Subject: [PATCH] arm64: Fix task tracing

From: [email-masked] (Christopher Covington)
Date: Mon, 08 Apr 2013 10:42:24 -0400

Hi Will,

On 04/03/2013 02:04 PM, Will Deacon wrote:
> 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 <[email-masked]>
>> ---
>>  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.

The problem with the existing implementation is that it doesn't seem to
compensate for how cpu_switch_to changes the stack pointer. Consider the
following sequence.

cpu_switch_to(prev=A, next=B)
cpu_switch_to(prev=B, next=C)
cpu_switch_to(prev=C, next=A)

After the third call, using A's stack, next will be B, and its thread ID will
be written to CONTEXTIDR. An easy way to see this in a simulator is to just
instrument the code with some printk's.

Thanks,
Christopher

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