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Date: Wed, 26 Nov 2014 09:31:36 -0800
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Pranith Kumar <bobby.prani@...il.com>
Cc: Alexander Viro <viro@...iv.linux.org.uk>,
Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>,
Darren Hart <dvhart@...ux.intel.com>,
Davidlohr Bueso <dave@...olabs.net>,
Heiko Carstens <heiko.carstens@...ibm.com>,
Jason Low <jason.low2@...com>, linux-fsdevel@...r.kernel.org,
open list <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] set_mb: Use smp_store_release() instead of set_mb()
On Wed, Nov 26, 2014 at 11:57:36AM -0500, Pranith Kumar wrote:
> set_mb() and smp_store_release() perform the same function. Also there are only
> a few users of set_mb(). We can convert these users to use smp_store_release()
> and delete the set_mb() definition.
>
> The following patch changes the users and if this is OK I will go ahead and
> delete the set_mb() definition. Comments and suggestions welcome.
The set_mb() and smp_store_release() operations are not quite identical:
#define set_mb(var, value) do { var = value; smp_mb(); } while (0)
#define smp_store_release(p, v) \
do { \
compiletime_assert_atomic_type(*p); \
smp_mb(); \
ACCESS_ONCE(*p) = (v); \
} while (0)
Note that set_mb() has the barrier -after- the store, but smp_store_release()
has the barrier -before- the store.
Thanx, Paul
> Thanks!
> Pranith
>
> Signed-off-by: Pranith Kumar <bobby.prani@...il.com>
> ---
> fs/select.c | 6 +++---
> include/linux/sched.h | 14 +++++++-------
> kernel/futex.c | 4 ++--
> kernel/sched/wait.c | 4 ++--
> 4 files changed, 14 insertions(+), 14 deletions(-)
>
> diff --git a/fs/select.c b/fs/select.c
> index 467bb1c..959a908 100644
> --- a/fs/select.c
> +++ b/fs/select.c
> @@ -189,7 +189,7 @@ static int __pollwake(wait_queue_t *wait, unsigned mode, int sync, void *key)
> * doesn't imply write barrier and the users expect write
> * barrier semantics on wakeup functions. The following
> * smp_wmb() is equivalent to smp_wmb() in try_to_wake_up()
> - * and is paired with set_mb() in poll_schedule_timeout.
> + * and is paired with smp_store_release() in poll_schedule_timeout.
> */
> smp_wmb();
> pwq->triggered = 1;
> @@ -244,7 +244,7 @@ int poll_schedule_timeout(struct poll_wqueues *pwq, int state,
> /*
> * Prepare for the next iteration.
> *
> - * The following set_mb() serves two purposes. First, it's
> + * The following smp_store_release() serves two purposes. First, it's
> * the counterpart rmb of the wmb in pollwake() such that data
> * written before wake up is always visible after wake up.
> * Second, the full barrier guarantees that triggered clearing
> @@ -252,7 +252,7 @@ int poll_schedule_timeout(struct poll_wqueues *pwq, int state,
> * this problem doesn't exist for the first iteration as
> * add_wait_queue() has full barrier semantics.
> */
> - set_mb(pwq->triggered, 0);
> + smp_store_release(pwq->triggered, 0);
>
> return rc;
> }
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 8db31ef..4621d0b 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -253,7 +253,7 @@ extern char ___assert_task_state[1 - 2*!!(
> #define set_task_state(tsk, state_value) \
> do { \
> (tsk)->task_state_change = _THIS_IP_; \
> - set_mb((tsk)->state, (state_value)); \
> + smp_store_release((tsk)->state, (state_value)); \
> } while (0)
>
> /*
> @@ -272,10 +272,10 @@ extern char ___assert_task_state[1 - 2*!!(
> current->task_state_change = _THIS_IP_; \
> current->state = (state_value); \
> } while (0)
> -#define set_current_state(state_value) \
> - do { \
> - current->task_state_change = _THIS_IP_; \
> - set_mb(current->state, (state_value)); \
> +#define set_current_state(state_value) \
> + do { \
> + current->task_state_change = _THIS_IP_; \
> + smp_store_release(current->state, (state_value)); \
> } while (0)
>
> #else
> @@ -283,7 +283,7 @@ extern char ___assert_task_state[1 - 2*!!(
> #define __set_task_state(tsk, state_value) \
> do { (tsk)->state = (state_value); } while (0)
> #define set_task_state(tsk, state_value) \
> - set_mb((tsk)->state, (state_value))
> + smp_store_release((tsk)->state, (state_value))
>
> /*
> * set_current_state() includes a barrier so that the write of current->state
> @@ -299,7 +299,7 @@ extern char ___assert_task_state[1 - 2*!!(
> #define __set_current_state(state_value) \
> do { current->state = (state_value); } while (0)
> #define set_current_state(state_value) \
> - set_mb(current->state, (state_value))
> + smp_store_release(current->state, (state_value))
>
> #endif
>
> diff --git a/kernel/futex.c b/kernel/futex.c
> index 63678b5..0604355 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -2055,8 +2055,8 @@ static void futex_wait_queue_me(struct futex_hash_bucket *hb, struct futex_q *q,
> {
> /*
> * The task state is guaranteed to be set before another task can
> - * wake it. set_current_state() is implemented using set_mb() and
> - * queue_me() calls spin_unlock() upon completion, both serializing
> + * wake it. set_current_state() is implemented using smp_store_release()
> + * and queue_me() calls spin_unlock() upon completion, both serializing
> * access to the hash list and forcing another memory barrier.
> */
> set_current_state(TASK_INTERRUPTIBLE);
> diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c
> index 852143a..7d990c0 100644
> --- a/kernel/sched/wait.c
> +++ b/kernel/sched/wait.c
> @@ -341,7 +341,7 @@ long wait_woken(wait_queue_t *wait, unsigned mode, long timeout)
> * condition being true _OR_ WQ_FLAG_WOKEN such that we will not miss
> * an event.
> */
> - set_mb(wait->flags, wait->flags & ~WQ_FLAG_WOKEN); /* B */
> + smp_store_release(wait->flags, wait->flags & ~WQ_FLAG_WOKEN); /* B */
>
> return timeout;
> }
> @@ -354,7 +354,7 @@ int woken_wake_function(wait_queue_t *wait, unsigned mode, int sync, void *key)
> * doesn't imply write barrier and the users expects write
> * barrier semantics on wakeup functions. The following
> * smp_wmb() is equivalent to smp_wmb() in try_to_wake_up()
> - * and is paired with set_mb() in wait_woken().
> + * and is paired with smp_store_release() in wait_woken().
> */
> smp_wmb(); /* C */
> wait->flags |= WQ_FLAG_WOKEN;
> --
> 1.9.1
>
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