Re: [PATCH v2 0/9] Remove spin_unlock_wait()

From	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH v2 0/9] Remove spin_unlock_wait()
Date	2017-07-06 19:20 +0200
Message-ID	<u0iBA-8tF-17@gated-at.bofh.it> (permalink)
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On Thu, Jul 06, 2017 at 06:50:36PM +0200, Peter Zijlstra wrote:
> On Thu, Jul 06, 2017 at 09:20:24AM -0700, Paul E. McKenney wrote:
> > On Thu, Jul 06, 2017 at 06:05:55PM +0200, Peter Zijlstra wrote:
> > > On Thu, Jul 06, 2017 at 02:12:24PM +0000, David Laight wrote:
> > > > From: Paul E. McKenney
> > 
> > [ . . . ]
> > 
> > > Now on the one hand I feel like Oleg that it would be a shame to loose
> > > the optimization, OTOH this thing is really really tricky to use,
> > > and has lead to a number of bugs already.
> > 
> > I do agree, it is a bit sad to see these optimizations go.  So, should
> > this make mainline, I will be tagging the commits that spin_unlock_wait()
> > so that they can be easily reverted should someone come up with good
> > semantics and a compelling use case with compelling performance benefits.
> 
> Ha!, but what would constitute 'good semantics' ?

At this point, it beats the heck out of me!  ;-)

> The current thing is something along the lines of:
> 
>   "Waits for the currently observed critical section
>    to complete with ACQUIRE ordering such that it will observe
>    whatever state was left by said critical section."
> 
> With the 'obvious' benefit of limited interference on those actually
> wanting to acquire the lock, and a shorter wait time on our side too,
> since we only need to wait for completion of the current section, and
> not for however many contender are before us.
> 
> Not sure I have an actual (micro) benchmark that shows a difference
> though.
> 
> 
> 
> Is this all good enough to retain the thing, I dunno. Like I said, I'm
> conflicted on the whole thing. On the one hand its a nice optimization,
> on the other hand I don't want to have to keep fixing these bugs.

Yeah, if I had seen a compelling use case...  Oleg's task_work case was
closest, but given that it involved a task-local lock that shouldn't
be all -that- heavily contended, it is hard to see there being all that
much difference.

But maybe I am missing something here?  Wouldn't be the first time...

							Thanx, Paul

Thread

[PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-06 01:40 +0200
  [PATCH v2 9/9] arch: Remove spin_unlock_wait() arch-specific definitions "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-06 01:40 +0200
  RE: [PATCH v2 0/9] Remove spin_unlock_wait() David Laight <David.Laight@ACULAB.COM> - 2017-07-06 16:20 +0200
    Re: [PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-06 17:30 +0200
      Re: [PATCH v2 0/9] Remove spin_unlock_wait() Peter Zijlstra <peterz@infradead.org> - 2017-07-06 18:20 +0200
        Re: [PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-06 18:30 +0200
          Re: [PATCH v2 0/9] Remove spin_unlock_wait() Peter Zijlstra <peterz@infradead.org> - 2017-07-06 18:50 +0200
            Re: [PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-06 19:10 +0200
          Re: [PATCH v2 0/9] Remove spin_unlock_wait() Alan Stern <stern@rowland.harvard.edu> - 2017-07-06 18:50 +0200
            Re: [PATCH v2 0/9] Remove spin_unlock_wait() Peter Zijlstra <peterz@infradead.org> - 2017-07-06 19:00 +0200
              Re: [PATCH v2 0/9] Remove spin_unlock_wait() Alan Stern <stern@rowland.harvard.edu> - 2017-07-06 21:40 +0200
    Re: [PATCH v2 0/9] Remove spin_unlock_wait() Peter Zijlstra <peterz@infradead.org> - 2017-07-06 18:10 +0200
      Re: [PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-06 18:30 +0200
        Re: [PATCH v2 0/9] Remove spin_unlock_wait() Peter Zijlstra <peterz@infradead.org> - 2017-07-06 19:00 +0200
          Re: [PATCH v2 0/9] Remove spin_unlock_wait() Will Deacon <will.deacon@arm.com> - 2017-07-06 19:10 +0200
            Re: [PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-06 19:30 +0200
          Re: [PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-06 19:20 +0200
          Re: [PATCH v2 0/9] Remove spin_unlock_wait() Ingo Molnar <mingo@kernel.org> - 2017-07-07 10:40 +0200
            Re: [PATCH v2 0/9] Remove spin_unlock_wait() Peter Zijlstra <peterz@infradead.org> - 2017-07-07 10:50 +0200
              Re: [PATCH v2 0/9] Remove spin_unlock_wait() Ingo Molnar <mingo@kernel.org> - 2017-07-07 12:40 +0200
                Re: [PATCH v2 0/9] Remove spin_unlock_wait() Peter Zijlstra <peterz@infradead.org> - 2017-07-07 13:30 +0200
            Re: [PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-07 16:50 +0200
              Re: [PATCH v2 0/9] Remove spin_unlock_wait() Ingo Molnar <mingo@kernel.org> - 2017-07-08 10:50 +0200
                Re: [PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-08 13:50 +0200
            Re: [PATCH v2 0/9] Remove spin_unlock_wait() Manfred Spraul <manfred@colorfullife.com> - 2017-07-07 19:50 +0200
              Re: [PATCH v2 0/9] Remove spin_unlock_wait() Ingo Molnar <mingo@kernel.org> - 2017-07-08 10:40 +0200
                Re: [PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-08 13:50 +0200
                Re: [PATCH v2 0/9] Remove spin_unlock_wait() Ingo Molnar <mingo@kernel.org> - 2017-07-08 14:40 +0200
                Re: [PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-08 16:50 +0200
                Re: [PATCH v2 0/9] Remove spin_unlock_wait() Alan Stern <stern@rowland.harvard.edu> - 2017-07-08 18:30 +0200
                Re: [PATCH v2 0/9] Remove spin_unlock_wait() Manfred Spraul <manfred@colorfullife.com> - 2017-07-10 19:30 +0200
      Re: [PATCH v2 0/9] Remove spin_unlock_wait() Ingo Molnar <mingo@kernel.org> - 2017-07-07 10:10 +0200
        Re: [PATCH v2 0/9] Remove spin_unlock_wait() Ingo Molnar <mingo@kernel.org> - 2017-07-07 11:40 +0200
  [PATCH v3 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-07 21:30 +0200
    [PATCH v3 9/9] arch: Remove spin_unlock_wait() arch-specific definitions "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-07 21:40 +0200
    [PATCH v3 5/9] exit: Replace spin_unlock_wait() with lock/unlock pair "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-07 21:40 +0200
    [PATCH v3 8/9] locking: Remove spin_unlock_wait() generic definitions "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-07 21:40 +0200

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