linux-kernel - [PATCH] zsmalloc: fix migrate_zspage-zs

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Date:	Fri, 15 Jan 2016 09:36:24 +0900
From:	Junil Lee <junil0814.lee@....com>
To:	<minchan@...nel.org>, <ngupta@...are.org>
CC:	sergey.senozhatsky.work@...il.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Junil Lee <junil0814.lee@....com>
Subject: [PATCH] zsmalloc: fix migrate_zspage-zs_free race condition

To prevent unlock at the not correct situation, tagging the new obj to
assure lock in migrate_zspage() before right unlock path.

Two functions are in race condition by tag which set 1 on last bit of
obj, however unlock succrently when update new obj to handle before call
unpin_tag() which is right unlock path.

summarize this problem by call flow as below:

		CPU0								CPU1
migrate_zspage
find_alloced_obj()
	trypin_tag() -- obj |= HANDLE_PIN_BIT
obj_malloc() -- new obj is not set			zs_free
record_obj() -- unlock and break sync		pin_tag() -- get lock
unpin_tag()

Signed-off-by: Junil Lee <junil0814.lee@....com>
---
 mm/zsmalloc.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index e7414ce..bb459ef 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -1635,6 +1635,7 @@ static int migrate_zspage(struct zs_pool *pool, struct size_class *class,
 		free_obj = obj_malloc(d_page, class, handle);
 		zs_object_copy(free_obj, used_obj, class);
 		index++;
+		free_obj |= BIT(HANDLE_PIN_BIT);
 		record_obj(handle, free_obj);
 		unpin_tag(handle);
 		obj_free(pool, class, used_obj);
-- 
2.6.2