linux-kernel - Re: [PATCH] Documentation: atomic_t.txt: Explain ordering provided by smp_mb__{before,after}

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Message-ID: <20190423134003.GV4038@hirez.programming.kicks-ass.net>
Date:   Tue, 23 Apr 2019 15:40:03 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     Nicholas Piggin <npiggin@...il.com>,
        LKMM Maintainers -- Akira Yokosawa <akiyks@...il.com>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Boqun Feng <boqun.feng@...il.com>,
        David Howells <dhowells@...hat.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Kernel development list <linux-kernel@...r.kernel.org>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Alan Stern <stern@...land.harvard.edu>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH] Documentation: atomic_t.txt: Explain ordering provided
 by smp_mb__{before,after}_atomic()

On Tue, Apr 23, 2019 at 06:30:10AM -0700, Paul E. McKenney wrote:
> There are a great many that look like this:
> 
> 	smp_mb__before_atomic();
> 	clear_bit(NFSD4_CLIENT_UPCALL_LOCK, &clp->cl_flags);
> 	smp_mb__after_atomic();

Ooh, marvel at the comment describing the ordering there. Oh wait :-(
So much for checkpatch.pl I suppose.

I think the first is a release order for the 'LOCK' bit and the second
is because of wake_up_bit() being a shitty interface.

So maybe that should've been:

	clear_bit_unlock(NFSD4_CLIENT_UPCALL_LOCK, &clp->cl_flags);
	smp_mb__after_atomic();
	wake_up_bit(&clp->cl_flags, NFSD4_CLIENT_UPCALL_LOCK);

instead?