On Tue, 2008-06-10 at 10:44 -0500, James Bottomley wrote: > On Tue, 2008-06-10 at 15:51 +0100, Catalin Marinas wrote: > > I was thinking whether this condition can be removed and allow the > > smp_call_function*() to be called with IRQs disabled. At a quick look, > > it seems to be possible if the csd_flag_wait() function calls the IPI > > handlers directly when the IRQs are disabled (see the patch below). [...] > > Please let me know what you think or whether deadlocks are still > > possible (or any other solution apart from hardware fixes :-)). Thanks. > > I don't see how your proposal fixes the deadlocks. The problem is that > on a lot of arch's IPIs are normal interrupts. If interrupts are > disabled, you don't see them. ARM uses normal interrupts for IPIs as well. > The deadlock scenario is CPU1 enters smp_call_function() with IRQ's > disabled as CPU2 does the same thing and spins on the call_lock. Now > CPU1 is waiting for an ack for its IPI to CPU2, but CPU2 will never see > the IPI until it enables interrupts. I can see in the generic IPI patches that the call_function_lock is only held for list_add_tail_rcu and not while waiting for the other CPU to complete (both arch_send_call_function_ipi and csd_flag_wait are outside the locking region). The patch I posted polls for an incoming IPI in the csd_flag_wait() function if the interrupts are disabled so that it clears the wait flag even if it doesn't get the IPI. The disadvantage might be a spurious IPI (but I can leave with this). If interrupts are enabled, there is no drawback, apart from a call to irq_disabled(). > One way to mitigate the effects of this is to enable interrupts if the > architecture code finds the call_lock (x86 implementation) held against > it, then re-disable before trying to get the lock again. But really, in > order to make smp_call_function work in interrupt disabled sections, the > interrupt handler has to be modified to bar all non-IPI interrupts until > the critical section is over (otherwise there's no point allowing it > with disabled interrupts because an smp_call_function becomes a de facto > interrupt enable again). If you really want to see how something like > this works, the voyager code has it (because interrupt disabling in the > VIC is expensive). But it's quite a lot of code ... I still think it can be less complicated that this. I'll look at Paul's post to get some ideas. As I said, I need this functionality on current ARM SMP systems, even if it means implementing it separately. Thanks. -- Catalin -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
