From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Waiman Long <Waiman.Long@hp.com>, Ingo Molnar <mingo@elte.hu>,
Andrew Morton <akpm@linux-foundation.org>,
Andrea Arcangeli <aarcange@redhat.com>,
Alex Shi <alex.shi@linaro.org>, Andi Kleen <andi@firstfloor.org>,
Michel Lespinasse <walken@google.com>,
Davidlohr Bueso <davidlohr.bueso@hp.com>,
Matthew R Wilcox <matthew.r.wilcox@intel.com>,
Dave Hansen <dave.hansen@intel.com>,
Peter Zijlstra <a.p.zijlstra@chello.nl>,
Rik van Riel <riel@redhat.com>,
Peter Hurley <peter@hurleysoftware.com>,
linux-kernel@vger.kernel.org, linux-mm <linux-mm@kvack.org>
Subject: Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and locking code into its own file
Date: Fri, 27 Sep 2013 13:38:58 -0700 [thread overview]
Message-ID: <20130927203858.GB9093@linux.vnet.ibm.com> (raw)
In-Reply-To: <1380310733.3467.118.camel@schen9-DESK>
On Fri, Sep 27, 2013 at 12:38:53PM -0700, Tim Chen wrote:
> On Fri, 2013-09-27 at 08:29 -0700, Paul E. McKenney wrote:
> > On Wed, Sep 25, 2013 at 03:10:49PM -0700, Tim Chen wrote:
> > > We will need the MCS lock code for doing optimistic spinning for rwsem.
> > > Extracting the MCS code from mutex.c and put into its own file allow us
> > > to reuse this code easily for rwsem.
> > >
> > > Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
> > > Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
> > > ---
> > > include/linux/mcslock.h | 58 +++++++++++++++++++++++++++++++++++++++++++++++
> > > kernel/mutex.c | 58 +++++-----------------------------------------
> > > 2 files changed, 65 insertions(+), 51 deletions(-)
> > > create mode 100644 include/linux/mcslock.h
> > >
> > > diff --git a/include/linux/mcslock.h b/include/linux/mcslock.h
> > > new file mode 100644
> > > index 0000000..20fd3f0
> > > --- /dev/null
> > > +++ b/include/linux/mcslock.h
> > > @@ -0,0 +1,58 @@
> > > +/*
> > > + * MCS lock defines
> > > + *
> > > + * This file contains the main data structure and API definitions of MCS lock.
> > > + */
> > > +#ifndef __LINUX_MCSLOCK_H
> > > +#define __LINUX_MCSLOCK_H
> > > +
> > > +struct mcs_spin_node {
> > > + struct mcs_spin_node *next;
> > > + int locked; /* 1 if lock acquired */
> > > +};
> > > +
> > > +/*
> > > + * We don't inline mcs_spin_lock() so that perf can correctly account for the
> > > + * time spent in this lock function.
> > > + */
> > > +static noinline
> > > +void mcs_spin_lock(struct mcs_spin_node **lock, struct mcs_spin_node *node)
> > > +{
> > > + struct mcs_spin_node *prev;
> > > +
> > > + /* Init node */
> > > + node->locked = 0;
> > > + node->next = NULL;
> > > +
> > > + prev = xchg(lock, node);
> > > + if (likely(prev == NULL)) {
> > > + /* Lock acquired */
> > > + node->locked = 1;
> > > + return;
> > > + }
> > > + ACCESS_ONCE(prev->next) = node;
> > > + smp_wmb();
>
> BTW, is the above memory barrier necessary? It seems like the xchg
> instruction already provided a memory barrier.
>
> Now if we made the changes that Jason suggested:
>
>
> /* Init node */
> - node->locked = 0;
> node->next = NULL;
>
> prev = xchg(lock, node);
> if (likely(prev == NULL)) {
> /* Lock acquired */
> - node->locked = 1;
> return;
> }
> + node->locked = 0;
> ACCESS_ONCE(prev->next) = node;
> smp_wmb();
>
> We are probably still okay as other cpus do not read the value of
> node->locked, which is a local variable.
I don't immediately see the need for the smp_wmb() in either case.
> Tim
>
> > > + /* Wait until the lock holder passes the lock down */
> > > + while (!ACCESS_ONCE(node->locked))
> > > + arch_mutex_cpu_relax();
However, you do need a full memory barrier here in order to ensure that
you see the effects of the previous lock holder's critical section.
Thanx, Paul
> > > +}
> > > +
> > > +static void mcs_spin_unlock(struct mcs_spin_node **lock, struct mcs_spin_node *node)
> > > +{
> > > + struct mcs_spin_node *next = ACCESS_ONCE(node->next);
> > > +
> > > + if (likely(!next)) {
> > > + /*
> > > + * Release the lock by setting it to NULL
> > > + */
> > > + if (cmpxchg(lock, node, NULL) == node)
> > > + return;
> > > + /* Wait until the next pointer is set */
> > > + while (!(next = ACCESS_ONCE(node->next)))
> > > + arch_mutex_cpu_relax();
> > > + }
> > > + ACCESS_ONCE(next->locked) = 1;
> > > + smp_wmb();
> >
> > Shouldn't the memory barrier precede the "ACCESS_ONCE(next->locked) = 1;"?
> > Maybe in an "else" clause of the prior "if" statement, given that the
> > cmpxchg() does it otherwise.
> >
> > Otherwise, in the case where the "if" conditionn is false, the critical
> > section could bleed out past the unlock.
> >
> > Thanx, Paul
> >
>
>
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next prev parent reply other threads:[~2013-09-27 20:39 UTC|newest]
Thread overview: 64+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <cover.1380144003.git.tim.c.chen@linux.intel.com>
2013-09-25 22:10 ` [PATCH v6 0/6] rwsem: performance optimizations Tim Chen
2013-09-25 22:10 ` [PATCH v6 1/6] rwsem: check the lock before cpmxchg in down_write_trylock Tim Chen
2013-09-25 22:10 ` [PATCH v6 2/6] rwsem: remove 'out' label in do_wake Tim Chen
2013-09-25 22:10 ` [PATCH v6 3/6] rwsem: remove try_reader_grant label do_wake Tim Chen
2013-09-25 22:10 ` [PATCH v6 4/6] rwsem/wake: check lock before do atomic update Tim Chen
2013-09-25 22:10 ` [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and locking code into its own file Tim Chen
2013-09-26 6:46 ` Ingo Molnar
2013-09-26 8:40 ` Peter Zijlstra
2013-09-26 9:37 ` Ingo Molnar
2013-09-26 18:18 ` Tim Chen
2013-09-26 19:27 ` Jason Low
2013-09-26 20:06 ` Davidlohr Bueso
2013-09-26 20:23 ` Jason Low
2013-09-26 20:40 ` Davidlohr Bueso
2013-09-26 21:09 ` Jason Low
2013-09-26 21:41 ` Tim Chen
2013-09-26 22:42 ` Jason Low
2013-09-26 22:57 ` Tim Chen
2013-09-27 6:02 ` Ingo Molnar
2013-09-27 6:26 ` Jason Low
2013-09-27 11:23 ` Peter Zijlstra
2013-09-27 13:44 ` Joe Perches
2013-09-27 13:48 ` Peter Zijlstra
2013-09-27 14:05 ` Joe Perches
2013-09-27 14:18 ` Peter Zijlstra
2013-09-27 14:14 ` [PATCH] checkpatch: Make the memory barrier test noisier Joe Perches
2013-09-27 14:26 ` Peter Zijlstra
2013-09-27 14:34 ` Joe Perches
2013-09-27 14:50 ` Peter Zijlstra
2013-09-27 15:17 ` Paul E. McKenney
2013-09-27 15:34 ` Peter Zijlstra
2013-09-27 16:04 ` Paul E. McKenney
2013-09-27 23:40 ` Oliver Neukum
2013-09-28 7:54 ` Peter Zijlstra
2013-09-27 16:12 ` [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and locking code into its own file Jason Low
2013-09-27 16:19 ` Tim Chen
2013-10-02 19:19 ` Waiman Long
2013-10-02 19:30 ` Jason Low
2013-10-02 19:37 ` Waiman Long
2013-09-26 22:22 ` Davidlohr Bueso
2013-09-27 15:29 ` Paul E. McKenney
2013-09-27 18:09 ` Tim Chen
2013-09-28 2:58 ` Waiman Long
2013-09-27 19:38 ` Tim Chen
2013-09-27 20:16 ` Jason Low
2013-09-27 20:38 ` Paul E. McKenney [this message]
2013-09-27 22:46 ` Tim Chen
2013-09-27 23:01 ` Paul E. McKenney
2013-09-27 23:54 ` Jason Low
2013-09-28 0:02 ` Davidlohr Bueso
2013-09-28 2:19 ` Paul E. McKenney
2013-09-28 4:34 ` Jason Low
2013-09-30 15:51 ` Waiman Long
2013-09-30 16:10 ` Jason Low
2013-09-30 16:36 ` Waiman Long
2013-10-01 16:48 ` Tim Chen
2013-10-01 20:01 ` Waiman Long
2013-10-01 21:16 ` Tim Chen
2013-10-02 1:25 ` Waiman Long
2013-10-02 18:43 ` Tim Chen
2013-10-02 19:32 ` Waiman Long
2013-09-30 16:28 ` Tim Chen
2013-09-25 22:10 ` [PATCH v6 6/6] rwsem: do optimistic spinning for writer lock acquisition Tim Chen
2013-09-26 6:53 ` Ingo Molnar
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