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SubjectRe: [PATCH] arm64: Fix task tracing
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 <cov@codeaurora.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.

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

--
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
hosted by the Linux Foundation.


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