Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage

[NeilBrown]

On Tue, 19 May 2015 15:07:25 -0700 "Paul E. McKenney"
<paul...@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 <ne...@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.marl...@gmail.com>
Signed-off-by: NeilBrown <ne...@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);

pgpMbWdBiMk4u.pgp
Description: OpenPGP digital signature