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authorCarlos Maiolino <cmaiolino@redhat.com>2012-08-10 15:01:51 -0300
committerBen Myers <bpm@sgi.com>2012-08-29 15:01:11 -0500
commit6fb8a90aa3f2319a25f3396b1e9273300f8903b8 (patch)
tree3a74bce9dbe7a6ef8b0c952335c1963a649eefa2 /fs/xfs/xfs_buf.c
parenta672e1be30d5bc848cd0067c55ed29b2015b7c17 (diff)
xfs: fix race while discarding buffers [V4]
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> Reviewed-by: Ben Myers <bpm@sgi.com> Reviewed-by: Dave Chinner <dchinner@redhat.com> Signed-off-by: Ben Myers <bpm@sgi.com>
Diffstat (limited to 'fs/xfs/xfs_buf.c')
-rw-r--r--fs/xfs/xfs_buf.c5
1 files changed, 4 insertions, 1 deletions
diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
index d7a9dd735e1..933b7930b86 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);