[PATCH v2] xfs: remove xfs_buf_cache.bc_lock

[PATCH v2] xfs: remove xfs_buf_cache.bc_lock

[Christoph Hellwig]

xfs_buf_cache.bc_lock serializes adding buffers to and removing them from
the hashtable.  But as the rhashtable code already uses fine grained
internal locking for inserts and removals the extra protection isn't
actually required.

It also happens to fix a lock order inversion vs b_lock added by the
recent lookup race fix.

Fixes: ee10f6fcdb96 ("xfs: fix buffer lookup vs release race")
Reported-by: "Lai, Yi" <yi1.lai@linux.intel.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
---

Changes since v1:
 - document the initial buffer state vs lockless lookups

 fs/xfs/xfs_buf.c | 31 +++++++++++++++++--------------
 fs/xfs/xfs_buf.h |  1 -
 2 files changed, 17 insertions(+), 15 deletions(-)

diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
index d1d4a0a22e13..5db1b9022865 100644
--- a/fs/xfs/xfs_buf.c
+++ b/fs/xfs/xfs_buf.c
@@ -41,8 +41,7 @@ struct kmem_cache *xfs_buf_cache;
  *
  * xfs_buf_rele:
  *	b_lock
- *	  pag_buf_lock
- *	    lru_lock
+ *	  lru_lock
  *
  * xfs_buftarg_drain_rele
  *	lru_lock
@@ -220,14 +219,21 @@ _xfs_buf_alloc(
 	 */
 	flags &= ~(XBF_UNMAPPED | XBF_TRYLOCK | XBF_ASYNC | XBF_READ_AHEAD);
 
-	spin_lock_init(&bp->b_lock);
+	/*
+	 * A new buffer is held and locked by the owner.  This ensures that the
+	 * buffer is owned by the caller and racing RCU lookups right after
+	 * inserting into the hash table are safe (and will have to wait for
+	 * the unlock to do anything non-trivial).
+	 */
 	bp->b_hold = 1;
+	sema_init(&bp->b_sema, 0); /* held, no waiters */
+
+	spin_lock_init(&bp->b_lock);
 	atomic_set(&bp->b_lru_ref, 1);
 	init_completion(&bp->b_iowait);
 	INIT_LIST_HEAD(&bp->b_lru);
 	INIT_LIST_HEAD(&bp->b_list);
 	INIT_LIST_HEAD(&bp->b_li_list);
-	sema_init(&bp->b_sema, 0); /* held, no waiters */
 	bp->b_target = target;
 	bp->b_mount = target->bt_mount;
 	bp->b_flags = flags;
@@ -502,7 +508,6 @@ int
 xfs_buf_cache_init(
 	struct xfs_buf_cache	*bch)
 {
-	spin_lock_init(&bch->bc_lock);
 	return rhashtable_init(&bch->bc_hash, &xfs_buf_hash_params);
 }
 
@@ -652,17 +657,20 @@ xfs_buf_find_insert(
 	if (error)
 		goto out_free_buf;
 
-	spin_lock(&bch->bc_lock);
+	/* The new buffer keeps the perag reference until it is freed. */
+	new_bp->b_pag = pag;
+
+	rcu_read_lock();
 	bp = rhashtable_lookup_get_insert_fast(&bch->bc_hash,
 			&new_bp->b_rhash_head, xfs_buf_hash_params);
 	if (IS_ERR(bp)) {
+		rcu_read_unlock();
 		error = PTR_ERR(bp);
-		spin_unlock(&bch->bc_lock);
 		goto out_free_buf;
 	}
 	if (bp && xfs_buf_try_hold(bp)) {
 		/* found an existing buffer */
-		spin_unlock(&bch->bc_lock);
+		rcu_read_unlock();
 		error = xfs_buf_find_lock(bp, flags);
 		if (error)
 			xfs_buf_rele(bp);
@@ -670,10 +678,8 @@ xfs_buf_find_insert(
 			*bpp = bp;
 		goto out_free_buf;
 	}
+	rcu_read_unlock();
 
-	/* The new buffer keeps the perag reference until it is freed. */
-	new_bp->b_pag = pag;
-	spin_unlock(&bch->bc_lock);
 	*bpp = new_bp;
 	return 0;
 
@@ -1090,7 +1096,6 @@ xfs_buf_rele_cached(
 	}
 
 	/* we are asked to drop the last reference */
-	spin_lock(&bch->bc_lock);
 	__xfs_buf_ioacct_dec(bp);
 	if (!(bp->b_flags & XBF_STALE) && atomic_read(&bp->b_lru_ref)) {
 		/*
@@ -1102,7 +1107,6 @@ xfs_buf_rele_cached(
 			bp->b_state &= ~XFS_BSTATE_DISPOSE;
 		else
 			bp->b_hold--;
-		spin_unlock(&bch->bc_lock);
 	} else {
 		bp->b_hold--;
 		/*
@@ -1120,7 +1124,6 @@ xfs_buf_rele_cached(
 		ASSERT(!(bp->b_flags & _XBF_DELWRI_Q));
 		rhashtable_remove_fast(&bch->bc_hash, &bp->b_rhash_head,
 				xfs_buf_hash_params);
-		spin_unlock(&bch->bc_lock);
 		if (pag)
 			xfs_perag_put(pag);
 		freebuf = true;
diff --git a/fs/xfs/xfs_buf.h b/fs/xfs/xfs_buf.h
index 7e73663c5d4a..3b4ed42e11c0 100644
--- a/fs/xfs/xfs_buf.h
+++ b/fs/xfs/xfs_buf.h
@@ -80,7 +80,6 @@ typedef unsigned int xfs_buf_flags_t;
 #define XFS_BSTATE_IN_FLIGHT	 (1 << 1)	/* I/O in flight */
 
 struct xfs_buf_cache {
-	spinlock_t		bc_lock;
 	struct rhashtable	bc_hash;
 };
 
-- 
2.45.2

Re: [PATCH v2] xfs: remove xfs_buf_cache.bc_lock

[Dave Chinner]

On Tue, Jan 28, 2025 at 06:22:58AM +0100, Christoph Hellwig wrote:
> xfs_buf_cache.bc_lock serializes adding buffers to and removing them from
> the hashtable.  But as the rhashtable code already uses fine grained
> internal locking for inserts and removals the extra protection isn't
> actually required.
> 
> It also happens to fix a lock order inversion vs b_lock added by the
> recent lookup race fix.
> 
> Fixes: ee10f6fcdb96 ("xfs: fix buffer lookup vs release race")
> Reported-by: "Lai, Yi" <yi1.lai@linux.intel.com>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
> ---
> 
> Changes since v1:
>  - document the initial buffer state vs lockless lookups

Looks fine.

Reviewed-by: Dave Chinner <dchinner@redhat.com>

-- 
Dave Chinner
david@fromorbit.com

Re: [PATCH v2] xfs: remove xfs_buf_cache.bc_lock

[Carlos Maiolino]

On Tue, 28 Jan 2025 06:22:58 +0100, Christoph Hellwig wrote:
> xfs_buf_cache.bc_lock serializes adding buffers to and removing them from
> the hashtable.  But as the rhashtable code already uses fine grained
> internal locking for inserts and removals the extra protection isn't
> actually required.
> 
> It also happens to fix a lock order inversion vs b_lock added by the
> recent lookup race fix.
> 
> [...]

Applied to for-next, thanks!

[1/1] xfs: remove xfs_buf_cache.bc_lock
      commit: a9ab28b3d21aec6d0f56fe722953e20ce470237b

Best regards,
-- 
Carlos Maiolino <cem@kernel.org>