Re: [PATCH] ceph: fix deadlock bugs by making iput() calls asynchronous

[Dave Chinner <david@fromorbit.com>]

On Wed, Sep 17, 2025 at 02:44:04PM +0200, Max Kellermann wrote:
> The iput() function is a dangerous one - if the reference counter goes
> to zero, the function may block for a long time due to:
> 
> - inode_wait_for_writeback() waits until writeback on this inode
>   completes
> 
> - the filesystem-specific "evict_inode" callback can do similar
>   things; e.g. all netfs-based filesystems will call
>   netfs_wait_for_outstanding_io() which is similar to
>   inode_wait_for_writeback()
> 
> Therefore, callers must carefully evaluate the context they're in and
> check whether invoking iput() is a good idea at all.
> 
> Most of the time, this is not a problem because the dcache holds
> references to all inodes, and the dcache is usually the one to release
> the last reference.  But this assumption is fragile.  For example,
> under (memcg) memory pressure, the dcache shrinker is more likely to
> release inode references, moving the inode eviction to contexts where
> that was extremely unlikely to occur.
> 
> Our production servers "found" at least two deadlock bugs in the Ceph
> filesystem that were caused by this iput() behavior:
> 
> 1. Writeback may lead to iput() calls in Ceph (e.g. from
>    ceph_put_wrbuffer_cap_refs()) which deadlocks in
>    inode_wait_for_writeback().  Waiting for writeback completion from
>    within writeback will obviously never be able to make any progress.
>    This leads to blocked kworkers like this:
> 
>     INFO: task kworker/u777:6:1270802 blocked for more than 122 seconds.
>           Not tainted 6.16.7-i1-es #773
>     task:kworker/u777:6  state:D stack:0 pid:1270802 tgid:1270802 ppid:2
>           task_flags:0x4208060 flags:0x00004000
>     Workqueue: writeback wb_workfn (flush-ceph-3)
>     Call Trace:
>      <TASK>
>      __schedule+0x4ea/0x17d0
>      schedule+0x1c/0xc0
>      inode_wait_for_writeback+0x71/0xb0
>      evict+0xcf/0x200
>      ceph_put_wrbuffer_cap_refs+0xdd/0x220
>      ceph_invalidate_folio+0x97/0xc0
>      ceph_writepages_start+0x127b/0x14d0
>      do_writepages+0xba/0x150
>      __writeback_single_inode+0x34/0x290
>      writeback_sb_inodes+0x203/0x470
>      __writeback_inodes_wb+0x4c/0xe0
>      wb_writeback+0x189/0x2b0
>      wb_workfn+0x30b/0x3d0
>      process_one_work+0x143/0x2b0
>      worker_thread+0x30a/0x450
> 
> 2. In the Ceph messenger thread (net/ceph/messenger*.c), any iput()
>    call may invoke ceph_evict_inode() which will deadlock in
>    netfs_wait_for_outstanding_io(); since this blocks the messenger
>    thread, completions from the Ceph servers will not ever be received
>    and handled.
> 
> It looks like these deadlock bugs have been in the Ceph filesystem
> code since forever (therefore no "Fixes" tag in this patch).  There
> may be various ways to solve this:
> 
> - make iput() asynchronous and defer the actual eviction like fput()
>   (may add overhead)
> 
> - make iput() only asynchronous if I_SYNC is set (doesn't solve random
>   things happening inside the "evict_inode" callback)
> 
> - add iput_deferred() to make this asynchronous behavior/overhead
>   optional and explicit
> 
> - refactor Ceph to avoid iput() calls from within writeback and
>   messenger (if that is even possible)
> 
> - add a Ceph-specific workaround

- wait for Josef to finish his inode refcount rework patchset that
  gets rid of this whole "writeback doesn't hold an inode reference"
  problem that is the root cause of this the deadlock.

All that adding a whacky async iput work around does right now is
make it harder for Josef to land the patchset that makes this
problem go away entirely....

> diff --git a/fs/ceph/inode.c b/fs/ceph/inode.c
> index f67025465de0..385d5261632d 100644
> --- a/fs/ceph/inode.c
> +++ b/fs/ceph/inode.c
> @@ -2191,6 +2191,48 @@ void ceph_queue_inode_work(struct inode *inode, int work_bit)
>  	}
>  }
>  
> +/**
> + * Queue an asynchronous iput() call in a worker thread.  Use this
> + * instead of iput() in contexts where evicting the inode is unsafe.
> + * For example, inode eviction may cause deadlocks in
> + * inode_wait_for_writeback() (when called from within writeback) or
> + * in netfs_wait_for_outstanding_io() (when called from within the
> + * Ceph messenger).
> + *
> + * @n: how many references to put
> + */
> +void ceph_iput_n_async(struct inode *inode, int n)
> +{
> +	if (unlikely(!inode))
> +		return;
> +
> +	if (likely(atomic_sub_return(n, &inode->i_count) > 0))
> +		/* somebody else is holding another reference -
> +		 * nothing left to do for us
> +		 */
> +		return;
> +
> +	doutc(ceph_inode_to_fs_client(inode)->client, "%p %llx.%llx\n", inode, ceph_vinop(inode));
> +
> +	/* the reference counter is now 0, i.e. nobody else is holding
> +	 * a reference to this inode; restore it to 1 and donate it to
> +	 * ceph_inode_work() which will call iput() at the end
> +	 */
> +	atomic_set(&inode->i_count, 1);

If you must do this, please have a look at how
btrfs_add_delayed_iput() handles detecting the last inode reference
and punting it to an async queue without needing to drop the last
reference at all.

-Dave.
-- 
Dave Chinner
david@fromorbit.com