netdev - Re: [PATCH bpf-next 2/3] tools, perf: use smp_{rmb,mb} barriers instead of {rmb,mb}

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Message-ID: <20181017155050.GM3121@hirez.programming.kicks-ass.net>
Date:   Wed, 17 Oct 2018 17:50:50 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     alexei.starovoitov@...il.com, paulmck@...ux.vnet.ibm.com,
        will.deacon@....com, acme@...hat.com, yhs@...com,
        john.fastabend@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH bpf-next 2/3] tools, perf: use smp_{rmb,mb} barriers
 instead of {rmb,mb}

On Wed, Oct 17, 2018 at 04:41:55PM +0200, Daniel Borkmann wrote:
> @@ -73,7 +73,8 @@ static inline u64 perf_mmap__read_head(struct perf_mmap *mm)
>  {
>  	struct perf_event_mmap_page *pc = mm->base;
>  	u64 head = READ_ONCE(pc->data_head);
> -	rmb();
> +
> +	smp_rmb();
>  	return head;
>  }
>  
> @@ -84,7 +85,7 @@ static inline void perf_mmap__write_tail(struct perf_mmap *md, u64 tail)
>  	/*
>  	 * ensure all reads are done before we write the tail out.
>  	 */
> -	mb();
> +	smp_mb();
>  	pc->data_tail = tail;

Ideally that would be a WRITE_ONCE() to avoid store tearing.

Alternatively, I think we can use smp_store_release() here, all we care
about is that the prior loads stay prior.

Similarly, I suppose, we could use smp_load_acquire() for the data_head
load above.

>  }
>  
> -- 
> 2.9.5
>