Re: [PATCH v4 3/3] locking/mutex: Ensure forward progress of waiter-spinner

[Waiman Long <waiman.long@hpe.com>]

On 08/08/2016 01:37 PM, Peter Zijlstra wrote:
> On Mon, Jul 18, 2016 at 04:39:26PM -0400, Waiman Long wrote:
>> As both an optimistic spinner and a waiter-spinner (a woken task from
>> the wait queue spinning) can be spinning on the lock at the same time,
>> we cannot ensure forward progress for the waiter-spinner. Therefore,
>> it is possible for the waiter-spinner to be starved of getting the
>> lock, though not likely.
> Right; yet your previous two changelogs/comments implied otherwise.
>
>> This patch adds a flag to indicate that a waiter-spinner is
>> spinning and hence has priority over the acquisition of the lock. A
>> waiter-spinner sets this flag while spinning. An optimistic spinner
>> will check this flag and yield if set. This essentially makes the
>> waiter-spinner jump to the head of the optimistic spinning queue to
>> acquire the lock.
>>
>> There will be no increase in size for the mutex structure for 64-bit
>> architectures. For 32-bit architectures, there will be a size increase
>> of 4 bytes.
> Alternative might be to use the LSB of mutex::owner, but that's going to
> be somewhat icky too.

I was thinking about doing that. However, the owner field is used in 
quite a number of places. It may be a bit risky to change all of them.

> I'm not sure the 32bit platforms are going to be excited about growing
> struct mutex...

Or we can make this a 64-bit architecture specific change if the 
increase in mutex size is a real concern. Actually, we don't need to use 
a list_head structure for wait_list. It can be just a pointer to 
mutex_waiter that has the list_head structure. This can save a pointer 
from the structure.

Cheers,
Longman