Messages in this thread | | | Date | Wed, 20 May 2015 06:28:35 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage |
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On Wed, May 20, 2015 at 03:09:19PM +1000, NeilBrown wrote: > On Tue, 19 May 2015 15:07:25 -0700 "Paul E. McKenney" > <paulmck@linux.vnet.ibm.com> wrote: > > > > The code in md probably needs to change in any case, as otherwise we are > > invoking rcu_dereference_whatever() on a full struct list_head rather > > than on a single pointer. Or am I missing something here? > > I think it would be > rcu_dereference_whatever(&mddev->disks) > > I don't know what you mean by "on a full struct list_head", but there is > nothing actually being dereferenced here - right? Just pointer arithmetic on > 'mddev'.
It really does dereference. Strange but true.
> I should probably just > > diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c > index 2bc56e2a3526..b1d237bf8b3b 100644 > --- a/drivers/md/bitmap.c > +++ b/drivers/md/bitmap.c > @@ -181,7 +181,7 @@ static struct md_rdev *next_active_rdev(struct md_rdev *rdev, struct mddev *mdde > rcu_read_lock(); > if (rdev == NULL) > /* start at the beginning */ > - rdev = list_entry_rcu(&mddev->disks, struct md_rdev, same_set); > + rdev = list_entry(&mddev->disks, struct md_rdev, same_set); > else { > /* release the previous rdev and start from there. */ > rdev_dec_pending(rdev, mddev); > > as there really are no RCU issues with getting that address. Maybe I should > move it outside the rcu_read_lock() just to be blatant.... but that would > make the code a lot more clumsy as the rdev_dec_pending must be inside the > rcu_read_lock.. > > So this.
Fair enough -- if you aren't using RCU, there is really no point in using the RCU API. I will drop this patch from my tree. You are pushing yours, I am guessing?
Thanx, Paul
> Thanks, > NeilBrown > > From: NeilBrown <neilb@suse.de> > Date: Wed, 20 May 2015 15:05:09 +1000 > Subject: [PATCH] md/bitmap: remove rcu annotation from pointer arithmetic. > > Evaluating "&mddev->disks" is simple pointer arithmetic, so > it does not need 'rcu' annotations - no dereferencing is happening. > > Also enhance the comment to explain that 'rdev' in that case > is not actually a pointer to an rdev. > > Reported-by: Patrick Marlier <patrick.marlier@gmail.com> > Signed-off-by: NeilBrown <neilb@suse.de> > > diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c > index 2bc56e2a3526..135a0907e9de 100644 > --- a/drivers/md/bitmap.c > +++ b/drivers/md/bitmap.c > @@ -177,11 +177,16 @@ static struct md_rdev *next_active_rdev(struct md_rdev *rdev, struct mddev *mdde > * nr_pending is 0 and In_sync is clear, the entries we return will > * still be in the same position on the list when we re-enter > * list_for_each_entry_continue_rcu. > + * > + * Note that if entered with 'rdev == NULL' to start at the > + * beginning, we temporarily assign 'rdev' to an address which > + * isn't really an rdev, but which can be used by > + * list_for_each_entry_continue_rcu() to find the first entry. > */ > rcu_read_lock(); > if (rdev == NULL) > /* start at the beginning */ > - rdev = list_entry_rcu(&mddev->disks, struct md_rdev, same_set); > + rdev = list_entry(&mddev->disks, struct md_rdev, same_set); > else { > /* release the previous rdev and start from there. */ > rdev_dec_pending(rdev, mddev);
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