Patch "zswap: don't param_set_charp while holding spinlock" 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

zswap: don't param_set_charp while holding spinlock

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:
zswap-don-t-param_set_charp-while-holding-spinlock.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: Dan Streetman <[email protected]>
Date: Mon, 27 Feb 2017 14:26:53 -0800
Subject: zswap: don't param_set_charp while holding spinlock

From: Dan Streetman <[email protected]>

commit fd5bb66cd934987e49557455b6497fc006521940 upstream.

Change the zpool/compressor param callback function to release the zswap_pools_lock spinlock before calling param_set_charp, since that function may sleep when it calls kmalloc with GFP_KERNEL.

While this problem has existed for a while, I wasn't able to trigger it using a tight loop changing either/both the zpool and compressor params; I think it's very unlikely to be an issue on the stable kernels, especially since most zswap users will change the compressor and/or zpool from sysfs only one time each boot - or zero times, if they add the params to the kernel boot.

---
 mm/zswap.c |   12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

@@ -752,18 +752,22 @@ static int __zswap_param_set(const char
 	pool = zswap_pool_find_get(type, compressor);
 	if (pool) {
 		zswap_pool_debug("using existing", pool);
+		WARN_ON(pool == zswap_pool_current());
 		list_del_rcu(&pool->list);
-	} else {
-		spin_unlock(&zswap_pools_lock);
-		pool = zswap_pool_create(type, compressor);
-		spin_lock(&zswap_pools_lock);
 	}
 
+	spin_unlock(&zswap_pools_lock);
+
+	if (!pool)
+		pool = zswap_pool_create(type, compressor);
+
 	if (pool)
 		ret = param_set_charp(s, kp);
 	else
 		ret = -EINVAL;
 
+	spin_lock(&zswap_pools_lock);
+
 	if (!ret) {
 		put_pool = zswap_pool_current();
 		list_add_rcu(&pool->list, &zswap_pools);

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

queue-4.4/mm-zswap-use-workqueue-to-destroy-pool.patch queue-4.4/zswap-don-t-param_set_charp-while-holding-spinlock.patch