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

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Message-ID: <20130403180443.GB21167@mudshark.cambridge.arm.com>
Date:	Wed, 3 Apr 2013 19:04:43 +0100
From:	Will Deacon <will.deacon@....com>
To:	Christopher Covington <cov@...eaurora.org>
Cc:	Catalin Marinas <Catalin.Marinas@....com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arm64: Fix task tracing

Hi Christopher,

On Wed, Apr 03, 2013 at 07:01:01PM +0100, Christopher Covington wrote:
> For accurate accounting call contextidr_thread_switch before a
> task is scheduled, rather than after.
> 
> 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..c2cc249 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -311,11 +311,11 @@ struct task_struct *__switch_to(struct task_struct *prev,
>  	fpsimd_thread_switch(next);
>  	tls_thread_switch(next);
>  	hw_breakpoint_thread_switch(next);
> +	contextidr_thread_switch(next);
>  
>  	/* the actual thread switch */
>  	last = cpu_switch_to(prev, next);
>  
> -	contextidr_thread_switch(next);
>  	return last;
>  }

Catalin and I wondered about this and decided to go with the current
approach in case a debugger, in response to the contextidr write, decided to
go off and mine information about the *new* task using the sp.

If we update the register before we've switched the registers, there's a
sizeable window where the debugger will get confused.

Do you have a real use-case that has triggered this patch?

Cheers,

Will
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