linux-kernel - Re: [PATCH for-4.17 2/2] powerpc: Remove smp_mb() from arch_spin_is

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Date:   Wed, 28 Mar 2018 11:17:36 +0200
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH for-4.17 2/2] powerpc: Remove smp_mb() from
 arch_spin_is_locked()

On Wed, Mar 28, 2018 at 08:51:35AM +1100, Benjamin Herrenschmidt wrote:
> On Tue, 2018-03-27 at 15:13 +0200, Andrea Parri wrote:
> > > 
> > > So unless it's very performance sensitive, I'd rather have things like
> > > spin_is_locked be conservative by default and provide simpler ordering
> > > semantics.
> > 
> > Well, it might not be "very performance sensitive" but allow me to say
> > that "40+ SYNCs in stuff like BUG_ON or such" is sadness to my eyes ;),
> 
> In the fast path or the trap case ? Because the latter doesn't matter
> at all...

Both: you must execute the sync "before" issuing the load of lock.slock.


> 
> > especially when considered that our "high level API" provides means to
> > avoid this situation (e.g., smp_mb__after_spinlock(); BTW, if you look
> > at architectures for which this macro is "non-trivial", you can get an
> > idea of the architectures which "wouldn't work"; of course, x86 is not
> > among these).  Yes, we do appear to have different views on what is to
> > be considered the "simpler ordering semantics".  I'm willing to change
> > mine _as soon as_ this gets documented: would you be willing to send a
> > patch (on the lines of my [1]) to describe/document such semantics?
> 
> Not really :-) Just expressing an opinion. I don't fully object to your
> approach, just saying it's open for debate.

Always (open for debate).

I'm sending a v2 of this series shortly, integrating some feedback;
I prefer to keep this patch in the series (feel free to ignore).

  Andrea


> 
> At this point, I have too many other things to chase to follow up too
> much on this.
> 
> Cheers,
> Ben.
>