On Mon, Oct 22, 2012 at 07:39:16PM -0400, Mikulas Patocka wrote:
> Use rcu_read_lock_sched / rcu_read_unlock_sched / synchronize_sched
> instead of rcu_read_lock / rcu_read_unlock / synchronize_rcu.
>
> This is an optimization. The RCU-protected region is very small, so
> there will be no latency problems if we disable preempt in this region.
>
> So we use rcu_read_lock_sched / rcu_read_unlock_sched that translates
> to preempt_disable / preempt_disable. It is smaller (and supposedly
> faster) than preemptible rcu_read_lock / rcu_read_unlock.
>
> Signed-off-by: Mikulas Patocka <mpato...@redhat.com>
OK, as promised/threatened, I finally got a chance to take a closer look.
The light_mb() and heavy_mb() definitions aren't doing much for me,
the code would be cleared with them expanded inline. And while the
approach of pairing barrier() with synchronize_sched() is interesting,
it would be simpler to rely on RCU's properties. The key point is that
if RCU cannot prove that a given RCU-sched read-side critical section
is seen by all CPUs to have started after a given synchronize_sched(),
then that synchronize_sched() must wait for that RCU-sched read-side
critical section to complete.
This means, as discussed earlier, that there will be a memory barrier
somewhere following the end of that RCU-sched read-side critical section,
and that this memory barrier executes before the completion of the
synchronize_sched().
So I suggest something like the following (untested!) implementation:
------------------------------------------------------------------------
struct percpu_rw_semaphore {
unsigned __percpu *counters;
bool locked;
struct mutex mtx;
wait_queue_head_t wq;
};
static inline void percpu_down_read(struct percpu_rw_semaphore *p)
{
rcu_read_lock_sched();
if (unlikely(p->locked)) {
rcu_read_unlock_sched();
/*
* There might (or might not) be a writer. Acquire &p->mtx,
* it is always safe (if a bit slow) to do so.
*/
mutex_lock(&p->mtx);
this_cpu_inc(*p->counters);
mutex_unlock(&p->mtx);
return;
}
/* No writer, proceed locklessly. */
this_cpu_inc(*p->counters);
rcu_read_unlock_sched();
}
static inline void percpu_up_read(struct percpu_rw_semaphore *p)
{
/*
* Decrement our count, but protected by RCU-sched so that
* the writer can force proper serialization.
*/
rcu_read_lock_sched();
this_cpu_dec(*p->counters);
rcu_read_unlock_sched();
}
static inline unsigned __percpu_count(unsigned __percpu *counters)
{
unsigned total = 0;
int cpu;
for_each_possible_cpu(cpu)
total += ACCESS_ONCE(*per_cpu_ptr(counters, cpu));
return total;
}
static inline void percpu_down_write(struct percpu_rw_semaphore *p)
{
mutex_lock(&p->mtx);
/* Wait for a previous writer, if necessary. */
wait_event(p->wq, !ACCESS_ONCE(p->locked));
/* Force the readers to acquire the lock when manipulating counts. */
ACCESS_ONCE(p->locked) = true;
/* Wait for all pre-existing readers' checks of ->locked to finish. */
synchronize_sched();
/*
* At this point, all percpu_down_read() invocations will
* acquire p->mtx.
*/
/*
* Wait for all pre-existing readers to complete their
* percpu_up_read() calls. Because ->locked is set and
* because we hold ->mtx, there cannot be any new readers.
* ->counters will therefore monotonically decrement to zero.
*/
while (__percpu_count(p->counters))
msleep(1);
/*
* Invoke synchronize_sched() in order to force the last
* caller of percpu_up_read() to exit its RCU-sched read-side
* critical section. On SMP systems, this also forces the CPU
* that invoked that percpu_up_read() to execute a full memory
* barrier between the time it exited the RCU-sched read-side
* critical section and the time that synchronize_sched() returns,
* so that the critical section begun by this invocation of
* percpu_down_write() will happen after the critical section
* ended by percpu_up_read().
*/
synchronize_sched();
}
static inline void percpu_up_write(struct percpu_rw_semaphore *p)
{
/* Allow others to proceed, but not yet locklessly. */
mutex_unlock(&p->mtx);
/*
* Ensure that all calls to percpu_down_read() that did not
* start unambiguously after the above mutex_unlock() still
* acquire the lock, forcing their critical sections to be
* serialized with the one terminated by this call to
* percpu_up_write().
*/
synchronize_sched();
/* Now it is safe to allow readers to proceed locklessly. */
ACCESS_ONCE(p->locked) = false;
/*
* If there is another writer waiting, wake it up. Note that
* p->mtx properly serializes its critical section with the
* critical section terminated by this call to percpu_up_write().
*/
wake_up(&p->wq);
}
static inline int percpu_init_rwsem(struct percpu_rw_semaphore *p)
{
p->counters = alloc_percpu(unsigned);
if (unlikely(!p->counters))
return -ENOMEM;
p->locked = false;
mutex_init(&p->mtx);
init_waitqueue_head(&p->wq);
return 0;
}
static inline void percpu_free_rwsem(struct percpu_rw_semaphore *p)
{
free_percpu(p->counters);
p->counters = NULL; /* catch use after free bugs */
}
------------------------------------------------------------------------
Of course, it would be nice to get rid of the extra synchronize_sched().
One way to do this is to use SRCU, which allows blocking operations in
its read-side critical sections (though also increasing read-side overhead
a bit, and also untested):
------------------------------------------------------------------------
struct percpu_rw_semaphore {
bool locked;
struct mutex mtx; /* Could also be rw_semaphore. */
struct srcu_struct s;
wait_queue_head_t wq;
};
static inline int percpu_down_read(struct percpu_rw_semaphore *p)
{
int idx;
idx = srcu_read_lock(&p->s);
if (unlikely(p->locked)) {
srcu_read_unlock(&p->s, idx);
/*
* There might (or might not) be a writer. Acquire &p->mtx,
* it is always safe (if a bit slow) to do so.
*/
mutex_lock(&p->mtx);
return -1; /* srcu_read_lock() cannot return -1. */
}
return idx;
}
static inline void percpu_up_read(struct percpu_rw_semaphore *p, int idx)
{
if (idx == -1)
mutex_unlock(&p->mtx);
else
srcu_read_unlock(&p->s, idx);
}
static inline void percpu_down_write(struct percpu_rw_semaphore *p)
{
mutex_lock(&p->mtx);
/* Wait for a previous writer, if necessary. */
wait_event(p->wq, !ACCESS_ONCE(p->locked));
/* Force new readers to acquire the lock when manipulating counts. */
ACCESS_ONCE(p->locked) = true;
/* Wait for all pre-existing readers' checks of ->locked to finish. */
synchronize_srcu(&p->s);
/* At this point, all lockless readers have completed. */
}
static inline void percpu_up_write(struct percpu_rw_semaphore *p)
{
/* Allow others to proceed, but not yet locklessly. */
mutex_unlock(&p->mtx);
/*
* Ensure that all calls to percpu_down_read() that did not
* start unambiguously after the above mutex_unlock() still
* acquire the lock, forcing their critical sections to be
* serialized with the one terminated by this call to
* percpu_up_write().
*/
synchronize_sched();
/* Now it is safe to allow readers to proceed locklessly. */
ACCESS_ONCE(p->locked) = false;
/*
* If there is another writer waiting, wake it up. Note that
* p->mtx properly serializes its critical section with the
* critical section terminated by this call to percpu_up_write().
*/
wake_up(&p->wq);
}
static inline int percpu_init_rwsem(struct percpu_rw_semaphore *p)
{
p->locked = false;
mutex_init(&p->mtx);
if (unlikely(!init_srcu_struct(&p->s)));
return -ENOMEM;
init_waitqueue_head(&p->wq);
return 0;
}
static inline void percpu_free_rwsem(struct percpu_rw_semaphore *p)
{
cleanup_srcu_struct(&p->s);
}
------------------------------------------------------------------------
Of course, there was a question raised as to whether something already
exists that does this job...
And you guys did ask!
Thanx, Paul
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