Re: [PATCH 7/9] w1: don't hold list_mutex while a slave is torn down

[Krzysztof Kozlowski <krzk@kernel.org>]

On 27/10/2026 17:04, Greg Kroah-Hartman wrote:
> From: Josef Bacik <josef@toxicpanda.com>
> 
> When the last reference to a slave is dropped, w1_unref_slave() holds
> list_mutex of the master until the slave has been freed.  In between it
> calls the ->remove_slave() callback of the family driver and
> device_unregister().  Both wait for others: for the sysfs callbacks
> which are running, and in the case of w1_therm also for the file
> therm_bulk_read of the master and for everybody who is talking to the
> sensor.  None of those can therefore take list_mutex, although the
> callbacks of therm_bulk_read walk the list which it protects.
> 
> The mutex is only needed to make the decrement of the reference count and
> the removal from the list one step for w1_search_slave() and
> w1_slave_search_device(), which look the slave up in the list and, in the
> first case, take a reference.  Once the slave is off the list, nobody can
> find it any more.  Drop the mutex at that point.
> 
> The callers which hold the mutex of the master, which serialises
> attaching and detaching, are not affected.  The one which does not is
> w1_process_cb(), when it drops the last reference to a slave which has
> been detached while a netlink command for it was queued.  Somebody who
> attaches a slave with the same ID through sysfs or netlink at that very
> moment used to wait until the old device was gone.  Now the registration
> of the new device may fail because the name is still in use.
> 
> This is a preparation for "w1: w1_therm: lock the list of slaves for the
> bulk read".
> 
> Cc: stable <stable@kernel.org>
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>


Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>

Best regards,
Krzysztof