On Thu, Apr 12, 2018 at 03:14:40PM +0200, h...@lst.de wrote:
> > At least the SCSI ULP drivers drop the last reference to a disk after
> > the blk_cleanup_queue() call. As explained in the description of commit
> > a063057d7c73, removing a request queue from blkcg must happen before
> > blk_cleanup_queue() finishes because a block driver may free the
> > request queue spinlock immediately after blk_cleanup_queue() returns.
> > So I don't think that we can move the code that removes a request
> > queue from blkcg into put_disk(). Another challenge is that some block
> > drivers (e.g. skd) clear the disk->queue pointer if device_add_disk()
> > has not been called to avoid that put_disk() causes a request queue
> > reference count imbalance.
> Which sounds like a very good reason not to use a driver controller
> lock for internals like blkcq.
> In fact splitting the lock used for synchronizing access to queue
> fields from the driver controller lock used to synchronize I/O
> in the legacy path in long overdue.

It'd be probably a lot easier to make sure the shared lock doesn't go
away till all the request_queues using it go away.  The choice is
between refcnting something which carries the lock and double locking
in hot paths.  Can't think of many downsides of the former approach.



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