Messages in this thread | | | Date | Tue, 9 Mar 2010 19:10:26 +0100 (CET) | From | Thomas Gleixner <> | Subject | Re: [RFC][PATCH] IRQ: Fix oneshot irq race between irq_finalize_oneshot and handle_level_irq |
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On Tue, 9 Mar 2010, Valdis.Kletnieks@vt.edu wrote: > On Tue, 09 Mar 2010 08:58:11 +0100, Thomas Gleixner said: > > On Tue, 9 Mar 2010, Lars-Peter Clausen wrote: > > > > > If the kernel has been compiled with preemtion support and handle_level_irq is > > > called from process context for a oneshot irq there is a race between > > > irq_finalize_oneshot and handle_level_irq which results in the irq not being > > > unmasked after its handlers have been run. > > > > > > irq_finalize_oneshot is expected to unmask the irq after the threaded irq > > > handler has been run. It only does so if IRQ_MASKED is set for the irqs status. > > > IRQ_MASKED gets set in the lower part of handle_level_irq after handle_IRQ_event > > > has been called. > > > handle_IRQ_event will wakeup the oneshot irqs threaded handler and if the > > > kernel has been build with preemption there is a chance that the threaded irq > > > handler will finish before execution is returned to handle_level_irq. > > > As a result irq_finalize_oneshot will not unmask the irq and handle_level_irq > > > will set the IRQ_MASKED flag. Thus the irq will stay masked and stalls. > > > > > > In case of an race the call-graph would look like this: > > > handle_level_irq > > > |- mask_ack_irq > > > |- handle_IRQ_event > > > |- wake_up_process > > > |- irq_thread > > > |- action->thread_fn > > > |- irq_finalize_oneshot # Does not unmask the irq > > > |- # Set IRQ_MASKED status flag > > > > Errm, a thread _CANNOT_ preempt a hard interrupt handler. > > What stops the thread from concurrently running on another CPU and racing > that way? I'm an idiot, use small words. :)
Right it's a valid SMP problem, but I got confused by the lengthy explanation of a thread preempting an hard interrupt handler. :)
Will have a look after dinner.
Thanks,
tglx
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