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

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Message-Id: <20080521025617.612D726FA1C@magilla.localdomain>
Date:	Tue, 20 May 2008 19:56:17 -0700 (PDT)
From:	Roland McGrath <roland@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Oleg Nesterov <oleg@...sign.ru>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] signals: collect_signal: remove the unneeded
 sigismember() check

> 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.

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).

drm_lock uses a mask containing only the stop signals (SIGSTOP, SIGTSTP,
SIGTTOU, SIGTTIN).  So for other signals, its hook (drm_notifier) never
gets called, and signal_pending() stays set, handlers get run, etc.

So I can't tell what it's actually good for (that's at all reliable
now).  But you just told me not to ask you how it's supposed to work,
and I respect that. ;-)

The best I can figure is that the user-level code in question really
does want to have signal handlers run, and to process fatal signals,
during those critical sections.  It just wants to prevent a ^Z or
SIGSTOP at the critical spot leaving the lock held while the process
goes into TASK_STOPPED for an arbitrary period.

Thanks,
Roland
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