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Date: Wed, 20 Jun 2007 15:14:15 +0400 From: Oleg Nesterov <oleg@...sign.ru> To: Benjamin Herrenschmidt <benh@...nel.crashing.org> Cc: Linus Torvalds <torvalds@...ux-foundation.org>, Davide Libenzi <davidel@...ilserver.org>, Nicholas Miell <nmiell@...cast.net>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org> Subject: Re: Fix signalfd interaction with thread-private signals On 06/20, Benjamin Herrenschmidt wrote: > > On Tue, 2007-06-19 at 18:06 +0400, Oleg Nesterov wrote: > > On 06/19, Benjamin Herrenschmidt wrote: > > > > > > On Tue, 2007-06-19 at 13:14 +0400, Oleg Nesterov wrote: > > > > > > > The commited "Fix signalfd interaction with thread-private signals" > > > > (commit caec4e8dc85e0644ec24aeb36285e1ba02da58cc) doesn't implement > > > > this. > > > > > > Indeed, if you want what Davide described, you need to also change > > > signalfd side. The patch I did merely prevents another thread from > > > dequeuing somebody else private signals. > > > > Yes I see, but why do we need this change? Yes, we can dequeue SIGSEGV > > from another thread. Just don't do it if you have a handler for SIGSEGV? > > Well, for such synchronous signals, it's a fairly stupid idea, > especially since you can't predict who will get it. Signals such as SEGV > are forced-in, which means they are force-unblocked. Thus, you can't > know for sure whome of signalfd or the target thread will get it first, > depending on who catches the siglock first I suppose. In one case, > you'll manage to steal it, in the other, you'll thread will be killed. Yes. As I said, I think this falls into the "just don't do that" category. But nothing bad happens from the kernel POV. Also, suppose that some thread does for (;;) signal(SIGSEGV, SIG_IGN); Now we have the same situation. do_sigaction() can steal SIGSEGV from another thread. Perhaps, the proposed behaviour > Multiple threads can wait on the signalfd. Each one will dequeue either > its own private signals (tsk->pending) or the process shared ones > (tsk->signal->shared_pending). is more convenient, I can't judge. If we implement this, sys_signalfd(-1) in essence means "attach to the thread group, not current". This also makes sense. But what we have now (with this patch applied) is a bit strange, imho. Oleg. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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