linux-kernel - Re: [PATCH v2] arm64: Fix task tracing

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Message-ID: <20130415101159.GA25095@localhost.cambridge.arm.com>
Date:	Mon, 15 Apr 2013 11:11:59 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Christopher Covington <cov@...eaurora.org>
Cc:	Will Deacon <Will.Deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>
Subject: Re: [PATCH v2] arm64: Fix task tracing

On Tue, Apr 09, 2013 at 01:33:34PM +0100, Christopher Covington wrote:
> For accurate accounting pass contextidr_thread_switch the prev
> task pointer, since cpu_switch_to has at that point changed the
> the stack pointer.
> 
> Signed-off-by: Christopher Covington <cov@...eaurora.org>
> ---
>  arch/arm64/kernel/process.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index 0337cdb..a49b25a 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -315,7 +315,7 @@ struct task_struct *__switch_to(struct task_struct *prev,
>  	/* the actual thread switch */
>  	last = cpu_switch_to(prev, next);
>  
> -	contextidr_thread_switch(next);
> +	contextidr_thread_switch(prev);

The original code was indeed wrong but using prev isn't any better. For
a newly created thread, prev is probably 0 (if it's in a register,
cpu_context has been zeroed by copy_thread()) or some random stack
value.

So we either use current or move the call before cpu_switch_to() (I
would go for the former).

-- 
Catalin
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