Re: [PATCH] md: do not use ++ in rcu_dereference() argument

Kulikov Vasiliy
2010-09-05T19:23:35Z

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@gmail.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