On Thu, Jul 18, 2019 at 02:43:58PM +0900, Masami Hiramatsu wrote:
> Remove rcu_read_lock()/rcu_read_unlock() from debug exception
> handlers since the software breakpoint can be hit on idle task.
The exception entry and exit use irq_enter() and irq_exit(), in this
case, correct? Otherwise RCU will be ignoring this CPU.
Thanx, Paul
> Actually, we don't need it because those handlers run in exception
> context where the interrupts are disabled. This means those are never
> preempted.
>
> Reported-by: Naresh Kamboju <naresh.kamb...@linaro.org>
> Cc: Paul E. McKenney <paul...@linux.ibm.com>
> Signed-off-by: Masami Hiramatsu <mhira...@kernel.org>
> ---
> 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;
> }
>
