LKML Archive on lore.kernel.org
 help / color / Atom feed
From: Peter Zijlstra <peterz@infradead.org>
To: Mark Rutland <mark.rutland@arm.com>
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: Fri, 11 May 2018 18:22:29 +0200
Message-ID: <20180511162229.GK12217@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20180511105931.yyarmtz2gjkbuq2a@lakrids.cambridge.arm.com>

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.

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.

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..

  reply index

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-10 13:06 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 [this message]
2018-05-14 11:05     ` Mark Rutland
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20180511162229.GK12217@hirez.programming.kicks-ass.net \
    --to=peterz@infradead.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mark.rutland@arm.com \
    --cc=mingo@redhat.com \
    --cc=will.deacon@arm.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

LKML Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/lkml/0 lkml/git/0.git
	git clone --mirror https://lore.kernel.org/lkml/1 lkml/git/1.git
	git clone --mirror https://lore.kernel.org/lkml/2 lkml/git/2.git
	git clone --mirror https://lore.kernel.org/lkml/3 lkml/git/3.git
	git clone --mirror https://lore.kernel.org/lkml/4 lkml/git/4.git
	git clone --mirror https://lore.kernel.org/lkml/5 lkml/git/5.git
	git clone --mirror https://lore.kernel.org/lkml/6 lkml/git/6.git
	git clone --mirror https://lore.kernel.org/lkml/7 lkml/git/7.git
	git clone --mirror https://lore.kernel.org/lkml/8 lkml/git/8.git
	git clone --mirror https://lore.kernel.org/lkml/9 lkml/git/9.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 lkml lkml/ https://lore.kernel.org/lkml \
		linux-kernel@vger.kernel.org
	public-inbox-index lkml

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-kernel


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git