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 08:51:35 +1100
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Andrea Parri <andrea.parri@...rulasolutions.com>
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 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...

> 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.

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

Cheers,
Ben.