Re: [PATCH v2 4/4] arm64: Remove unneeded rcu_read_lock from debug handlers

Paul E. McKenney
2019-07-22T12:08:53Z

On Mon, Jul 22, 2019 at 04:49:11PM +0900, Masami Hiramatsu wrote:
> Remove rcu_read_lock()/rcu_read_unlock() from debug exception
> handlers since we are sure those are not preemptible and
> interrupts are off.
>
> Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> Cc: Paul E. McKenney <paulmck@linux.ibm.com>

From an RCU viewpoint:

Acked-by: Paul E. McKenney <paulmck@linux.ibm.com>

> ---
> arch/arm64/kernel/debug-monitors.c | 14 ++++++++------
> 1 file changed, 8 insertions(+), 6 deletions(-)
>
> diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
> index f8719bd30850..48222a4760c2 100644
> --- a/arch/arm64/kernel/debug-monitors.c
> +++ b/arch/arm64/kernel/debug-monitors.c
> @@ -207,16 +207,16 @@ static int call_step_hook(struct pt_regs *regs, unsigned int esr)
>
> list = user_mode(regs) ? &user_step_hook : &kernel_step_hook;
>
> - rcu_read_lock();
> -
> + /*
> + * Since single-step exception disables interrupt, this function is
> + * entirely not preemptible, and we can use rcu list safely here.
> + */
> list_for_each_entry_rcu(hook, list, node) {
> retval = hook->fn(regs, esr);
> if (retval == DBG_HOOK_HANDLED)
> break;
> }
>
> - rcu_read_unlock();
> -
> return retval;
> }
> NOKPROBE_SYMBOL(call_step_hook);
> @@ -305,14 +305,16 @@ static int call_break_hook(struct pt_regs *regs, unsigned int esr)
>
> list = user_mode(regs) ? &user_break_hook : &kernel_break_hook;
>
> - rcu_read_lock();
> + /*
> + * Since brk exception disables interrupt, this function is
> + * entirely not preemptible, and we can use rcu list safely here.
> + */
> list_for_each_entry_rcu(hook, list, node) {
> unsigned int comment = esr & ESR_ELx_BRK64_ISS_COMMENT_MASK;
>
> if ((comment & ~hook->mask) == hook->imm)
> fn = hook->fn;
> }
> - rcu_read_unlock();
>
> return fn ? fn(regs, esr) : DBG_HOOK_ERROR;
> }