Messages in this thread | | | Date | Wed, 26 Oct 2011 16:10:05 +0100 | From | Russell King <> | Subject | Re: [RESEND] [RFC][PATCH X86_32 1/2]: Call do_notify_resume() with interrupts enabled |
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On Wed, Oct 26, 2011 at 02:38:39PM +0200, Linus Torvalds wrote: > Ingo, Thomas, I think this is your call, but it seems valid,
Hi Linus,
I guess I should've talked to you about this during a moment during the kernel summit, but as I'm now back home it'll have to be email.
I've been toying with a similar patch for ARM, but I keep feeling uneasy about having interrupts enabled in this path (even though they get enabled in the depths of the signal handling code.)
I worry about are race condition like the following:
syscall enter ... syscall returns -ERESTARTNOHAND check for signal signal pending, but no handler, setup for restart interrupt happens, sets need_resched need_resched set switch to another thread ... something happens which queues SIGIO switch back to this thread check for signal signal pending, has handler, but we've setup for a restart return to userspace run SIGIO handler restart syscall
This feels like it violates the expectations of the syscall being restarted - which explicitly asks to be restarted only if there wasn't a handler run.
I've been working on the assumption that this is a problem and we should do something about it - but it's non-trivial to solve all the corner cases. We can do a lot better with the restarting if we delay setting up for a restart until either we setup the user stack for the sig handler or immediately before returning to userspace (with a TIF flag.)
If you're interested in seeing where I got to, the patch is available at: https://lkml.org/lkml/2011/8/25/231
However, that doesn't solve the (probably unsolvable) case where an ERESTARTSYS syscall is interrupted by a SA_RESTART-marked handler, and while that handler is running it is then interrupted by a non-SA_RESTART- marked handler. I think that is far too an obscure case to care about though.
-- Russell King Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/ maintainer of:
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