linux-kernel - Re: [RFC PATCH v7 1/7] Restartable sequences system call

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Message-ID: <1431024781.7024.1470835564859.JavaMail.zimbra@efficios.com>
Date:	Wed, 10 Aug 2016 13:26:04 +0000 (UTC)
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Russell King <linux@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-api <linux-api@...r.kernel.org>,
	Paul Turner <pjt@...gle.com>, Andrew Hunter <ahh@...gle.com>,
	Andy Lutomirski <luto@...capital.net>,
	Andi Kleen <andi@...stfloor.org>,
	Dave Watson <davejwatson@...com>, Chris Lameter <cl@...ux.com>,
	Ben Maurer <bmaurer@...com>, rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Josh Triplett <josh@...htriplett.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Boqun Feng <boqun.feng@...il.com>
Subject: Re: [RFC PATCH v7 1/7] Restartable sequences system call

----- On Aug 10, 2016, at 3:50 AM, Peter Zijlstra peterz@...radead.org wrote:

> On Tue, Aug 09, 2016 at 10:41:47PM +0000, Mathieu Desnoyers wrote:
>> #ifdef __LP64__
>> 
>> static bool rseq_update_cpu_id_event_counter(struct task_struct *t)
>> {
>>         union rseq_cpu_event u;
>> 
>>         u.e.cpu_id = raw_smp_processor_id();
>>         u.e.event_counter = ++t->rseq_event_counter;
>>         if (__put_user(u.v, &t->rseq->u.v))
>>                 return false;
>>         trace_rseq_inc(t->rseq_event_counter);
>>         return true;
>> }
>> 
>> #else /* #ifdef __LP64__ */
>> 
>> static bool rseq_update_cpu_id_event_counter(struct task_struct *t)
>> {
>>         if (__put_user(raw_smp_processor_id(), &t->rseq->u.e.cpu_id))
>>                 return false;
>>         if (__put_user(++t->rseq_event_counter, &t->rseq->u.e.event_counter))
>>                 return false;
>>         trace_rseq_inc(t->rseq_event_counter);
>>         return true;
>> }
>> 
>> #endif /* #else #ifdef __LP64__ */
> 
> I don't think you need to guard it (and CONFIG_64BIT is the 'right'
> kernel symbol for that), 32bit should have u64 __put_user() only
> implemented as 2 u32 stores.

OK, I can then simplify the implementation to:

[...]
 * On 64-bit architectures, both cpu_id and event_counter can be updated
 * with a single 64-bit store. On 32-bit architectures, __put_user() is
 * expected to perform two 32-bit single-copy stores to guarantee
 * single-copy atomicity semantics for other threads.
 */
static bool rseq_update_cpu_id_event_counter(struct task_struct *t)
{
        union rseq_cpu_event u;

        u.e.cpu_id = raw_smp_processor_id();
        u.e.event_counter = ++t->rseq_event_counter;
        if (__put_user(u.v, &t->rseq->u.v))
                return false;
        trace_rseq_inc(t->rseq_event_counter);
        return true;
}

Thanks!

Mathieu

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