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

Peter Zijlstra
2013-09-27T11:24:00Z

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?