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Message-ID: <20160401114303.GA24771@twins.programming.kicks-ass.net>
Date: Fri, 1 Apr 2016 13:43:03 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Will Deacon <will.deacon@....com>
Cc: Waiman Long <waiman.long@....com>, Ingo Molnar <mingo@...hat.com>,
linux-kernel@...r.kernel.org,
Scott J Norton <scott.norton@....com>,
Douglas Hatch <doug.hatch@....com>
Subject: Re: [PATCH] locking/qrwlock: Allow multiple spinning readers
> Ah, yes, I forgot about that. Lemme go find that discussions and see
> what I can do there.
Completely untested..
---
include/linux/compiler.h | 20 ++++++++++++++------
kernel/locking/qspinlock.c | 12 ++++++------
kernel/sched/core.c | 9 +++++----
kernel/sched/sched.h | 2 +-
kernel/smp.c | 2 +-
5 files changed, 27 insertions(+), 18 deletions(-)
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index b5ff9881bef8..c64f5897664f 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -305,7 +305,8 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
})
/**
- * smp_cond_acquire() - Spin wait for cond with ACQUIRE ordering
+ * smp_cond_load_acquire() - Spin wait for cond with ACQUIRE ordering
+ * @ptr: pointer to the variable wait on
* @cond: boolean expression to wait for
*
* Equivalent to using smp_load_acquire() on the condition variable but employs
@@ -315,11 +316,18 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
* provides LOAD->LOAD order, together they provide LOAD->{LOAD,STORE} order,
* aka. ACQUIRE.
*/
-#define smp_cond_acquire(cond) do { \
- while (!(cond)) \
- cpu_relax(); \
- smp_rmb(); /* ctrl + rmb := acquire */ \
-} while (0)
+#define smp_cond_load_acquire(ptr, cond_expr) ({ \
+ typeof(ptr) __PTR = (ptr); \
+ typeof(*ptr) VAL; \
+ for (;;) { \
+ VAL = READ_ONCE(*__PTR); \
+ if (cond_expr) \
+ break; \
+ cpu_relax(); \
+ } \
+ smp_rmb(); /* ctrl + rmb := acquire */ \
+ VAL; \
+})
#endif /* __KERNEL__ */
diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index ce2f75e32ae1..e98e5bf679e9 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -358,7 +358,7 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
* sequentiality; this is because not all clear_pending_set_locked()
* implementations imply full barriers.
*/
- smp_cond_acquire(!(atomic_read(&lock->val) & _Q_LOCKED_MASK));
+ smp_cond_load_acquire(&lock->val.counter, !(VAL & _Q_LOCKED_MASK));
/*
* take ownership and clear the pending bit.
@@ -434,7 +434,7 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
*
* The PV pv_wait_head_or_lock function, if active, will acquire
* the lock and return a non-zero value. So we have to skip the
- * smp_cond_acquire() call. As the next PV queue head hasn't been
+ * smp_cond_load_acquire() call. As the next PV queue head hasn't been
* designated yet, there is no way for the locked value to become
* _Q_SLOW_VAL. So both the set_locked() and the
* atomic_cmpxchg_relaxed() calls will be safe.
@@ -445,7 +445,7 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
if ((val = pv_wait_head_or_lock(lock, node)))
goto locked;
- smp_cond_acquire(!((val = atomic_read(&lock->val)) & _Q_LOCKED_PENDING_MASK));
+ val = smp_cond_load_acquire(&lock->val.counter, !(VAL & _Q_LOCKED_PENDING_MASK));
locked:
/*
@@ -465,9 +465,9 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
break;
}
/*
- * The smp_cond_acquire() call above has provided the necessary
- * acquire semantics required for locking. At most two
- * iterations of this loop may be ran.
+ * The smp_cond_load_acquire() call above has provided the
+ * necessary acquire semantics required for locking. At most
+ * two iterations of this loop may be ran.
*/
old = atomic_cmpxchg_relaxed(&lock->val, val, _Q_LOCKED_VAL);
if (old == val)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 11594230ef4d..90426a5ff016 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1843,7 +1843,7 @@ static void ttwu_queue(struct task_struct *p, int cpu)
* chain to provide order. Instead we do:
*
* 1) smp_store_release(X->on_cpu, 0)
- * 2) smp_cond_acquire(!X->on_cpu)
+ * 2) smp_cond_load_acquire(&X->on_cpu, !VAL)
*
* Example:
*
@@ -1854,7 +1854,7 @@ static void ttwu_queue(struct task_struct *p, int cpu)
* sched-out X
* smp_store_release(X->on_cpu, 0);
*
- * smp_cond_acquire(!X->on_cpu);
+ * smp_cond_load_acquire(&X->on_cpu, !VAL);
* X->state = WAKING
* set_task_cpu(X,2)
*
@@ -1880,7 +1880,8 @@ static void ttwu_queue(struct task_struct *p, int cpu)
* This means that any means of doing remote wakeups must order the CPU doing
* the wakeup against the CPU the task is going to end up running on. This,
* however, is already required for the regular Program-Order guarantee above,
- * since the waking CPU is the one issueing the ACQUIRE (smp_cond_acquire).
+ * since the waking CPU is the one issueing the ACQUIRE
+ * (smp_cond_load_acquire).
*
*/
@@ -1953,7 +1954,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
* This ensures that tasks getting woken will be fully ordered against
* their previous state and preserve Program Order.
*/
- smp_cond_acquire(!p->on_cpu);
+ smp_cond_load_acquire(&p->on_cpu, !VAL);
p->sched_contributes_to_load = !!task_contributes_to_load(p);
p->state = TASK_WAKING;
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index a7cbad7b3ad2..2cbf86b6a72e 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1104,7 +1104,7 @@ static inline void finish_lock_switch(struct rq *rq, struct task_struct *prev)
* In particular, the load of prev->state in finish_task_switch() must
* happen before this.
*
- * Pairs with the smp_cond_acquire() in try_to_wake_up().
+ * Pairs with the smp_cond_load_acquire() in try_to_wake_up().
*/
smp_store_release(&prev->on_cpu, 0);
#endif
diff --git a/kernel/smp.c b/kernel/smp.c
index 74165443c240..36552beed397 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -107,7 +107,7 @@ void __init call_function_init(void)
*/
static __always_inline void csd_lock_wait(struct call_single_data *csd)
{
- smp_cond_acquire(!(csd->flags & CSD_FLAG_LOCK));
+ smp_cond_load_acquire(&csd->flags, !(VAL & CSD_FLAG_LOCK));
}
static __always_inline void csd_lock(struct call_single_data *csd)
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