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nilfs2: fix deadlock warnings caused by lock dependency in init_nilfs()
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After commit c0e473a ("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().

Link: https://lkml.kernel.org/r/20250503053327.12294-1-konishi.ryusuke@gmail.com
Fixes: c0e473a ("block: fix race between set_blocksize and read paths")
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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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Ryusuke Konishi authored and Andrew Morton committed May 8, 2025
1 parent dac2a4f commit fb881cd
Showing 1 changed file with 0 additions and 3 deletions.
3 changes: 0 additions & 3 deletions fs/nilfs2/the_nilfs.c
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Original file line number Diff line number Diff line change
Expand Up @@ -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");
Expand Down Expand Up @@ -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:
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