Messages in this thread Patches in this message | | | Date | Wed, 20 May 2015 15:09:19 +1000 | From | NeilBrown <> | Subject | Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage |
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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'.
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.
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);[unhandled content-type:application/pgp-signature] | |