_signal() does recalc_sigpending() and
sets TIF_SIGPENDING again. We return 0 to get_signal_to_deliver(),
and then return to user-space with TIF_SIGPENDING. Endless loop ?

Even if it works somehow... "blocking" SIG means that in fact we
"block" SIG+1, SIG+2, etc if SIG is pending.

> drm_lock uses a mask containing only the stop signals (SIGSTOP, SIGTSTP,
> SIGTTOU, SIGTTIN).

and of course this can't work for multithread programs, another
thread can dequeue SIGTSTP and initiate a group stop.

Oleg.

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