On Fri, Nov 08, 2013 at 11:52:38AM -0800, Tim Chen wrote:
> From: Waiman Long <Waiman.Long@xxxxxx>
>
> This patch corrects the way memory barriers are used in the MCS lock
> with smp_load_acquire and smp_store_release fucnction.
> It removes ones that are not needed.
>
> It uses architecture specific load-acquire and store-release
> primitives for synchronization, if available. Generic implementations
> are provided in case they are not defined even though they may not
> be optimal. These generic implementation could be removed later on
> once changes are made in all the relevant header files.
>
> Suggested-by: Michel Lespinasse <walken@xxxxxxxxxx>
> Signed-off-by: Waiman Long <Waiman.Long@xxxxxx>
> Signed-off-by: Jason Low <jason.low2@xxxxxx>
> Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Please see comments below.
Thanx, Paul
> ---
> kernel/locking/mcs_spinlock.c | 48 +++++++++++++++++++++++++++++++++++------
> 1 files changed, 41 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/locking/mcs_spinlock.c b/kernel/locking/mcs_spinlock.c
> index b6f27f8..df5c167 100644
> --- a/kernel/locking/mcs_spinlock.c
> +++ b/kernel/locking/mcs_spinlock.c
> @@ -23,6 +23,31 @@
> #endif
>
> /*
> + * Fall back to use the regular atomic operations and memory barrier if
> + * the acquire/release versions are not defined.
> + */
> +#ifndef xchg_acquire
> +# define xchg_acquire(p, v) xchg(p, v)
> +#endif
> +
> +#ifndef smp_load_acquire
> +# define smp_load_acquire(p) \
> + ({ \
> + typeof(*p) __v = ACCESS_ONCE(*(p)); \
> + smp_mb(); \
> + __v; \
> + })
> +#endif
> +
> +#ifndef smp_store_release
> +# define smp_store_release(p, v) \
> + do { \
> + smp_mb(); \
> + ACCESS_ONCE(*(p)) = v; \
> + } while (0)
> +#endif
> +
> +/*
> * In order to acquire the lock, the caller should declare a local node and
> * pass a reference of the node to this function in addition to the lock.
> * If the lock has already been acquired, then this will proceed to spin
> @@ -37,15 +62,19 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
> node->locked = 0;
> node->next = NULL;
>
> - prev = xchg(lock, node);
> + /* xchg() provides a memory barrier */
> + prev = xchg_acquire(lock, node);
But if this is xchg_acquire() with only acquire semantics, it need not
ensure that the initializations of node->locked and node->next above
will happen before the "ACCESS_ONCE(prev->next) = node" below. This
therefore needs to remain xchg(). Or you need an smp_store_release()
below instead of an ACCESS_ONCE() assignment.
As currently written, the poor CPU doing the unlock can be fatally
disappointed by seeing pre-initialized values of ->locked and ->next.
This could, among other things, result in a hang where the handoff
happens before the initialization.
> if (likely(prev == NULL)) {
> /* Lock acquired */
> return;
> }
> ACCESS_ONCE(prev->next) = node;
> - smp_wmb();
> - /* Wait until the lock holder passes the lock down */
> - while (!ACCESS_ONCE(node->locked))
> + /*
> + * Wait until the lock holder passes the lock down.
> + * Using smp_load_acquire() provides a memory barrier that
> + * ensures subsequent operations happen after the lock is acquired.
> + */
> + while (!(smp_load_acquire(&node->locked)))
> arch_mutex_cpu_relax();
OK, this smp_load_acquire() makes sense!
> }
> EXPORT_SYMBOL_GPL(mcs_spin_lock);
> @@ -54,7 +83,7 @@ EXPORT_SYMBOL_GPL(mcs_spin_lock);
> * Releases the lock. The caller should pass in the corresponding node that
> * was used to acquire the lock.
> */
> -static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
> +void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
> {
> struct mcs_spinlock *next = ACCESS_ONCE(node->next);
>
> @@ -68,7 +97,12 @@ static void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *nod
> while (!(next = ACCESS_ONCE(node->next)))
> arch_mutex_cpu_relax();
> }
> - ACCESS_ONCE(next->locked) = 1;
> - smp_wmb();
> + /*
> + * Pass lock to next waiter.
> + * smp_store_release() provides a memory barrier to ensure
> + * all operations in the critical section has been completed
> + * before unlocking.
> + */
> + smp_store_release(&next->locked , 1);
This smp_store_release() makes sense as well!
Could you please get rid of the extraneous space before the comma?
> }
> EXPORT_SYMBOL_GPL(mcs_spin_unlock);
> --
> 1.7.4.4
>
>
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