On Tue, Nov 27, 2012 at 05:51:12PM +0100, Frederic Weisbecker wrote: > 2012/11/27 Gleb Natapov <gleb@xxxxxxxxxx>: > > On Tue, Nov 27, 2012 at 04:56:30PM +0100, Frederic Weisbecker wrote: > >> 2012/11/27 Gleb Natapov <gleb@xxxxxxxxxx>: > >> > For KVM_PV_REASON_PAGE_NOT_PRESENT it behaves like an exception. > >> > >> Ok. > >> There seem to be a bug in kvm_async_pf_task_wait(). Using > >> idle_cpu(cpu) to find out if the current task is the idle task may not > >> work if there is pending wake up. Me may schedule another task but > >> when that other task sleeps later we can't schedule back to idle until > >> the fault is completed. > >> > >> The right way is to use is_idle_task(current) > > But if there is pending wake up then scheduling to the waked up task is > > exactly what we want. > > Ok, but what if that task goes to sleep soon after beeing scheduled > and there is no other task on the runqueue and the page fault has not > been handled yet? The only thing you can do is to schedule the idle > task. But the idle task is waiting for the fault completion so you > can't do that. Yes, I see now. So even though we have runnable task we can't schedule away from idle task. Wouldn't the patch below solve Sasha's and Li's RCU problems then (not even compiled): diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c index 4180a87..636800d 100644 --- a/arch/x86/kernel/kvm.c +++ b/arch/x86/kernel/kvm.c @@ -113,7 +113,7 @@ void kvm_async_pf_task_wait(u32 token) int cpu, idle; cpu = get_cpu(); - idle = idle_cpu(cpu); + idle = is_idle_task(current); put_cpu(); spin_lock(&b->lock); @@ -247,10 +247,7 @@ do_async_page_fault(struct pt_regs *regs, unsigned long error_code) break; case KVM_PV_REASON_PAGE_NOT_PRESENT: /* page is swapped out by the host. */ - rcu_irq_enter(); - exit_idle(); kvm_async_pf_task_wait((u32)read_cr2()); - rcu_irq_exit(); break; case KVM_PV_REASON_PAGE_READY: rcu_irq_enter(); -- Gleb. -- To unsubscribe from this list: send the line "unsubscribe linux-next" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
