[PATCH 06/11] block: fix queue freeze vs limits lock order in sysfs store methods

[John Garry]

On 09/01/2025 05:57, Christoph Hellwig wrote:
> queue_attr_store() always freezes a device queue before calling the
> attribute store operation. For attributes that control queue limits, the
> store operation will also lock the queue limits with a call to
> queue_limits_start_update(). However, some drivers (e.g. SCSI sd) may
> need to issue commands to a device to obtain limit values from the
> hardware with the queue limits locked. This creates a potential ABBA
> deadlock situation if a user attempts to modify a limit (thus freezing
> the device queue) while the device driver starts a revalidation of the
> device queue limits.
> 
> Avoid such deadlock by not freezing the queue before calling the
> ->store_limit() method in struct queue_sysfs_entry and instead use the
> queue_limits_commit_update_frozen helper to freeze the queue after taking
> the limits lock.
> 
> This also removes taking the sysfs lock for the store_limit method as
> it doesn't protect anything here, but creates even more nesting.
> Hopefully it will go away from the actual sysfs methods entirely soon.

Do you mean that the sysfs_lock could be removed in future? I would have 
thought that queue limits lock could be used for the same thing, but I 
am probably failing to see some lock nesting/ordering issues...