Re: Full hostlock pushdown available

[Jeff Garzik <jeff@garzik.org>]

On 11/02/2010 01:53 PM, Andi Kleen wrote:
>> Boaz' approach is OBVIOUSLY mechanical, correct and bisectable.
>> Yours is not.
>
> Sorry Jeff, but my patch has 100% the same locking order as Boaz.
> The only difference is in which function it is.

Incorrect.  Please re-read your own code.

Your code eliminates the drop+reacquire of the host_lock, choosing 
instead a brand new, untested, unreviewing locking scheme of holding the 
host_lock for the entirety of libata's queuecommand run.

In contrast, Boaz' proposed pattern of adding stub functions such as

	int queuecmd_unlocked(scsi_cmd cmd, callback done)
	{
		lock_irqsave
		get serial

		call driver's existing queuecommand

		unlock_irqrestore
	}

does not change libata's locking at all, because it does not modify a 
driver's queuecommand at all.  It is obviously correct.

Or to restate another way,

	Current libata locking
	----------------------
	spin_lock_irqsave(host_lock)
	spin_unlock(host_lock)
	spin_lock(ap lock)
	...
	spin_unlock(ap lock)
	spin_lock(host_lock)
	spin_unlock_irqrestore(host_lock)


	Andi's brand new locking scheme
	(missing the release of host lock)
	----------------------------------
	spin_lock_irqsave(host_lock)
	spin_lock(ap lock)
	...
	spin_unlock(ap lock)
	spin_unlock_irqrestore(host_lock)

The two versions are quite obviously NOT equivalent in any way, because 
you have ADDED the holding of host_lock for the duration of libata's 
queuecommand.

	Jeff