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

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Date:   Tue, 27 Mar 2018 15:13:39 +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 Tue, Mar 27, 2018 at 10:33:06PM +1100, Benjamin Herrenschmidt wrote:
> On Tue, 2018-03-27 at 12:25 +0200, Andrea Parri wrote:
> > > I would rather wait until it is properly documented. Debugging that IPC
> > > problem took a *LOT* of time and energy, I wouldn't want these issues
> > > to come and bite us again.
> > 
> > I understand. And I'm grateful for this debugging as well as for the (IMO)
> > excellent account of it you provided in 51d7d5205d338.
> > 
> > Said this ;) I cannot except myself from saying that I would probably have
> > resisted that solution (adding an smp_mb() in my arch_spin_is_locked), and
> > instead "blamed"/suggested that caller to fix his memory ordering...
> 
> This is debatable. I tend go for the conservative approach assuming
> most people using fairly high level APIs such as spin_is_locked() are
> not fully cognisant of the subtleties of our memory model.
> 
> After all, it works on x86 ...
> 
> The fact that the load can leak into the internals of spin_lock()
> implementation and re-order with the lock word load itself is quite
> hard to fathom for somebody who doesn't have years of experience
> dealing with that sort of issue.
> 
> So people will get it wrong.
> 
> 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 ;),
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?

  Andrea


> 
> Cheers,
> Ben.
>