Re: [PATCH] ocfs2: kill osb->system_file_mutex lock

[Heming Zhao <heming.zhao@suse.com>]

On Thu, May 14, 2026 at 04:09:25PM +0900, Tetsuo Handa wrote:
> Hi Heming,
> 
> I would like to clarify why the expectation of "being called only once" is logically
> incorrect, reply to your concern regarding the reference count leak and explain why
> this patch is completely safe and sufficient.
> 
> 1. get_local_system_inode() can fail under memory pressure:
>    get_local_system_inode() allocates memory internally. Under heavy memory pressure,
>    this allocation can fail and return NULL. When this happens, the caller
>    ocfs2_get_system_file_inode() must fall back to calling _ocfs2_get_system_file_inode()
>    again to read the inode from disk. Therefore, the filesystem design must inherently
>    support multiple calls to _ocfs2_get_system_file_inode().
> 
> 2. Why cmpxchg() is sufficient and safe without the mutex:
>    The only thing the system_file_mutex is needed was to prevent a race where two
>    threads concurrently execute _ocfs2_get_system_file_inode(), obtain the SAME inode
>    pointer (since the underlying VFS iget_locked() returns the identical address for
>    the same slot), and both mistakenly invoke igrab() on it, leading to a reference
>    count leak.
> 
>    This patch perfectly solves that race condition by using cmpxchg() on the target
>    pointer array slot:
> 
>      * The thread that wins the cmpxchg() successfully initializes the slot with the
>        fetched inode and get the extra refcount because it is the first time to store
>        into the slot.
> 
>      * The thread that loses the cmpxchg() detects that another thread has already
>        initialized the slot with the exact same inode. The loser thread returns
>        without getting the extra refcount because it is not the first time to store
>        into the slot.
> 
>    Therefore, the reference counting contract is strictly and atomically maintained.
>    No references are leaked, and the array slot is never corrupted.

Hi,

The logic here is incorrect. The purpose of the refcount is to track how many
consumers are using the inode.

In the original code, if two threads concurrently access ocfs2_get_system_file_inode()
while the inode is uninitialized, inode->i_count would ultimately be incremented
by 3. However, with your patch, i_count will only be incremented by 2.

To be more specific:
Your patch explicitly triggers a race condition: when the target local_system
inode is uninitialized and two threads enter simultaneously, Thread 1 wins the
cmpxchg() and increments the refcount before exiting. Thread 2, however, loses
the refcount increment simply because the atomic operation failed.

btw, The issue addressed in commit 43b10a20372d was that after two concurrent
threads returned, inode->i_count ended up being 4 when the correct value should
have been 3. With your patch, the value will end up being 2, which is insufficient.

In my opinion, the problem with the current code is that the scope of
mutex_lock(&osb->system_file_mutex) is too broad. This mutex only needs to be
held prior to calling _ocfs2_get_system_file_inode(). I previously highlighted
this point in my initial review comment on the patch.

Thanks,
Heming

> 
> 3. Standard filesystems do not use a global mutex for this:
>    Standard filesystems (like Ext4's ext4_get_journal_inode or XFS's
>    xfs_qm_init_quotainos) rely entirely on the VFS layer's internal hashing/locking (e.g.,
>    iget_locked) to serialize metadata/system inode lookups. OCFS2's system_file_mutex is a
>    redundant global lock that heavily pollutes the lock dependency graph, triggering
>    possible deadlock warnings that block us from testing and fixing genuine deadlocks.
> 
> Since the cmpxchg() approach guarantees atomic slot initialization + igrab(), the global
> mutex is completely redundant and should be removed.
> 
> Regards.
>