linux-kernel - Re: [PATCH 1/2] perf: Correctly align perf event tracing buffer

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Date:	Mon, 29 Mar 2010 10:51:31 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	David Miller <davem@...emloft.net>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 1/2] perf: Correctly align perf event tracing buffer

On Sun, 2010-03-28 at 07:11 +0200, Frederic Weisbecker wrote:
> The trace event buffer used by perf to record raw sample events
> is typed as an array of char and may then not be aligned to 8
> by alloc_percpu().
> 
> But we need it to be aligned to 8 in sparc64 because we cast
> this buffer into a random structure type built by the TRACE_EVENT()
> macro to store the traces. So if a random 64 bits field is accessed
> inside, it may be not under an expected good alignment.
> 
> Use an array of long instead to force the appropriate alignment, and
> perform a compile time check to ensure the size in byte of the buffer
> is a multiple of sizeof(long) so that its actual size doesn't get
> shrinked under us.
> 
> This fixes unaligned accesses reported while using perf lock
> in sparc 64.
> 
> Suggested-by: David Miller <davem@...emloft.net>
> Suggested-by: Tejun Heo <htejun@...il.com>
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
> Cc: Paul Mackerras <paulus@...ba.org>
> Cc: Ingo Molnar <mingo@...e.hu>
> Cc: David Miller <davem@...emloft.net>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> ---
>  kernel/trace/trace_event_perf.c |   11 +++++++++--
>  1 files changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/trace/trace_event_perf.c b/kernel/trace/trace_event_perf.c
> index 0709e4f..69941f3 100644
> --- a/kernel/trace/trace_event_perf.c
> +++ b/kernel/trace/trace_event_perf.c
> @@ -15,7 +15,12 @@ EXPORT_PER_CPU_SYMBOL_GPL(perf_trace_regs);
>  static char *perf_trace_buf;
>  static char *perf_trace_buf_nmi;
>  
> -typedef typeof(char [PERF_MAX_TRACE_SIZE]) perf_trace_t ;
> +/*
> + * Force it to be aligned to unsigned long to avoid misaligned accesses
> + * suprises
> + */
> +typedef typeof(unsigned long [PERF_MAX_TRACE_SIZE / sizeof(unsigned long)])
> +	perf_trace_t;
>  

wouldn't __aligned(8) be simpler?

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