* [Qemu-devel] [PATCH] rcu: optimize rcu_read_lock
@ 2015-12-16 11:32 Paolo Bonzini
2015-12-17 1:16 ` Wen Congyang
0 siblings, 1 reply; 2+ messages in thread
From: Paolo Bonzini @ 2015-12-16 11:32 UTC (permalink / raw)
To: qemu-devel
rcu_read_lock cannot change rcu_gp_ongoing from true to false
(the previous value of p_rcu_reader->ctr is zero), hence
there is no need to check p_rcu_reader->waiting and wake up
a concurrent synchronize_rcu.
While at it mark the wakeup as unlikely in rcu_read_unlock.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
include/qemu/rcu.h | 6 +-----
1 file changed, 1 insertion(+), 5 deletions(-)
diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h
index f6d1d56..7c7cca7 100644
--- a/include/qemu/rcu.h
+++ b/include/qemu/rcu.h
@@ -88,10 +88,6 @@ static inline void rcu_read_lock(void)
ctr = atomic_read(&rcu_gp_ctr);
atomic_xchg(&p_rcu_reader->ctr, ctr);
- if (atomic_read(&p_rcu_reader->waiting)) {
- atomic_set(&p_rcu_reader->waiting, false);
- qemu_event_set(&rcu_gp_event);
- }
}
static inline void rcu_read_unlock(void)
@@ -104,7 +100,7 @@ static inline void rcu_read_unlock(void)
}
atomic_xchg(&p_rcu_reader->ctr, 0);
- if (atomic_read(&p_rcu_reader->waiting)) {
+ if (unlikely(atomic_read(&p_rcu_reader->waiting))) {
atomic_set(&p_rcu_reader->waiting, false);
qemu_event_set(&rcu_gp_event);
}
--
2.5.0
^ permalink raw reply related [flat|nested] 2+ messages in thread* Re: [Qemu-devel] [PATCH] rcu: optimize rcu_read_lock
2015-12-16 11:32 [Qemu-devel] [PATCH] rcu: optimize rcu_read_lock Paolo Bonzini
@ 2015-12-17 1:16 ` Wen Congyang
0 siblings, 0 replies; 2+ messages in thread
From: Wen Congyang @ 2015-12-17 1:16 UTC (permalink / raw)
To: Paolo Bonzini, qemu-devel
On 12/16/2015 07:32 PM, Paolo Bonzini wrote:
> rcu_read_lock cannot change rcu_gp_ongoing from true to false
> (the previous value of p_rcu_reader->ctr is zero), hence
> there is no need to check p_rcu_reader->waiting and wake up
> a concurrent synchronize_rcu.
>
> While at it mark the wakeup as unlikely in rcu_read_unlock.
>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Wen Congyang <wency@cn.fujitsu.com>
> ---
> include/qemu/rcu.h | 6 +-----
> 1 file changed, 1 insertion(+), 5 deletions(-)
>
> diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h
> index f6d1d56..7c7cca7 100644
> --- a/include/qemu/rcu.h
> +++ b/include/qemu/rcu.h
> @@ -88,10 +88,6 @@ static inline void rcu_read_lock(void)
>
> ctr = atomic_read(&rcu_gp_ctr);
> atomic_xchg(&p_rcu_reader->ctr, ctr);
> - if (atomic_read(&p_rcu_reader->waiting)) {
> - atomic_set(&p_rcu_reader->waiting, false);
> - qemu_event_set(&rcu_gp_event);
> - }
> }
>
> static inline void rcu_read_unlock(void)
> @@ -104,7 +100,7 @@ static inline void rcu_read_unlock(void)
> }
>
> atomic_xchg(&p_rcu_reader->ctr, 0);
> - if (atomic_read(&p_rcu_reader->waiting)) {
> + if (unlikely(atomic_read(&p_rcu_reader->waiting))) {
> atomic_set(&p_rcu_reader->waiting, false);
> qemu_event_set(&rcu_gp_event);
> }
>
^ permalink raw reply [flat|nested] 2+ messages in thread
end of thread, other threads:[~2015-12-17 1:17 UTC | newest]
Thread overview: 2+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2015-12-16 11:32 [Qemu-devel] [PATCH] rcu: optimize rcu_read_lock Paolo Bonzini
2015-12-17 1:16 ` Wen Congyang
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).