From c69439a891ccb37ede5d68539636337c6bd92fab Mon Sep 17 00:00:00 2001 From: Christoph Hellwig Date: Mon, 18 May 2026 08:02:05 +0200 Subject: [PATCH] xfs: fix a buffer lookup against removal race When a buffer is freed either by LRU eviction or because it is unset, the lockref is marked as dead instantly, which prevents the buffer from being used after finding it in the buffer hash in xfs_buf_lookup and xfs_buf_find_insert. But the latter will then not add the new buffer to the hash because it already found an existing buffer. Fix this using in two places: Remove the buffer from the hash before marking the lockref dead so that that no buffer with a dead lockref can be found in the hash, but if we find one in xfs_buf_find_insert due to store reordering, handle this case correctly instead of returning an unhashed buffer. Fixes: 67fe4303972e ("xfs: don't keep a reference for buffers on the LRU") Reported-by: Andrey Albershteyn Reported-by: Carlos Maiolino Signed-off-by: Christoph Hellwig Reviewed-by: Andrey Albershteyn Reviewed-by: Carlos Maiolino Signed-off-by: Carlos Maiolino --- fs/xfs/xfs_buf.c | 34 ++++++++++++++++++++++++---------- 1 file changed, 24 insertions(+), 10 deletions(-) diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c index 580d40a5ee57..0cea458f1353 100644 --- a/fs/xfs/xfs_buf.c +++ b/fs/xfs/xfs_buf.c @@ -472,6 +472,7 @@ xfs_buf_find_insert( /* The new buffer keeps the perag reference until it is freed. */ new_bp->b_pag = pag; +retry: rcu_read_lock(); bp = rhashtable_lookup_get_insert_fast(&btp->bt_hash, &new_bp->b_rhash_head, xfs_buf_hash_params); @@ -480,8 +481,16 @@ xfs_buf_find_insert( error = PTR_ERR(bp); goto out_free_buf; } - if (bp && lockref_get_not_dead(&bp->b_lockref)) { - /* found an existing buffer */ + if (bp) { + /* + * If there is an existing buffer with a dead lockref, retry + * until the new buffer is added, or a usable buffer is found. + */ + if (!lockref_get_not_dead(&bp->b_lockref)) { + rcu_read_unlock(); + cpu_relax(); + goto retry; + } rcu_read_unlock(); error = xfs_buf_find_lock(bp, flags); if (error) @@ -820,15 +829,20 @@ xfs_buf_destroy( ASSERT(__lockref_is_dead(&bp->b_lockref)); ASSERT(!(bp->b_flags & _XBF_DELWRI_Q)); + if (bp->b_pag) + xfs_perag_put(bp->b_pag); + xfs_buf_free(bp); +} + +static inline void +xfs_buf_kill( + struct xfs_buf *bp) +{ + lockref_mark_dead(&bp->b_lockref); if (!xfs_buf_is_uncached(bp)) { rhashtable_remove_fast(&bp->b_target->bt_hash, &bp->b_rhash_head, xfs_buf_hash_params); - - if (bp->b_pag) - xfs_perag_put(bp->b_pag); } - - xfs_buf_free(bp); } /* @@ -851,7 +865,7 @@ xfs_buf_rele( return; kill: - lockref_mark_dead(&bp->b_lockref); + xfs_buf_kill(bp); list_lru_del_obj(&bp->b_target->bt_lru, &bp->b_lru); spin_unlock(&bp->b_lockref.lock); @@ -1433,7 +1447,7 @@ xfs_buftarg_drain_rele( return LRU_SKIP; } - lockref_mark_dead(&bp->b_lockref); + xfs_buf_kill(bp); list_lru_isolate_move(lru, item, dispose); spin_unlock(&bp->b_lockref.lock); return LRU_REMOVED; @@ -1545,7 +1559,7 @@ xfs_buftarg_isolate( return LRU_ROTATE; } - lockref_mark_dead(&bp->b_lockref); + xfs_buf_kill(bp); list_lru_isolate_move(lru, item, dispose); spin_unlock(&bp->b_lockref.lock); return LRU_REMOVED;