[PATCH] xfs: remove xfs_buf_cache.bc_lock

[Christoph Hellwig <hch@xxxxxx>]

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@xxxxxxxxxxxxxxx>
Signed-off-by: Christoph Hellwig <hch@xxxxxx>
---
 fs/xfs/xfs_buf.c | 20 ++++++++------------
 fs/xfs/xfs_buf.h |  1 -
 2 files changed, 8 insertions(+), 13 deletions(-)

diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
index d1d4a0a22e13..1fffa2990bd9 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
@@ -502,7 +501,6 @@ 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 +650,20 @@ 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 +671,8 @@ 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 +1089,6 @@ 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 +1100,6 @@ 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 +1117,6 @@ 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