Re: [patch part-II V2 09/13] x86/entry/common: Split hardirq tracing into lockdep and ftrace parts

[Alexandre Chartre]

On 3/8/20 11:24 PM, Thomas Gleixner wrote:
> trace_hardirqs_off() is in fact a tracepoint which can be utilized by BPF,
> which is unsafe before calling enter_from_user_mode(), which in turn
> invokes context tracking. trace_hardirqs_off() also invokes
> lockdep_hardirqs_off() under the hood.
>
> OTOH lockdep needs to know about the interrupts disabled state before
> enter_from_user_mode(). lockdep_hardirqs_off() is safe to call at this
> point.
>
> Split it so lockdep knows about the state and invoke the tracepoint after
> the context is set straight.
>
> Even if the functions attached to a tracepoint would all be safe to be
> called in rcuidle having it split up is still giving a performance
> advantage because rcu_read_lock_sched() is avoiding the whole dance of:
>
>     scru_read_lock();
>     rcu_irq_enter_irqson();
>     ...
>     rcu_irq_exit_irqson();
>     scru_read_unlock();
>     
> around the tracepoint function invocation just to have the C entry points
> of syscalls call enter_from_user_mode() right after the above dance.
>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> ---
> V2: New patch
> ---
>   arch/x86/entry/common.c |   13 +++++++++++--
>   1 file changed, 11 insertions(+), 2 deletions(-)


Reviewed-by: Alexandre Chartre <alexandre.chartre@xxxxxxxxxx>

alex.


> --- a/arch/x86/entry/common.c
> +++ b/arch/x86/entry/common.c
> @@ -60,10 +60,19 @@ static __always_inline void syscall_entr
>   {
>   	/*
>   	 * Usermode is traced as interrupts enabled, but the syscall entry
> -	 * mechanisms disable interrupts. Tell the tracer.
> +	 * mechanisms disable interrupts. Tell lockdep before calling
> +	 * enter_from_user_mode(). This is safe vs. RCU while the
> +	 * tracepoint is not.
>   	 */
> -	trace_hardirqs_off();
> +	lockdep_hardirqs_on(CALLER_ADDR0);
> +
>   	enter_from_user_mode();
> +
> +	/*
> +	 * Tell the tracer about the irq state as well before enabling
> +	 * interrupts.
> +	 */
> +	__trace_hardirqs_off();
>   	local_irq_enable();
>   }
>