Date: Wed, 23 Jul 2014 12:34:58 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Petr =?ISO-8859-1?B?TWzhZGVr?= <pmladek@suse.cz>,
        Ingo Molnar <mingo@elte.hu>, Jiri Kosina <jkosina@suse.cz>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3] ring-buffer: Race when writing and swapping cpu
 buffer in parallel
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On Wed, 23 Jul 2014 18:28:48 +0200
Frederic Weisbecker <fweisbec@gmail.com> 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 Ml?dek 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
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