On Mon, Sep 06, 2010 at 12:32:55PM +1000, Neil Brown wrote:
> On Mon, 6 Sep 2010 12:14:08 +1000
> Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx> wrote:
>
> > Hi Paul,
> >
> > After merging the rcu tree, today's linux-next build
> > (powerpc ppc64_defconfig) produced this warning:
> >
> > drivers/md/raid1.c: In function 'read_balance':
> > drivers/md/raid1.c:445: warning: operation on 'new_disk' may be undefined
> >
> > I am picking on the rcu tree because the line above has not changed since
> > 2005. The line is:
> >
> > 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)) {
> >
> > Where new_disk is being updated in the parameter to the last
> > rcu_dereference.
> >
>
> Hi Stephen,
>
> There is a patch in linux-kernel
> "PATCH] md: do not use ++ in rcu_dereference() argument"
>
> that addresses this - it is a problem in md: I guess rcu as become more
> helpful in finding these problems.
Indeed, the sparse-based checking is a bit more picky about things,
even when you are not using sparse...
Thanx, Paul
> I'll probably fix it a little differently, but I'll have something in my
> for-next for tomorrow.
>
> Thanks,
> NeilBrown
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