From 6fb8a90aa3f2319a25f3396b1e9273300f8903b8 Mon Sep 17 00:00:00 2001 From: Carlos Maiolino Date: Fri, 10 Aug 2012 15:01:51 -0300 Subject: 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 Reviewed-by: Ben Myers Reviewed-by: Dave Chinner Signed-off-by: Ben Myers --- fs/xfs/xfs_buf.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) (limited to 'fs/xfs/xfs_buf.c') diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c index d7a9dd735e1e..933b7930b863 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); -- cgit