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SubjectRe: [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@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.

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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