[patch 04/78] zswap: don't param_set_charp while holding spinlock

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From: Dan Streetman <ddstreet@xxxxxxxx>
Subject: 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/20170126155821.4545-1-ddstreet@xxxxxxxx
Signed-off-by: Dan Streetman <dan.streetman@xxxxxxxxxxxxx>
Reported-by: Sergey Senozhatsky <sergey.senozhatsky.work@xxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 mm/zswap.c |   20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff -puN mm/zswap.c~zswap-dont-param_set_charp-while-holding-spinlock mm/zswap.c
--- a/mm/zswap.c~zswap-dont-param_set_charp-while-holding-spinlock
+++ a/mm/zswap.c
@@ -704,18 +704,22 @@ 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);
@@ -727,7 +731,11 @@ static int __zswap_param_set(const char
 		 */
 		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
@@ -738,8 +746,6 @@ static int __zswap_param_set(const char
 		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
 	 */
_
--
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