On Tue, Feb 23, 2016 at 10:51:42AM +0530, Vineet Gupta wrote: > On Friday 19 February 2016 12:17 PM, Vineet Gupta wrote: > > Hi Peter, > > > > I've been debugging a csd_lock_wait() deadlock on SMP+PREEMPT ARC HS38x2 and it > > turned out to be lot more interesting than I'd hoped for. This is stock v4.4 > > > > Trouble starts with an IPI to self which doesn't get delivered as the inter-core > > interrupt providing h/w is not capable of IPI to self (which I found as part of > > debugging this). Subsequent IPIs from other cores to this core get elided as well > > due to the IPI coalescing optimization in arch/arc/kernel/smp.c: ipi_send_msg_one() > > > > There are ways to use a different h/w mechanism to solve the trigger issue and I'd > > hoped to just implement arch_irq_work_raise(). Yes, there are other architectures that use other means for self-IPI, IIRC PowerPC has to program their timer in the past to generate a local interrupt. > > But the trouble is the call stack > > for this issue: IPI to self is triggered from > > > > sys_sched_setscheduler > > __balance_callback > > pull_rt_task > > irq_work_queue_on <-- called with @cpu == self > > > > Looking into irq_work.c, irq_work_queue() is what is semantically needed, > > specifically arch_irq_work_raise() will not be called, which means I need > > arch_send_call_function_single_ipi() to be able to IPI to self cpu also. Is that > > expected from arch code.... > > What I actually meant was is it OK for irq_work_queue_on() to be called locally > (is this a sched bug/optimization(. Further if it is OK to be called, does it need > to do behave more like irq_work_queue() i.e. call arch_irq_work_raise() or > arch_send_call_function_single_ipi() is expected to handle sending IPI to self ! Right, so I'm not actually sure we started out with this requirement. But you're not the first to run into this, see: lkml.kernel.org/r/CAJZ5v0gLankSuziQq25qTCyNqeOX43yD9jnJu_XXwbdyajfmKg at mail.gmail.com Initially I think irq_work_queue_on() was only used remotely, but I think it makes sense to allow the current cpu, esp. since people seem to be using it like that. Now the distinct difference between arch_irq_work_raise() and arch_send_call_function_single_ipi() is that arch_irq_work_raise() should be NMI-safe. So on x86 it has to be extra careful about the lapic state, whereas the regular IPI code doesn't. I seem to have forgotten the status of NMIs on ARC, but this is something to make a note of.
