Re: [PATCH] xfs: prevent race in zoned space reservations

[Christoph Hellwig <hch@lst.de>]

On Wed, Aug 26, 2026 at 02:32:19PM +0200, Hans Holmberg wrote:
> xfs_zoned_add_available() checks whether the reservation list is empty
> before adding blocks to the available-space counter.  This check is not
> serialized against a task adding itself to the reservation list however.
> 
> This allows the space provider to observe an empty list, after which a
> reserver can enqueue itself and retry the counter before the new space is
> added.  The provider then adds the space and returns without waking the
> now-eligible reserver, leaving it asleep until GC or another event
> provides a wakeup, potentially adding seconds to max write latency.
> 
> Take the reservation lock before updating the counter and checking the
> list.  Use list_empty() because the list is now inspected under its lock.
> 
> Taking a per-mount lock when handing back space is far from ideal, but
> benchmarking with null_blk showed no measurable performance regression.
> 

I kinda hate taking a global lock for each block free, but at least
the critical section is very small..

Reviewed-by: Christoph Hellwig <hch@lst.de>