Messages in this thread | | | Date | Mon, 20 Dec 2004 17:27:33 +0000 | From | Matthew Wilcox <> | Subject | Re: [KJ] Re: [PATCH] pcxx: replace cli()/sti() with spin_lock_irqsave()/spin_unlock_irqrestore() |
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On Mon, Dec 20, 2004 at 02:59:09PM +0000, Alan Cox wrote: > On Gwe, 2004-12-17 at 22:34, James Nelson wrote: > > - save_flags(flags); > > - cli(); > > + spin_lock_irqsave(&pcxx_lock, flags); > > del_timer_sync(&pcxx_timer); > > Not safe if the lock is grabbed by the timer between the lock and the > irqsave as it will spin on another cpu and the timer delete will never > finish.
Right, but wrong reason ...
James admitted he thought the driver was otherwise SMP-safe; he didn't know how to convert things from the old locking style to proper locking.
The problem with this code section is not the race between local interrupts and the lock, since irqs are disabled before the cpu tries to grab the lock. The problem is that if the lock is grabbed by this code path, and then the timer running on a different CPU attempts to acquire the lock, it will spin. del_timer_sync() will then spin waiting for the timer to complete. We're deadlocked.
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