zswap: don't param_set_charp while holding spinlock

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.

Fixes: c99b42c3529e ("zswap: use charp for zswap param strings")
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Dan Streetman <[e-mail]>
Reported-by: Sergey Senozhatsky <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Minchan Kim <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/zswap.c

···
 	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_tail_rcu(&pool->list, &zswap_pools);
 		put_pool = pool;
-	} else if (!zswap_has_pool) {
+	}
+
+	spin_unlock(&zswap_pools_lock);
+
+	if (!zswap_has_pool && !pool) {
 		/* if initial pool creation failed, and this pool creation also
 		 * failed, maybe both compressor and zpool params were bad.
 		 * Allow changing this param, so pool creation will succeed
···
 		 */
 		ret = param_set_charp(s, kp);
 	}
-
-	spin_unlock(&zswap_pools_lock);
 
 	/* drop the ref from either the old current pool,
 	 * or the new pool we failed to add