From: Davidlohr Bueso <dave@stgolabs.net>
To: Peter Zijlstra <peterz@infradead.org>
Cc: mingo@kernel.org, will@kernel.org, oleg@redhat.com,
tglx@linutronix.de, linux-kernel@vger.kernel.org,
bigeasy@linutronix.de, juri.lelli@redhat.com,
williams@redhat.com, bristot@redhat.com, longman@redhat.com,
jack@suse.com
Subject: Re: [PATCH 5/5] locking/percpu-rwsem: Remove the embedded rwsem
Date: Mon, 18 Nov 2019 15:19:35 -0800 [thread overview]
Message-ID: <20191118231935.7wvkozof3ocubxej@linux-p48b> (raw)
In-Reply-To: <20191118195304.b3d6fg4jmmj7kmfh@linux-p48b>
On Mon, 18 Nov 2019, Davidlohr Bueso wrote:
>On Wed, 13 Nov 2019, Peter Zijlstra wrote:
3>>bool __percpu_down_read(struct percpu_rw_semaphore *sem, bool try)
>>{
>> if (__percpu_down_read_trylock(sem))
>>@@ -89,20 +156,10 @@ bool __percpu_down_read(struct percpu_rw
>> if (try)
>> return false;
>>
>>- /*
>>- * We either call schedule() in the wait, or we'll fall through
>>- * and reschedule on the preempt_enable() in percpu_down_read().
>>- */
>>- preempt_enable_no_resched();
>>-
>>- /*
>>- * Avoid lockdep for the down/up_read() we already have them.
>>- */
>>- __down_read(&sem->rw_sem);
>>- this_cpu_inc(*sem->read_count);
>>- __up_read(&sem->rw_sem);
>>-
>>+ preempt_enable();
>>+ percpu_rwsem_wait(sem, /* .reader = */ true );
>> preempt_disable();
>>+
>> return true;
>>}
>>EXPORT_SYMBOL_GPL(__percpu_down_read);
>
>Do we really need to export symbol here? This function is only called
>from percpu-rwsem.h.
Similarly, afaict we can get rid of __percpu_up_read() and put the
slowpath all into percpu_up_read(). Also explicitly mention the
single task nature of the writer (which is a better comment for
the rcuwait_wake_up()).
diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h
index f5ecf6a8a1dd..eda545f42fb8 100644
--- a/include/linux/percpu-rwsem.h
+++ b/include/linux/percpu-rwsem.h
@@ -43,7 +43,6 @@ is_static struct percpu_rw_semaphore name = { \
__DEFINE_PERCPU_RWSEM(name, static)
extern bool __percpu_down_read(struct percpu_rw_semaphore *, bool);
-extern void __percpu_up_read(struct percpu_rw_semaphore *);
static inline void percpu_down_read(struct percpu_rw_semaphore *sem)
{
@@ -103,10 +102,23 @@ static inline void percpu_up_read(struct percpu_rw_semaphore *sem)
/*
* Same as in percpu_down_read().
*/
- if (likely(rcu_sync_is_idle(&sem->rss)))
+ if (likely(rcu_sync_is_idle(&sem->rss))) {
__this_cpu_dec(*sem->read_count);
- else
- __percpu_up_read(sem); /* Unconditional memory barrier */
+ goto done;
+ }
+
+ /*
+ * slowpath; reader will only ever wake a single blocked writer.
+ */
+ smp_mb(); /* B matches C */
+ /*
+ * In other words, if they see our decrement (presumably to
+ * aggregate zero, as that is the only time it matters) they
+ * will also see our critical section.
+ */
+ __this_cpu_dec(*sem->read_count);
+ rcuwait_wake_up(&sem->writer);
+done:
preempt_enable();
}
diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
index 851038468efb..a5150a876626 100644
--- a/kernel/locking/percpu-rwsem.c
+++ b/kernel/locking/percpu-rwsem.c
@@ -164,21 +164,6 @@ bool __percpu_down_read(struct percpu_rw_semaphore *sem, bool try)
}
EXPORT_SYMBOL_GPL(__percpu_down_read);
-void __percpu_up_read(struct percpu_rw_semaphore *sem)
-{
- smp_mb(); /* B matches C */
- /*
- * In other words, if they see our decrement (presumably to aggregate
- * zero, as that is the only time it matters) they will also see our
- * critical section.
- */
- __this_cpu_dec(*sem->read_count);
-
- /* Prod writer to re-evaluate readers_active_check() */
- rcuwait_wake_up(&sem->writer);
-}
-EXPORT_SYMBOL_GPL(__percpu_up_read);
-
#define per_cpu_sum(var) \
({ \
typeof(var) __sum = 0; \
next prev parent reply other threads:[~2019-11-18 23:23 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-13 10:21 [PATCH 0/5] locking: Percpu-rwsem rewrite Peter Zijlstra
2019-11-13 10:21 ` [PATCH 1/5] locking/percpu-rwsem, lockdep: Make percpu-rwsem use its own lockdep_map Peter Zijlstra
2019-11-15 20:39 ` Davidlohr Bueso
2020-01-08 1:33 ` [PATCH] locking/percpu-rwsem: Add might_sleep() for writer locking Davidlohr Bueso
2020-01-08 1:33 ` Davidlohr Bueso
2020-02-11 12:48 ` [tip: locking/core] " tip-bot2 for Davidlohr Bueso
2019-11-13 10:21 ` [PATCH 2/5] locking/percpu-rwsem: Convert to bool Peter Zijlstra
2019-11-13 10:21 ` [PATCH 3/5] locking/percpu-rwsem: Move __this_cpu_inc() into the slowpath Peter Zijlstra
2019-11-13 10:21 ` [PATCH 4/5] locking/percpu-rwsem: Extract __percpu_down_read_trylock() Peter Zijlstra
2019-11-18 16:28 ` Oleg Nesterov
2019-11-13 10:21 ` [PATCH 5/5] locking/percpu-rwsem: Remove the embedded rwsem Peter Zijlstra
2019-11-18 19:53 ` Davidlohr Bueso
2019-11-18 23:19 ` Davidlohr Bueso [this message]
2019-12-17 10:45 ` Peter Zijlstra
2019-12-17 10:35 ` Peter Zijlstra
2019-11-18 21:52 ` Waiman Long
2019-12-17 10:28 ` Peter Zijlstra
2019-11-19 13:50 ` Waiman Long
2019-11-19 15:58 ` Oleg Nesterov
2019-11-19 16:28 ` Waiman Long
2019-12-17 10:26 ` Peter Zijlstra
2019-12-17 10:28 ` Peter Zijlstra
2019-11-15 17:14 ` [PATCH 0/5] locking: Percpu-rwsem rewrite Juri Lelli
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