linux-kernel - Re: [PATCH] md: do not use ++ in rcu

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Date:	Sun, 5 Sep 2010 23:23:35 +0400
From:	Kulikov Vasiliy <segooon@...il.com>
To:	Sam Ravnborg <sam@...nborg.org>
Cc:	kernel-janitors@...r.kernel.org, Neil Brown <neilb@...e.de>,
	Jens Axboe <jaxboe@...ionio.com>, linux-raid@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] md: do not use ++ in rcu_dereference() argument

On Sun, Sep 05, 2010 at 21:01 +0200, Sam Ravnborg wrote:
> On Sun, Sep 05, 2010 at 10:32:18PM +0400, Kulikov Vasiliy wrote:
> > From: Vasiliy Kulikov <segooon@...il.com>
> > 
> > rcu_dereference() is macro, so it might use its argument twice.
> > Argument must not has side effects.
> > 
> > It was found by compiler warning:
> > drivers/md/raid1.c: In function ‘read_balance’:
> > drivers/md/raid1.c:445: warning: operation on ‘new_disk’ may be undefined
> 
> This change looks wrong.
> In the original implementation new_disk is incremented and
> then we do the array lookup.
> With your implementation it looks like we increment it after
> the array lookup.

No, the original code increments new_disk and then dereferences mirrors.

The full code:

		for (rdev = rcu_dereference(conf->mirrors[new_disk].rdev);
		     r1_bio->bios[new_disk] == IO_BLOCKED ||
		     !rdev || !test_bit(In_sync, &rdev->flags)
			     || test_bit(WriteMostly, &rdev->flags);
		     rdev = rcu_dereference(conf->mirrors[++new_disk].rdev)) {

			if (rdev && test_bit(In_sync, &rdev->flags) &&
				r1_bio->bios[new_disk] != IO_BLOCKED)
				wonly_disk = new_disk;

			if (new_disk == conf->raid_disks - 1) {
				new_disk = wonly_disk;
				break;
			}
		}

    so,

    for (a; b; c = f(++g)) {
       ...
    } 

    ==

    a;
    while (b) {
       ...
       l_continue:
       c = f(++g);
    }

    ==

    a;
    while (b) {
       ...
       l_continue:
       g++;
       c = f(g);
    }

    ==

    for (a; b; c = f(g)) {
       ...
       g++;
    } 

Or you mean smth more?

-- 
Vasiliy
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