Re: [PATCH V3 7/9] dax: fix holder_ops race in fs_put_dax()

[Dave Jiang <dave.jiang@intel.com>]

On 5/30/26 9:51 AM, John Groves wrote:
> From: John Groves <John@Groves.net>
> 
> Clear holder_ops before holder_data so that a concurrent fs_dax_get()
> cannot have its newly installed holder_ops overwritten. cmpxchg()
> provides release ordering on weakly-ordered architectures, ensuring the
> WRITE_ONCE(holder_ops, NULL) store is visible to any CPU that observes
> the holder_data release.
> 
> Add WARN_ON() on the cmpxchg result to catch two API contract
> violations: fs_put_dax() called by a non-holder, or called twice by
> the same holder (double-put). Either way holder_ops has already been
> cleared, so WARN_ON() does not prevent the damage but makes the bug
> visible. (Note: "damage" is only if a non-holder causes holder_ops
> to be cleared)
> 
> Also add a kerneldoc comment documenting that fs_put_dax() must only
> be called by the current holder.
> 
> Fixes: eec38f5d86d27 ("dax: Add fs_dax_get() func to prepare dax for fs-dax usage")
> Signed-off-by: John Groves <john@groves.net>
> ---
>  drivers/dax/super.c | 35 ++++++++++++++++++++++++++++++++---
>  1 file changed, 32 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/dax/super.c b/drivers/dax/super.c
> index 25cf99dd9360b..4c56ac2faacdb 100644
> --- a/drivers/dax/super.c
> +++ b/drivers/dax/super.c
> @@ -116,11 +116,40 @@ EXPORT_SYMBOL_GPL(fs_dax_get_by_bdev);
>  
>  #if IS_ENABLED(CONFIG_FS_DAX)
>  
> +/**
> + * fs_put_dax() - release holder ownership of a dax_device
> + * @dax_dev: dax device to release (may be NULL)
> + * @holder: the holder pointer previously passed to fs_dax_get() or
> + *          fs_dax_get_by_bdev(); must match exactly, as it is used
> + *          in a cmpxchg to atomically release ownership
> + *
> + * Must only be called by the current holder. Clears holder_ops before
> + * holder_data to avoid a race where a concurrent fs_dax_get() could have
> + * its newly installed holder_ops overwritten.
> + */
>  void fs_put_dax(struct dax_device *dax_dev, void *holder)
>  {
> -	if (dax_dev && holder &&
> -	    cmpxchg(&dax_dev->holder_data, holder, NULL) == holder)
> -		dax_dev->holder_ops = NULL;
> +	if (dax_dev && holder) {
> +		/*
> +		 * Clear holder_ops before releasing holder_data. A concurrent
> +		 * dax_holder_notify_failure() that sees NULL ops returns
> +		 * -EOPNOTSUPP cleanly. A concurrent fs_dax_get() that acquires
> +		 * holder_data after the cmpxchg below is guaranteed to observe
> +		 * holder_ops=NULL first (cmpxchg provides release ordering), so
> +		 * its subsequent store of new ops will not be overwritten.
> +		 *
> +		 * Two cases will trigger the WARN_ON():
> +		 * - Caller is not the current holder; this is an API contract
> +		 *   violation, and the holder will no longer get callbacks
> +		 * - Holder calls this function twice; also a contract violation
> +		 *
> +		 * A lock would be necessary to guard against the contract
> +		 * violations, but we WARN_ON() instead since violating the
> +		 * contract is a bug
> +		 */
> +		WRITE_ONCE(dax_dev->holder_ops, NULL);
> +		WARN_ON(cmpxchg(&dax_dev->holder_data, holder, NULL) != holder);
> +	}
>  	put_dax(dax_dev);
>  }
>  EXPORT_SYMBOL_GPL(fs_put_dax);


This is what Claude Opus 4.8 said:

  The added WARN_ON(cmpxchg(...) != holder) fires on the supported
  device-removal-while-mounted path. kill_dax() (super.c:457) clears holder_data
  = NULL while a holder is still attached — it explicitly tests holder_data !=
  NULL to deliver MF_MEM_PRE_REMOVE first. For xfs on pmem:

  1. pmem_remove() → kill_dax() → MF_MEM_PRE_REMOVE →
  xfs_force_shutdown(SHUTDOWN_FORCE_UMOUNT); the handler does not call
  fs_put_dax. kill_dax then clears holder_data.
  2. Forced unmount → xfs_free_buftarg() → fs_put_dax(bt_daxdev, mp).
  3. cmpxchg(&holder_data, mp, NULL) returns NULL (already cleared) != mp → WARN
  fires, despite xfs being the legitimate holder doing a single put.

  The old == holder form skipped silently in this case. On panic_on_warn systems
  this turns a supported device removal into a panic.

  The commit message's claim that the WARN catches only "non-holder" or
  "double-put" contract violations is incomplete — it also catches the holder
  racing with kill_dax(), which is not a contract violation.

This is the suggested fix:
  void fs_put_dax(struct dax_device *dax_dev, void *holder)
  {
        if (dax_dev && holder) {
                void *prev;

                /*
                 * Clear holder_ops before releasing holder_data so a
                 * concurrent fs_dax_get() that wins holder_data observes
                 * holder_ops == NULL and its store is not overwritten.
                 */
                WRITE_ONCE(dax_dev->holder_ops, NULL);
                prev = cmpxchg(&dax_dev->holder_data, holder, NULL);

                /*
                 * prev == holder: normal release.
                 * prev == NULL:   already released by kill_dax() when the
                 *                 device was removed under a live holder;
                 *                 not a bug.
                 * prev != holder (non-NULL): fs_put_dax() called by something
                 *                 that is not the current holder.
                 */
                WARN_ON(prev && prev != holder);
        }
        put_dax(dax_dev);
  }