dax: fix holder_ops race in fs_put_dax()

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 a WARN_ON() that fires only when the cmpxchg observes a non-NULL
value that is not @holder, i.e. fs_put_dax() called by something that
is not the current holder. That is an API contract violation; the
WARN_ON() does not prevent the damage but makes the bug visible.

A NULL cmpxchg result is deliberately tolerated: kill_dax() clears
holder_data while a holder is still attached when a device is removed
out from under a mounted filesystem (after delivering MF_MEM_PRE_REMOVE).
The holder's subsequent fs_put_dax() - e.g. xfs_free_buftarg() after a
forced shutdown - then legitimately finds holder_data already NULL, so
warning on that case would turn supported device removal into a splat
(or a panic with panic_on_warn).

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>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Link: https://patch.msgid.link/0100019ecc09dcab-2f4aa175-0b84-4b36-9e54-ebff302ebb0a-000000@email.amazonses.com
Signed-off-by: Alison Schofield <alison.schofield@intel.com>

0 files changed, 0 insertions, 0 deletions