[PATCH v2] sysv: don't call sb_bread() with pointers_lock held

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

syzbot is reporting sleep in atomic context in SysV filesystem [1], for
sb_bread() is called with rw_spinlock held.

A "write_lock(&pointers_lock) => read_lock(&pointers_lock) deadlock" bug
and a "sb_bread() with write_lock(&pointers_lock)" bug were introduced by
"Replace BKL for chain locking with sysvfs-private rwlock" in Linux 2.5.12.

Then, "[PATCH] err1-40: sysvfs locking fix" in Linux 2.6.8 fixed the
former bug by moving pointers_lock lock to the callers, but instead
introduced a "sb_bread() with read_lock(&pointers_lock)" bug (which made
this problem easier to hit).

Al Viro suggested that why not to do like get_branch()/get_block()/
find_shared() in Minix filesystem does. And doing like that is a revert
of "[PATCH] err1-40: sysvfs locking fix" except that get_branch() from
find_shared() is called without write_lock(&pointers_lock).

Reported-by: syzbot <syzbot+69b40dc5fd40f32c199f@syzkaller.appspotmail.com>
Link: https://syzkaller.appspot.com/bug?extid=69b40dc5fd40f32c199f
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
Changes in v2:
  Correct patch description.

 fs/sysv/itree.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/fs/sysv/itree.c b/fs/sysv/itree.c
index b22764fe669c..ab4756c94755 100644
--- a/fs/sysv/itree.c
+++ b/fs/sysv/itree.c
@@ -82,9 +82,6 @@ static inline sysv_zone_t *block_end(struct buffer_head *bh)
 	return (sysv_zone_t*)((char*)bh->b_data + bh->b_size);
 }
 
-/*
- * Requires read_lock(&pointers_lock) or write_lock(&pointers_lock)
- */
 static Indirect *get_branch(struct inode *inode,
 			    int depth,
 			    int offsets[],
@@ -104,15 +101,18 @@ static Indirect *get_branch(struct inode *inode,
 		bh = sb_bread(sb, block);
 		if (!bh)
 			goto failure;
+		read_lock(&pointers_lock);
 		if (!verify_chain(chain, p))
 			goto changed;
 		add_chain(++p, bh, (sysv_zone_t*)bh->b_data + *++offsets);
+		read_unlock(&pointers_lock);
 		if (!p->key)
 			goto no_block;
 	}
 	return NULL;
 
 changed:
+	read_unlock(&pointers_lock);
 	brelse(bh);
 	*err = -EAGAIN;
 	goto no_block;
@@ -214,9 +214,7 @@ static int get_block(struct inode *inode, sector_t iblock, struct buffer_head *b
 		goto out;
 
 reread:
-	read_lock(&pointers_lock);
 	partial = get_branch(inode, depth, offsets, chain, &err);
-	read_unlock(&pointers_lock);
 
 	/* Simplest case - block found, no allocation needed */
 	if (!partial) {
@@ -286,9 +284,9 @@ static Indirect *find_shared(struct inode *inode,
 	*top = 0;
 	for (k = depth; k > 1 && !offsets[k-1]; k--)
 		;
+	partial = get_branch(inode, k, offsets, chain, &err);
 
 	write_lock(&pointers_lock);
-	partial = get_branch(inode, k, offsets, chain, &err);
 	if (!partial)
 		partial = chain + k-1;
 	/*
-- 
2.34.1