[PATCH] nilfs2: fix deadlock warnings caused by lock dependency in init_nilfs()

[Ryusuke Konishi <konishi.ryusuke@gmail.com>]

After commit c0e473a0d226 ("block: fix race between set_blocksize and
read paths") was merged, set_blocksize() called by sb_set_blocksize()
now locks the inode of the backing device file.  As a result of this
change, syzbot started reporting deadlock warnings due to a circular
dependency involving the semaphore "ns_sem" of the nilfs object, the
inode lock of the backing device file, and the locks that this inode
lock is transitively dependent on.

This is caused by a new lock dependency added by the above change,
since init_nilfs() calls sb_set_blocksize() in the lock section of
"ns_sem".  However, these warnings are false positives because
init_nilfs() is called in the early stage of the mount operation and
the filesystem has not yet started.

The reason why "ns_sem" is locked in init_nilfs() was to avoid a race
condition in nilfs_fill_super() caused by sharing a nilfs object among
multiple filesystem instances (super block structures) in the early
implementation.  However, nilfs objects and super block structures
have long ago become one-to-one, and there is no longer any need to
use the semaphore there.

So, fix this issue by removing the use of the semaphore "ns_sem" in
init_nilfs().

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Reported-by: syzbot+00f7f5b884b117ee6773@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=00f7f5b884b117ee6773
Tested-by: syzbot+00f7f5b884b117ee6773@syzkaller.appspotmail.com
Reported-by: syzbot+f30591e72bfc24d4715b@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=f30591e72bfc24d4715b
Tested-by: syzbot+f30591e72bfc24d4715b@syzkaller.appspotmail.com
Fixes: c0e473a0d226 ("block: fix race between set_blocksize and read paths")
---
Hi Andrew, please apply this as a regression fix.

This resolves some deadlock warnings reported by syzbot since a change
in 6.15-rc4.

Thanks,
Ryusuke Konishi

 fs/nilfs2/the_nilfs.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/fs/nilfs2/the_nilfs.c b/fs/nilfs2/the_nilfs.c
index cb01ea81724d..d0bcf744c553 100644
--- a/fs/nilfs2/the_nilfs.c
+++ b/fs/nilfs2/the_nilfs.c
@@ -705,8 +705,6 @@ int init_nilfs(struct the_nilfs *nilfs, struct super_block *sb)
 	int blocksize;
 	int err;
 
-	down_write(&nilfs->ns_sem);
-
 	blocksize = sb_min_blocksize(sb, NILFS_MIN_BLOCK_SIZE);
 	if (!blocksize) {
 		nilfs_err(sb, "unable to set blocksize");
@@ -779,7 +777,6 @@ int init_nilfs(struct the_nilfs *nilfs, struct super_block *sb)
 	set_nilfs_init(nilfs);
 	err = 0;
  out:
-	up_write(&nilfs->ns_sem);
 	return err;
 
  failed_sbh:
-- 
2.43.0