Patch "scsi: sg: reset 'res_in_use' after unlinking reserved array" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

scsi: sg: reset 'res_in_use' after unlinking reserved array

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
scsi-sg-reset-res_in_use-after-unlinking-reserved-array.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Hannes Reinecke <[email protected]>
Date: Mon, 24 Apr 2017 10:26:36 +0200
Subject: scsi: sg: reset 'res_in_use' after unlinking reserved array

From: Hannes Reinecke <[email protected]>

commit e791ce27c3f6a1d3c746fd6a8f8e36c9540ec6f9 upstream.

Once the reserved page array is unused we can reset the 'res_in_use' state; here we can do a lazy update without holding the mutex as we only need to check against concurrent access, not concurrent release.

[mkp: checkpatch]

---
 drivers/scsi/sg.c |    2 ++
 1 file changed, 2 insertions(+)

@@ -2052,6 +2052,8 @@ sg_unlink_reserve(Sg_fd * sfp, Sg_reques
 	req_schp->sglist_len = 0;
 	sfp->save_scat_len = 0;
 	srp->res_used = 0;
+	/* Called without mutex lock to avoid deadlock */
+	sfp->res_in_use = 0;
 }
 
 static Sg_request *

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/scsi-sg-protect-accesses-to-reserved-page-array.patch queue-4.4/scsi-sg-reset-res_in_use-after-unlinking-reserved-array.patch

On Thu, 2017-08-31 at 08:20 +0200, [email protected] wrote:
[...]

[...]

This description makes no sense. A simple write to a variable cannot be used to synchronise anything as it can be reordered with other memory operations, including those that write to the buffer you're trying to protect. This write needs to be combined with a release barrier or preceded by an smp_wmb(). Every read of res_in_use needs to be combined with an acquire barrier or followed by an smp_rmb().

Ben.

Ben Hutchings
Software Developer, Codethink Ltd.