[PATCH v2] fs: jfs: fix a possible data race in metapage_writepage()

[Jia-Ju Bai <baijiaju1990@gmail.com>]

The functions metapage_writepage() and lmPostGC() can be concurrently 
executed in the following call contexts:

Thread1:
  metapage_writepage()

Thread2:
  lbmIODone()
    lmPostGC()

In metapage_writepage():
  if (mp->log && !(mp->log->cflag & logGC_PAGEOUT))

In lmPostGC():
  spin_lock_irqsave(&log->gclock, flags);
  ...
  log->cflag &= ~logGC_PAGEOUT
  ...
  spin_unlock_irqrestore(&log->gclock, flags);

The memory addresses of mp->log->cflag and log->cflag can be identical,
and thus a data race can occur.
This data race was found by our concurrency fuzzer.

Thus use the spin lock "mp->log->gclock" for the assignment of 
the data structure member "log->cflag" to a local variable 
in this function implementation.

Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
---
v2:
* Change the description.
  Thank Markus Elfring for good advice.

 fs/jfs/jfs_metapage.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/fs/jfs/jfs_metapage.c b/fs/jfs/jfs_metapage.c
index a2f5338a5ea1..026c11b2572d 100644
--- a/fs/jfs/jfs_metapage.c
+++ b/fs/jfs/jfs_metapage.c
@@ -351,6 +351,7 @@ static int metapage_writepage(struct page *page, struct writeback_control *wbc)
 	unsigned long bio_offset = 0;
 	int offset;
 	int bad_blocks = 0;
+	uint cflag;
 
 	page_start = (sector_t)page->index <<
 		     (PAGE_SHIFT - inode->i_blkbits);
@@ -370,8 +371,14 @@ static int metapage_writepage(struct page *page, struct writeback_control *wbc)
 			 * Make sure this page isn't blocked indefinitely.
 			 * If the journal isn't undergoing I/O, push it
 			 */
-			if (mp->log && !(mp->log->cflag & logGC_PAGEOUT))
-				jfs_flush_journal(mp->log, 0);
+
+			if (mp->log) {
+				spin_lock_irq(&mp->log->gclock);
+				cflag = mp->log->cflag;
+				spin_unlock_irq(&mp->log->gclock);
+				if (!(cflag & logGC_PAGEOUT))
+					jfs_flush_journal(mp->log, 0);
+			}
 			continue;
 		}
 
-- 
2.17.1