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

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  Re: [PATCH v4 3/3] locking/mutex: Ensure forward progress of  waiter-spinner Peter Zijlstra <peterz@infradead.org> - 2016-08-08 19:40 +0200
    Re: [PATCH v4 3/3] locking/mutex: Ensure forward progress of  waiter-spinner Peter Zijlstra <peterz@infradead.org> - 2016-08-10 21:20 +0200

#1457949 — Re: [PATCH v4 3/3] locking/mutex: Ensure forward progress of waiter-spinner

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

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#1459617

On Tue, Aug 09, 2016 at 02:00:00PM -0400, Waiman Long wrote:
> >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.

Agreed.

> >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.

Just grow the thing, we can poke at it later if we get complaints.

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