linux-kernel - Re: [PATCH 01/20] asm-generic/mmiowb: Add generic implementation of mmiowb() tracking

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Date:   Thu, 7 Mar 2019 10:13:18 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Nicholas Piggin <npiggin@...il.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        Will Deacon <will.deacon@....com>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Arnd Bergmann <arnd@...db.de>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Rich Felker <dalias@...c.org>,
        David Howells <dhowells@...hat.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        "Maciej W. Rozycki" <macro@...ux-mips.org>,
        Ingo Molnar <mingo@...nel.org>,
        Palmer Dabbelt <palmer@...ive.com>,
        Paul Burton <paul.burton@...s.com>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Tony Luck <tony.luck@...el.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>
Subject: Re: [PATCH 01/20] asm-generic/mmiowb: Add generic implementation of
 mmiowb() tracking

On Thu, Mar 07, 2019 at 11:47:53AM +1100, Michael Ellerman wrote:
> The mutex unlock fast path is just:
> 
> 	if (atomic_long_cmpxchg_release(&lock->owner, curr, 0UL) == curr)
> 		return true;
> 
> And because it's the "release" variant we just use lwsync, which doesn't
> order MMIO. If it was just atomic_long_cmpxchg() that would work because
> we use sync for those.
> 
> __up_write() uses atomic_long_sub_return_release(), so same story.

As does spin_unlock() of course, which is a great segway into...

  my RCsc desires :-)

If all your unlocks were to have SYNC, your locks would, aside from
ordering MMIO, also be RCsc, Win-Win :-)

There is, of course, that pesky little performance detail that keeps
getting in the way.