linux-kernel - Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and locking code into its own file

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Message-ID: <20130927112323.GJ3657@laptop.programming.kicks-ass.net>
Date:	Fri, 27 Sep 2013 13:23:23 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Tim Chen <tim.c.chen@...ux.intel.com>,
	Jason Low <jason.low2@...com>,
	Davidlohr Bueso <davidlohr@...com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Alex Shi <alex.shi@...aro.org>,
	Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Matthew R Wilcox <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	linux-kernel@...r.kernel.org, linux-mm <linux-mm@...ck.org>,
	Joe Perches <joe@...ches.com>
Subject: Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and
 locking code into its own file

On Fri, Sep 27, 2013 at 08:02:13AM +0200, Ingo Molnar wrote:
> Would be nice to have this as a separate, add-on patch. Every single 
> instruction removal that has no downside is an upside!
> 
> You can add a comment that explains it.

If someone is going to do add-on patches to the mcslock.h file, please
also consider doing a patch that adds comments to the memory barriers in
there.

Also, checkpatch.pl should really warn about that; and it appears there
code in there for that; however:

# grep -C3 smp_mb scripts/checkpatch.pl 
                        }
                }
# check for memory barriers without a comment.
                if ($line =~ /\b(mb|rmb|wmb|read_barrier_depends|smp_mb|smp_rmb|smp_wmb|smp_read_barrier_depends)\(/) {
                        if (!ctx_has_comment($first_line, $linenr)) {
                                CHK("MEMORY_BARRIER",
                                    "memory barrier without comment\n" . $herecurr);
# grep -C3 smp_wmb kernel/mutex.c
                return;
        }
        ACCESS_ONCE(prev->next) = node;
        smp_wmb();
        /* Wait until the lock holder passes the lock down */
        while (!ACCESS_ONCE(node->locked))
                arch_mutex_cpu_relax();
--
                        arch_mutex_cpu_relax();
        }
        ACCESS_ONCE(next->locked) = 1;
        smp_wmb();
}

/*
# scripts/checkpatch.pl -f kernel/mutex.c 2>&1 | grep memory
#

so that appears to be completely broken :/

Joe, any clue what's up with that?
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