linux-kernel - Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry

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Date:	Wed, 20 May 2015 15:09:19 +1000
From:	NeilBrown <neilb@...e.de>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Patrick Marlier <patrick.marlier@...il.com>,
	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com,
	wangyun@...ux.vnet.ibm.com
Subject: Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage

On Tue, 19 May 2015 15:07:25 -0700 "Paul E. McKenney"
<paulmck@...ux.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@...e.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@...il.com>
Signed-off-by: NeilBrown <neilb@...e.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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