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Date: Wed, 10 Aug 2016 09:50:44 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Russell King <linux@arm.linux.org.uk>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
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        Boqun Feng <boqun.feng@gmail.com>
Subject: Re: [RFC PATCH v7 1/7] Restartable sequences system call
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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.