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From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	maged michael <maged.michael@gmail.com>,
	Dave Watson <davejwatson@fb.com>,
	Will Deacon <will.deacon@arm.com>,
	"Russell King, ARM Linux" <linux@armlinux.org.uk>,
	David Sehr <sehr@google.com>, Paul Mackerras <paulus@samba.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	linux-arch <linux-arch@vger.kernel.org>, x86 <x86@kernel.org>,
	Andrew Hunter <ahh@google.com>,
	Greg Hackmann <ghackmann@google.com>,
	Alan Stern <stern@rowland.harvard.edu>,
	Paul <paulmck@linux.vnet.ibm.com>,
	Andrea Parri <parri.andrea@gmail.com>,
	Avi Kivity <avi@scylladb.com>, Boqun Feng <boqun.feng@gmail.com>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
	Nicholas Piggin <npiggin@gmail.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Andy Lutomirski <luto@kernel.org>,
	linux-api <linux-api@vger.kernel.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH for 4.16 v7 02/11] powerpc: membarrier: Skip memory barrier in switch_mm()
Date: Fri, 18 Jun 2021 13:26:32 -0400 (EDT)	[thread overview]
Message-ID: <2077369633.12794.1624037192994.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <8b200dd5-f37b-b208-82fb-2775df7bcd49@csgroup.eu>

----- On Jun 18, 2021, at 1:13 PM, Christophe Leroy christophe.leroy@csgroup.eu wrote:
[...]
> 
> I don't understand all that complexity to just replace a simple
> 'smp_mb__after_unlock_lock()'.
> 
> #define smp_mb__after_unlock_lock()	smp_mb()
> #define smp_mb()	barrier()
> # define barrier() __asm__ __volatile__("": : :"memory")
> 
> 
> Am I missing some subtility ?

On powerpc CONFIG_SMP, smp_mb() is actually defined as:

#define smp_mb()        __smp_mb()
#define __smp_mb()      mb()
#define mb()   __asm__ __volatile__ ("sync" : : : "memory")

So the original motivation here was to skip a "sync" instruction whenever
switching between threads which are part of the same process. But based on
recent discussions, I suspect my implementation may be inaccurately doing
so though.

Thanks,

Mathieu


> 
> Thanks
> Christophe

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

WARNING: multiple messages have this Message-ID (diff)
From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: maged michael <maged.michael@gmail.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Dave Watson <davejwatson@fb.com>,
	Will Deacon <will.deacon@arm.com>, Andrew Hunter <ahh@google.com>,
	David Sehr <sehr@google.com>, Paul Mackerras <paulus@samba.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	linux-arch <linux-arch@vger.kernel.org>, x86 <x86@kernel.org>,
	"Russell King, ARM Linux" <linux@armlinux.org.uk>,
	Greg Hackmann <ghackmann@google.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Ingo Molnar <mingo@redhat.com>,
	Alan Stern <stern@rowland.harvard.edu>,
	Paul <paulmck@linux.vnet.ibm.com>,
	Andrea Parri <parri.andrea@gmail.com>,
	Avi Kivity <avi@scylladb.com>, Boqun Feng <boqun.feng@gmail.com>,
	Nicholas Piggin <npiggin@gmail.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Andy Lutomirski <luto@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-api <linux-api@vger.kernel.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
Subject: Re: [PATCH for 4.16 v7 02/11] powerpc: membarrier: Skip memory barrier in switch_mm()
Date: Fri, 18 Jun 2021 13:26:32 -0400 (EDT)	[thread overview]
Message-ID: <2077369633.12794.1624037192994.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <8b200dd5-f37b-b208-82fb-2775df7bcd49@csgroup.eu>

----- On Jun 18, 2021, at 1:13 PM, Christophe Leroy christophe.leroy@csgroup.eu wrote:
[...]
> 
> I don't understand all that complexity to just replace a simple
> 'smp_mb__after_unlock_lock()'.
> 
> #define smp_mb__after_unlock_lock()	smp_mb()
> #define smp_mb()	barrier()
> # define barrier() __asm__ __volatile__("": : :"memory")
> 
> 
> Am I missing some subtility ?

On powerpc CONFIG_SMP, smp_mb() is actually defined as:

#define smp_mb()        __smp_mb()
#define __smp_mb()      mb()
#define mb()   __asm__ __volatile__ ("sync" : : : "memory")

So the original motivation here was to skip a "sync" instruction whenever
switching between threads which are part of the same process. But based on
recent discussions, I suspect my implementation may be inaccurately doing
so though.

Thanks,

Mathieu


> 
> Thanks
> Christophe

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

  reply	other threads:[~2021-06-18 17:26 UTC|newest]

Thread overview: 59+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-01-29 20:20 [PATCH for 4.16 00/11] membarrier updates for 4.16 Mathieu Desnoyers
2018-01-29 20:20 ` Mathieu Desnoyers
2018-01-29 20:20 ` [PATCH for 4.16 v2 01/11] membarrier: selftest: Test private expedited cmd Mathieu Desnoyers
2018-01-29 20:20   ` [Linux-kselftest-mirror] " Mathieu Desnoyers
2018-01-29 20:20   ` mathieu.desnoyers
2018-01-29 20:20   ` Mathieu Desnoyers
2018-02-05 21:25   ` [tip:sched/urgent] membarrier/selftest: Test private expedited command tip-bot for Mathieu Desnoyers
2018-01-29 20:20 ` [PATCH for 4.16 v7 02/11] powerpc: membarrier: Skip memory barrier in switch_mm() Mathieu Desnoyers
2018-01-29 20:20   ` Mathieu Desnoyers
2018-02-05 20:22   ` Ingo Molnar
2018-02-05 20:22     ` Ingo Molnar
     [not found]     ` <20180205202242.6ilyjh35byycf3ez-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2018-02-05 20:32       ` Mathieu Desnoyers
2018-02-05 20:32         ` Mathieu Desnoyers
2018-02-05 21:26   ` [tip:sched/urgent] powerpc, " tip-bot for Mathieu Desnoyers
2021-06-18 17:13   ` [PATCH for 4.16 v7 02/11] powerpc: " Christophe Leroy
2021-06-18 17:13     ` Christophe Leroy
2021-06-18 17:26     ` Mathieu Desnoyers [this message]
2021-06-18 17:26       ` Mathieu Desnoyers
2021-06-19  9:35       ` Christophe Leroy
2021-06-19  9:35         ` Christophe Leroy
2021-06-19 15:02         ` Segher Boessenkool
2021-06-19 15:02           ` Segher Boessenkool
2021-06-21 14:11           ` Christophe Leroy
2021-06-21 14:11             ` Christophe Leroy
2021-06-22  0:15             ` Segher Boessenkool
2021-06-22  0:15               ` Segher Boessenkool
2018-01-29 20:20 ` [PATCH for 4.16 v5 03/11] membarrier: Document scheduler barrier requirements Mathieu Desnoyers
2018-01-29 20:20   ` Mathieu Desnoyers
2018-02-05 21:26   ` [tip:sched/urgent] " tip-bot for Mathieu Desnoyers
2018-01-29 20:20 ` [PATCH for 4.16 v2 05/11] membarrier: selftest: Test global expedited cmd Mathieu Desnoyers
2018-01-29 20:20   ` [Linux-kselftest-mirror] " Mathieu Desnoyers
2018-01-29 20:20   ` mathieu.desnoyers
2018-01-29 20:20   ` Mathieu Desnoyers
2018-02-05 21:27   ` [tip:sched/urgent] membarrier/selftest: Test global expedited command tip-bot for Mathieu Desnoyers
2018-01-29 20:20 ` [PATCH for 4.16 v2 06/11] Introduce sync_core_before_usermode Mathieu Desnoyers
2018-01-29 20:20   ` Mathieu Desnoyers
2018-02-05 21:28   ` [tip:sched/urgent] locking: Introduce sync_core_before_usermode() tip-bot for Mathieu Desnoyers
2018-01-29 20:20 ` [PATCH for 4.16 v3 07/11] x86: Implement sync_core_before_usermode Mathieu Desnoyers
2018-01-29 20:20   ` Mathieu Desnoyers
2018-02-05 21:28   ` [tip:sched/urgent] lockin/x86: Implement sync_core_before_usermode() tip-bot for Mathieu Desnoyers
2018-01-29 20:20 ` [PATCH for 4.16 v4 09/11] membarrier: x86: Provide core serializing command Mathieu Desnoyers
2018-01-29 20:20   ` Mathieu Desnoyers
2018-02-05 21:29   ` [tip:sched/urgent] membarrier/x86: " tip-bot for Mathieu Desnoyers
2018-01-29 20:20 ` [PATCH for 4.16 10/11] membarrier: arm64: " Mathieu Desnoyers
2018-01-29 20:20   ` Mathieu Desnoyers
2018-02-05 21:30   ` [tip:sched/urgent] membarrier/arm64: " tip-bot for Mathieu Desnoyers
     [not found] ` <20180129202020.8515-1-mathieu.desnoyers-vg+e7yoeK/dWk0Htik3J/w@public.gmane.org>
2018-01-29 20:20   ` [PATCH for 4.16 v3 04/11] membarrier: provide GLOBAL_EXPEDITED command Mathieu Desnoyers
2018-01-29 20:20     ` Mathieu Desnoyers
2018-02-05 21:27     ` [tip:sched/urgent] membarrier: Provide " tip-bot for Mathieu Desnoyers
2018-01-29 20:20   ` [PATCH for 4.16 v3 08/11] membarrier: Provide core serializing command Mathieu Desnoyers
2018-01-29 20:20     ` Mathieu Desnoyers
2018-02-05 21:29     ` [tip:sched/urgent] membarrier: Provide core serializing command, *_SYNC_CORE tip-bot for Mathieu Desnoyers
2018-01-29 20:20   ` [PATCH for 4.16 11/11] membarrier: selftest: Test private expedited sync core cmd Mathieu Desnoyers
2018-01-29 20:20     ` [Linux-kselftest-mirror] " Mathieu Desnoyers
2018-01-29 20:20     ` mathieu.desnoyers
2018-01-29 20:20     ` Mathieu Desnoyers
2018-02-05 21:30     ` [tip:sched/urgent] membarrier/selftest: Test private expedited sync core command tip-bot for Mathieu Desnoyers
2018-02-05 21:21   ` [PATCH for 4.16 00/11] membarrier updates for 4.16 Ingo Molnar
2018-02-05 21:21     ` Ingo Molnar

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