On Wed, 23 Jul 2014 18:28:48 +0200 Frederic Weisbecker <[email protected]> wrote:
> On Mon, Jul 21, 2014 at 08:43:17AM -0700, Paul E. McKenney wrote: > > On Mon, Jul 21, 2014 at 04:43:24PM +0200, Petr Mládek wrote: > > > 2. Go back, do the swap on any CPU, and do memory barriers via IPI. > > > > > > I wonder if the needed memory barrier in rb_reserve_next_event() > > > could be avoided by calling IPI from ring_buffer_swap_cpu(). > > > > > > I mean that rb_reserve_next_event() will include the current check > > > for swapped ring buffer without barriers. But > > > ring_buffer_swap_cpu() will interrupt the affected CPU and > > > basically do the barrier there only when needed. > > > > > > But I am not sure how this is different from calling > > > smp_call_function_single() from ring_buffer_swap_cpu(). > > > And I am back on the question why it is dangerous with disabled > > > interrupts. I can't find any clue in git history. And I miss this > > > part of the picture :-( > > > > IIRC, deadlock in the case where two CPUs attempt to invoke > > smp_call_function_single() at each other, but both have > > interrupts disabled. It might be possible to avoid this by telling > > smp_call_function_single() not to wait for a response, but this often > > just re-introduces the deadlock at a higher level. > > FWIW, this is what smp_call_function_single_async() does. But then the call > must synchronized such that no concurrent call happen until the IPI > completion. > > Otherwise you also have irq_work_queue_on() (not yet upstream but in > tip/timers/nohz > and tip/sched/core). Well, the code in question must wait for the IPI to finish, thus as Paul said, we just push the issue to the caller. -- Steve -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
