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

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Message-ID: <20140627074658.GA23205@pathway.suse.cz>
Date:	Fri, 27 Jun 2014 09:46:58 +0200
From:	Petr Mládek <pmladek@...e.cz>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Jiri Kosina <jkosina@...e.cz>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ring-buffer: Race when writing and swapping cpu buffer
 in parallel

On Thu 2014-06-26 20:55:00, Steven Rostedt wrote:
> On Thu, 26 Jun 2014 09:58:31 -0400
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > What we can do is force ring_buffer_swap_cpu() to only work for the CPU
> > that it is on. As we have snapshot in per_cpu buffers, to make that
> > work, we will need to change the per_cpu version of snapshot to do a
> > smp_call_function_single() to the CPU that it wants to take a snapshot
> > of, and run the swap there.
> > 
> > To force this, we can remove the cpu parameter from the
> > ring_buffer_swap_cpu(). By doing this, we may be able to remove some of
> > the CONFIG_RING_BUFFER_ALLOW_SWAP hacks too!
> > 
> > I'm not going to sacrifice the general performance of the ring buffer
> > for a feature that is seldom (if ever) used.

Fair enough. I see the point.

> Did you want to do the above, or do you want me to write something up?

I could look at it the following week.

When I think about it, the race is not that critical. In the worst
case, it slightly modifies the swapped buffer but there should not
be danger of any data corruption. Anyway, I will try to get rid of
it the way you suggested. It might be more important when the
ringbuffer has more users.

Best Regards,
Petr
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