[PATCH] xfs: fix race while discarding buffers [V4]

Carlos Maiolino
2012-08-10T18:02:01Z

While xfs_buftarg_shrink() is freeing buffers from the dispose list (filled with
buffers from lru list), there is a possibility to have xfs_buf_stale() racing
with it, and removing buffers from dispose list before xfs_buftarg_shrink() does
it.

This happens because xfs_buftarg_shrink() handle the dispose list without
locking and the test condition in xfs_buf_stale() checks for the buffer being in
*any* list:

if (!list_empty(&bp->b_lru)

If the buffer happens to be on dispose list, this causes the buffer counter of
lru list (btp->bt_lru_nr) to be decremented twice (once in xfs_buftarg_shrink()
and another in xfs_buf_stale()) causing a wrong account usage of the lru list.

This may cause xfs_buftarg_shrink() to return a wrong value to the memory
shrinker shrink_slab(), and such account error may also cause an underflowed
value to be returned; since the counter is lower than the current number of
items in the lru list, a decrement may happen when the counter is 0, causing
an underflow on the counter.

The fix uses a new flag field (and a new buffer flag) to serialize buffer
handling during the shrink process. The new flag field has been designed to use
btp->bt_lru_lock/unlock instead of xfs_buf_lock/unlock mechanism.

dchinner, sandeen, aquini and aris also deserve credits for this.

Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
---
fs/xfs/xfs_buf.c | 5 ++++-
fs/xfs/xfs_buf.h | 41 ++++++++++++++++++++++++-----------------
2 files changed, 28 insertions(+), 18 deletions(-)

diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
index d7a9dd7..933b793 100644
--- a/fs/xfs/xfs_buf.c
+++ b/fs/xfs/xfs_buf.c
@@ -96,6 +96,7 @@ xfs_buf_lru_add(
atomic_inc(&bp->b_hold);
list_add_tail(&bp->b_lru, &btp->bt_lru);
btp->bt_lru_nr++;
+ bp->b_lru_flags &= ~_XBF_LRU_DISPOSE;
}
spin_unlock(&btp->bt_lru_lock);
}
@@ -154,7 +155,8 @@ xfs_buf_stale(
struct xfs_buftarg *btp = bp->b_target;

spin_lock(&btp->bt_lru_lock);
- if (!list_empty(&bp->b_lru)) {
+ if (!list_empty(&bp->b_lru) &&
+ !(bp->b_lru_flags & _XBF_LRU_DISPOSE)) {
list_del_init(&bp->b_lru);
btp->bt_lru_nr--;
atomic_dec(&bp->b_hold);
@@ -1501,6 +1503,7 @@ xfs_buftarg_shrink(
*/
list_move(&bp->b_lru, &dispose);
btp->bt_lru_nr--;
+ bp->b_lru_flags |= _XBF_LRU_DISPOSE;
}
spin_unlock(&btp->bt_lru_lock);

diff --git a/fs/xfs/xfs_buf.h b/fs/xfs/xfs_buf.h
index d03b73b..7c0b6a0 100644
--- a/fs/xfs/xfs_buf.h
+++ b/fs/xfs/xfs_buf.h
@@ -38,27 +38,28 @@ typedef enum {
XBRW_ZERO = 3, /* Zero target memory */
} xfs_buf_rw_t;

-#define XBF_READ (1 << 0) /* buffer intended for reading from device */
-#define XBF_WRITE (1 << 1) /* buffer intended for writing to device */
-#define XBF_READ_AHEAD (1 << 2) /* asynchronous read-ahead */
-#define XBF_ASYNC (1 << 4) /* initiator will not wait for completion */
-#define XBF_DONE (1 << 5) /* all pages in the buffer uptodate */
-#define XBF_STALE (1 << 6) /* buffer has been staled, do not find it */
+#define XBF_READ (1 << 0) /* buffer intended for reading from device */
+#define XBF_WRITE (1 << 1) /* buffer intended for writing to device */
+#define XBF_READ_AHEAD (1 << 2) /* asynchronous read-ahead */
+#define XBF_ASYNC (1 << 4) /* initiator will not wait for completion */
+#define XBF_DONE (1 << 5) /* all pages in the buffer uptodate */
+#define XBF_STALE (1 << 6) /* buffer has been staled, do not find it */

/* I/O hints for the BIO layer */
-#define XBF_SYNCIO (1 << 10)/* treat this buffer as synchronous I/O */
-#define XBF_FUA (1 << 11)/* force cache write through mode */
-#define XBF_FLUSH (1 << 12)/* flush the disk cache before a write */
+#define XBF_SYNCIO (1 << 10)/* treat this buffer as synchronous I/O */
+#define XBF_FUA (1 << 11)/* force cache write through mode */
+#define XBF_FLUSH (1 << 12)/* flush the disk cache before a write */

/* flags used only as arguments to access routines */
-#define XBF_TRYLOCK (1 << 16)/* lock requested, but do not wait */
-#define XBF_UNMAPPED (1 << 17)/* do not map the buffer */
+#define XBF_TRYLOCK (1 << 16)/* lock requested, but do not wait */
+#define XBF_UNMAPPED (1 << 17)/* do not map the buffer */

/* flags used only internally */
-#define _XBF_PAGES (1 << 20)/* backed by refcounted pages */
-#define _XBF_KMEM (1 << 21)/* backed by heap memory */
-#define _XBF_DELWRI_Q (1 << 22)/* buffer on a delwri queue */
-#define _XBF_COMPOUND (1 << 23)/* compound buffer */
+#define _XBF_PAGES (1 << 20)/* backed by refcounted pages */
+#define _XBF_KMEM (1 << 21)/* backed by heap memory */
+#define _XBF_DELWRI_Q (1 << 22)/* buffer on a delwri queue */
+#define _XBF_COMPOUND (1 << 23)/* compound buffer */
+#define _XBF_LRU_DISPOSE (1 << 24)/* buffer being discarded */

typedef unsigned int xfs_buf_flags_t;

@@ -72,12 +73,13 @@ typedef unsigned int xfs_buf_flags_t;
{ XBF_SYNCIO, "SYNCIO" }, \
{ XBF_FUA, "FUA" }, \
{ XBF_FLUSH, "FLUSH" }, \
- { XBF_TRYLOCK, "TRYLOCK" }, /* should never be set */\
+ { XBF_TRYLOCK, "TRYLOCK" }, /* should never be set */\
{ XBF_UNMAPPED, "UNMAPPED" }, /* ditto */\
{ _XBF_PAGES, "PAGES" }, \
{ _XBF_KMEM, "KMEM" }, \
{ _XBF_DELWRI_Q, "DELWRI_Q" }, \
- { _XBF_COMPOUND, "COMPOUND" }
+ { _XBF_COMPOUND, "COMPOUND" }, \
+ { _XBF_LRU_DISPOSE, "LRU_DISPOSE" }

typedef struct xfs_buftarg {
dev_t bt_dev;
@@ -124,7 +126,12 @@ typedef struct xfs_buf {
xfs_buf_flags_t b_flags; /* status flags */
struct semaphore b_sema; /* semaphore for lockables */

+ /*
+ * concurrent access to b_lru and b_lru_flags are protected by
+ * bt_lru_lock and not by b_sema
+ */
struct list_head b_lru; /* lru list */
+ xfs_buf_flags_t b_lru_flags; /* internal lru status flags */
wait_queue_head_t b_waiters; /* unpin waiters */
struct list_head b_list;
struct xfs_perag *b_pag; /* contains rbtree root */
--
1.7.11.2