From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jiri Olsa Subject: Re: [PATCHv5 2/2] memory barrier: adding smp_mb__after_lock Date: Tue, 7 Jul 2009 12:18:16 +0200 Message-ID: <20090707101816.GA6619@jolsa.lab.eng.brq.redhat.com> References: <20090703081219.GE2902@jolsa.lab.eng.brq.redhat.com> <20090703081445.GG2902@jolsa.lab.eng.brq.redhat.com> <20090703090606.GA3902@elte.hu> <4A4DCD54.1080908@gmail.com> <20090703092438.GE3902@elte.hu> <20090703095659.GA4518@jolsa.lab.eng.brq.redhat.com> <20090703102530.GD32128@elte.hu> <20090703111848.GA10267@jolsa.lab.eng.brq.redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: Eric Dumazet , Peter Zijlstra , Mathieu Desnoyers , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, fbl@redhat.com, nhorman@redhat.com, davem@redhat.com, htejun@gmail.com, jarkao2@gmail.com, oleg@redhat.com, davidel@xmailserver.org To: Ingo Molnar Return-path: Received: from mx2.redhat.com ([66.187.237.31]:55410 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751358AbZGGKSi (ORCPT ); Tue, 7 Jul 2009 06:18:38 -0400 Content-Disposition: inline In-Reply-To: <20090703111848.GA10267@jolsa.lab.eng.brq.redhat.com> Sender: netdev-owner@vger.kernel.org List-ID: On Fri, Jul 03, 2009 at 01:18:48PM +0200, Jiri Olsa wrote: > On Fri, Jul 03, 2009 at 12:25:30PM +0200, Ingo Molnar wrote: > >=20 > > * Jiri Olsa wrote: > >=20 > > > On Fri, Jul 03, 2009 at 11:24:38AM +0200, Ingo Molnar wrote: > > > >=20 > > > > * Eric Dumazet wrote: > > > >=20 > > > > > Ingo Molnar a =E9crit : > > > > > > * Jiri Olsa wrote: > > > > > >=20 > > > > > >> +++ 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() > > > > > >> =20 > > > > > >> +/* The {read|write|spin}_lock() on x86 are full memory ba= rriers. */ > > > > > >> +#define smp_mb__after_lock() do { } while (0) > > > > > >=20 > > > > > > Two small stylistic comments, please make this an inline fu= nction: > > > > > >=20 > > > > > > static inline void smp_mb__after_lock(void) { } > > > > > > #define smp_mb__after_lock > > > > > >=20 > > > > > > (untested) > > > > > >=20 > > > > > >> +/* The lock does not imply full memory barrier. */ > > > > > >> +#ifndef smp_mb__after_lock > > > > > >> +#define smp_mb__after_lock() smp_mb() > > > > > >> +#endif > > > > > >=20 > > > > > > ditto. > > > > > >=20 > > > > > > Ingo > > > > >=20 > > > > > This was following existing implementations of various smp_mb= __??? helpers : > > > > >=20 > > > > > # grep -4 smp_mb__before_clear_bit include/asm-generic/bitops= =2Eh > > > > >=20 > > > > > /* > > > > > * 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 > > > >=20 > > > > Did i mention that those should be fixed too? :-) > > > >=20 > > > > Ingo > > >=20 > > > ok, could I include it in the 2/2 or you prefer separate patch? > >=20 > > depends on whether it will regress ;-) > >=20 > > If it regresses, it's better to have it separate. If it wont, it ca= n=20 > > be included. If unsure, default to the more conservative option. > >=20 > > Ingo >=20 >=20 > how about this..=20 > and similar change for smp_mb__before_clear_bit in a separate patch >=20 >=20 > diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/s= pinlock.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() > =20 > +/* 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 > =20 > +/* 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 stru= ct sock *sk) > * in its cache, and so does the tp->rcv_nxt update on CPU2 side. T= he 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_loc= k, 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 *s= k) > * > * 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); > } > =20 any feedback on this?=20 I'd send v6 if this way is acceptable.. thanks, jirka