linux-kernel - Re: [patches] seqlock: add barrier-less special cases for seqcounts

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Message-ID: <AANLkTi=3xTcqasoN77o3Hk9kyfT4+MbFR7K7CZE=U36Y@mail.gmail.com>
Date:	Fri, 12 Nov 2010 15:52:55 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Nick Piggin <npiggin@...nel.dk>
Cc:	linux-kernel@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patches] seqlock: add barrier-less special cases for seqcounts

On Fri, Nov 12, 2010 at 3:06 PM, Nick Piggin <npiggin@...nel.dk> wrote:
. ...
> seq2 = read_seqlock_begin(&child->d_seq);
> if (read_seqcount_retry(&dentry->d_seq, seq))
>  /* bail out */

So the only issue is that this particular back-to-back sequence with
these kinds of "take one seqlock and release the previous one" where
you currently end up having basically one smp_rmb() at the end of
"read_seqlock_begin()", only to be followed immediately by another one
starting out the "read_seqcount_retry()"?

If so, I think we should make _that_ operation ("move from one seqlock
to another") be the special one, because it smells like in general,
using the special non-locking versions is going to be a very subtle
interface.

                      Linus
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