linux-kernel - Re: RT signal queue overflow (Was: Q: SEGSEGV && uc_mcontext->ip (Was: Signal delivery order))

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Date:	Wed, 18 Mar 2009 07:52:23 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Gábor Melis <mega@...es.hu>
cc:	Roland McGrath <roland@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Davide Libenzi <davidel@...ilserver.org>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Friesen <cfriesen@...tel.com>,
	linux-kernel@...r.kernel.org
Subject: Re: RT signal queue overflow (Was: Q: SEGSEGV && uc_mcontext->ip
 (Was: Signal delivery order))

On Wed, 18 Mar 2009, Gábor Melis wrote:
> 
> It was just a month or so ago when I finally made to change to use a 
> non-real-time signal for signalling stop-for-gc.

Ahh, and that is when you noticed the issue with SIGSEGV?

One thing you might try is to still use a non-real-time signal, but simply 
depend on the fact that signals are basically peeled off the signal 
bitmask in a signal number order (with the exception that fatal signals 
are handled specially).

IOW, on x86, just about the _only_ normal user-generated signal that can 
happen before SIGSEGV is SIGUSR1, because SIGSEGV is 11, and SIGUSR1 is 
10.

If you were to use SIGUSR2 (signal #12) you'd probably never see this.

Of course, there are other signals with numbers < 11, but they are 
generally meant for other synchronous traps (ie signals like 
SIGTRAP/AIGABRT/SIGFPE/SIGBUS), or for killing the process (signals
SIGHUP/SIGINT/SIGQUIT).

So maybe you'd be happy with just picking another signal? It's not a 
_pretty_ solution, but it should work across most kernel versions.

			Linus
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