linux-kernel - Re: [PATCHv5 2/2] memory barrier: adding smp_mb__after

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Message-ID: <20090703111848.GA10267@jolsa.lab.eng.brq.redhat.com>
Date:	Fri, 3 Jul 2009 13:18:48 +0200
From:	Jiri Olsa <jolsa@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	fbl@...hat.com, nhorman@...hat.com, davem@...hat.com,
	htejun@...il.com, jarkao2@...il.com, oleg@...hat.com,
	davidel@...ilserver.org
Subject: Re: [PATCHv5 2/2] memory barrier: adding smp_mb__after_lock

On Fri, Jul 03, 2009 at 12:25:30PM +0200, Ingo Molnar wrote:
> 
> * Jiri Olsa <jolsa@...hat.com> wrote:
> 
> > On Fri, Jul 03, 2009 at 11:24:38AM +0200, Ingo Molnar wrote:
> > > 
> > > * Eric Dumazet <eric.dumazet@...il.com> wrote:
> > > 
> > > > Ingo Molnar a écrit :
> > > > > * Jiri Olsa <jolsa@...hat.com> wrote:
> > > > > 
> > > > >> +++ b/arch/x86/include/asm/spinlock.h
> > > > >> @@ -302,4 +302,7 @@ static inline void __raw_write_unlock(raw_rwlock_t *rw)
> > > > >>  #define _raw_read_relax(lock)	cpu_relax()
> > > > >>  #define _raw_write_relax(lock)	cpu_relax()
> > > > >>  
> > > > >> +/* The {read|write|spin}_lock() on x86 are full memory barriers. */
> > > > >> +#define smp_mb__after_lock() do { } while (0)
> > > > > 
> > > > > Two small stylistic comments, please make this an inline function:
> > > > > 
> > > > > static inline void smp_mb__after_lock(void) { }
> > > > > #define smp_mb__after_lock
> > > > > 
> > > > > (untested)
> > > > > 
> > > > >> +/* The lock does not imply full memory barrier. */
> > > > >> +#ifndef smp_mb__after_lock
> > > > >> +#define smp_mb__after_lock() smp_mb()
> > > > >> +#endif
> > > > > 
> > > > > ditto.
> > > > > 
> > > > > 	Ingo
> > > > 
> > > > This was following existing implementations of various smp_mb__??? helpers :
> > > > 
> > > > # grep -4 smp_mb__before_clear_bit include/asm-generic/bitops.h
> > > > 
> > > > /*
> > > >  * clear_bit may not imply a memory barrier
> > > >  */
> > > > #ifndef smp_mb__before_clear_bit
> > > > #define smp_mb__before_clear_bit()      smp_mb()
> > > > #define smp_mb__after_clear_bit()       smp_mb()
> > > > #endif
> > > 
> > > Did i mention that those should be fixed too? :-)
> > > 
> > > 	Ingo
> > 
> > ok, could I include it in the 2/2 or you prefer separate patch?
> 
> depends on whether it will regress ;-)
> 
> If it regresses, it's better to have it separate. If it wont, it can 
> be included. If unsure, default to the more conservative option.
> 
> 	Ingo


how about this.. 
and similar change for smp_mb__before_clear_bit in a separate patch


diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index b7e5db8..4e77853 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -302,4 +302,8 @@ static inline void __raw_write_unlock(raw_rwlock_t *rw)
 #define _raw_read_relax(lock)	cpu_relax()
 #define _raw_write_relax(lock)	cpu_relax()
 
+/* The {read|write|spin}_lock() on x86 are full memory barriers. */
+static inline void smp_mb__after_lock(void) { }
+#define ARCH_HAS_SMP_MB_AFTER_LOCK
+
 #endif /* _ASM_X86_SPINLOCK_H */
diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h
index 252b245..4be57ab 100644
--- a/include/linux/spinlock.h
+++ b/include/linux/spinlock.h
@@ -132,6 +132,11 @@ do {								\
 #endif /*__raw_spin_is_contended*/
 #endif
 
+/* The lock does not imply full memory barrier. */
+#ifndef ARCH_HAS_SMP_MB_AFTER_LOCK
+static inline void smp_mb__after_lock(void) { smp_mb(); }
+#endif
+
 /**
  * spin_unlock_wait - wait until the spinlock gets unlocked
  * @lock: the spinlock in question.
diff --git a/include/net/sock.h b/include/net/sock.h
index 4eb8409..98afcd9 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -1271,6 +1271,9 @@ static inline int sk_has_allocations(const struct sock *sk)
  * in its cache, and so does the tp->rcv_nxt update on CPU2 side.  The CPU1
  * could then endup calling schedule and sleep forever if there are no more
  * data on the socket.
+ *
+ * The sk_has_helper is always called right after a call to read_lock, so we
+ * can use smp_mb__after_lock barrier.
  */
 static inline int sk_has_sleeper(struct sock *sk)
 {
@@ -1280,7 +1283,7 @@ static inline int sk_has_sleeper(struct sock *sk)
 	 *
 	 * This memory barrier is paired in the sock_poll_wait.
 	 */
-	smp_mb();
+	smp_mb__after_lock();
 	return sk->sk_sleep && waitqueue_active(sk->sk_sleep);
 }
 
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