From: Mark Rutland <mark.rutland@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH] perf/ring_buffer: ensure atomicity and order of updates
Date: Mon, 14 May 2018 12:05:33 +0100 [thread overview]
Message-ID: <20180514110532.kihs5ilrs67kvq7e@lakrids.cambridge.arm.com> (raw)
In-Reply-To: <20180511162229.GK12217@hirez.programming.kicks-ass.net>
On Fri, May 11, 2018 at 06:22:29PM +0200, Peter Zijlstra wrote:
> On Fri, May 11, 2018 at 11:59:32AM +0100, Mark Rutland wrote:
> > READ_ONCE() and WRITE_ONCE() "helpfully" make a silent fallback to a
> > memcpy in this case, so we're broken today, regardless of this change.
> >
> > I suspect that in practice we get single-copy-atomicity for the 32-bit
> > halves, and sessions likely produce less than 4GiB of ringbuffer data,
> > so failures would be rare.
>
> This should not be a problem because of the 32bit adress space limit,
> which would necessarily limit us to the low word.
For the wrapped values, yes.
I thought that the head and tail values were meant to be free-running,
but I can't see where I got that idea from now that I've gone digging
again.
> Also note that in perf_output_put_handle(), where we write ->data_head,
> the store is from an 'unsigned long'. So on 32bit that will result in a
> zero high word. Similarly, in __perf_output_begin() we read ->data_tail
> into an unsigned long, which will discard the high word.
Ah, that's a fair point. So it's just compat userspace that this is
potentially borked for. ;)
> So userspace should always read (head) a zero high word, irrespective of
> a split store (2x32bit), and the kernel will disregard the high word on
> reading (tail), irrespective of what userspace put there.
>
> This is all a bit subtle, and could probably use a comment, but it ought
> to work..
It would be nice to guarantee that we don't lose 32-bit atomicity by
virtue of {READ,WRITE}_ONCE() falling back to memcpy in this case, so
maybe we should wrap this in some helpers.
I'll see if I can come up with something which isn't hideous, or I might
just pretend I never stumbled across this. :)
Thanks,
Mark.
next prev parent reply other threads:[~2018-05-14 11:05 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-05-10 13:06 [PATCH] perf/ring_buffer: ensure atomicity and order of updates Mark Rutland
2018-05-11 1:19 ` kbuild test robot
2018-05-11 1:19 ` kbuild test robot
2018-05-11 10:59 ` Mark Rutland
2018-05-11 16:22 ` Peter Zijlstra
2018-05-14 11:05 ` Mark Rutland [this message]
2018-05-14 11:28 ` Peter Zijlstra
2018-05-14 15:02 ` Peter Zijlstra
2018-05-14 15:20 ` Mark Rutland
2018-05-14 15:24 ` Peter Zijlstra
2018-05-23 16:42 ` Will Deacon
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