linux-kernel - Re: [PATCH v3] ring-buffer: Race when writing and swapping cpu buffer in parallel

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20140723164918.GS20751@pathway.suse.cz>
Date:	Wed, 23 Jul 2014 18:49:18 +0200
From:	Petr Mládek <pmladek@...e.cz>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>, Jiri Kosina <jkosina@...e.cz>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] ring-buffer: Race when writing and swapping cpu
 buffer in parallel

On Wed 2014-07-23 12:34:58, Steven Rostedt wrote:
> On Wed, 23 Jul 2014 18:28:48 +0200
> Frederic Weisbecker <fweisbec@...il.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 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.

JFYI, I already have a variant based on https://lkml.org/lkml/2014/7/21/416
It seems to work fine. I just want to double check few things before sending.

Best Regards,
Petr

PS: I am a bit distracted now because my wife is about to give birth
to our twins :-)
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/