From: Eric Biggers <firstname.lastname@example.org>
To: Matthew Wilcox <email@example.com>
Cc: Dave Chinner <firstname.lastname@example.org>,
Subject: Re: [PATCH] fs/direct-io: avoid data race on ->s_dio_done_wq
Date: Wed, 15 Jul 2020 22:33:32 -0700 [thread overview]
Message-ID: <20200716053332.GH1167@sol.localdomain> (raw)
On Thu, Jul 16, 2020 at 03:47:17AM +0100, Matthew Wilcox wrote:
> On Thu, Jul 16, 2020 at 11:46:56AM +1000, Dave Chinner wrote:
> > And why should we compromise performance on hundreds of millions of
> > modern systems to fix an extremely rare race on an extremely rare
> > platform that maybe only a hundred people world-wide might still
> > use?
> I thought that wasn't the argument here. It was that some future
> compiler might choose to do something absolutely awful that no current
> compiler does, and that rather than disable the stupid "optimisation",
> we'd be glad that we'd already stuffed the source code up so that it
> lay within some tortuous reading of the C spec.
> The memory model is just too complicated. Look at the recent exchange
> between myself & Dan Williams. I spent literally _hours_ trying to
> figure out what rules to follow.
> Neither Dan nor I are exactly "new" to Linux kernel development. As Dave
> is saying here, having to understand the memory model is too high a bar.
> Hell, I don't know if what we ended up with for v4 is actually correct.
> It lokos good to me, but *shrug*
Looks like you still got it wrong :-( It needs:
diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 934c92dcb9ab..9a95fbe86e15 100644
@@ -1029,7 +1029,7 @@ static int devmem_init_inode(void)
/* publish /dev/mem initialized */
- WRITE_ONCE(devmem_inode, inode);
+ smp_store_release(&devmem_inode, inode);
It seems one source of confusion is that READ_ONCE() and WRITE_ONCE() don't
actually pair with each other, unless no memory barriers are needed at all.
Instead, READ_ONCE() pairs with a primitive that has "release" semantics, e.g.
smp_store_release() or cmpxchg_release(). But READ_ONCE() is only correct if
there's no control flow dependency; if there is, it needs to be upgraded to a
primitive with "acquire" semantics, e.g. smp_load_acquire().
The best approach might be to just say that the READ_ONCE() + "release" pairing
should be avoided, and we should stick to "acquire" + "release". (And I think
Dave may be saying he'd prefer that for ->s_dio_done_wq?)
next prev parent reply other threads:[~2020-07-16 5:33 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-13 3:33 [PATCH] fs/direct-io: avoid data race on ->s_dio_done_wq Eric Biggers
2020-07-15 1:30 ` Dave Chinner
2020-07-15 2:37 ` Eric Biggers
2020-07-15 8:01 ` Dave Chinner
2020-07-15 16:13 ` Eric Biggers
2020-07-15 16:41 ` Darrick J. Wong
2020-07-16 1:46 ` Dave Chinner
2020-07-16 2:39 ` Eric Biggers
2020-07-16 2:47 ` Matthew Wilcox
2020-07-16 3:19 ` Eric Biggers
2020-07-16 5:33 ` Eric Biggers [this message]
2020-07-16 8:16 ` Dave Chinner
2020-07-17 1:36 ` Darrick J. Wong
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:
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).