Re: [PATCH v10 11/12] arm64: factor work_pending state machine to C

[Will Deacon <will.deacon@arm.com>]

Hi Chris,

On Wed, Mar 02, 2016 at 03:09:35PM -0500, Chris Metcalf wrote:
> Currently ret_fast_syscall, work_pending, and ret_to_user form an ad-hoc
> state machine that can be difficult to reason about due to duplicated
> code and a large number of branch targets.
> 
> This patch factors the common logic out into the existing
> do_notify_resume function, converting the code to C in the process,
> making the code more legible.
> 
> This patch tries to closely mirror the existing behaviour while using
> the usual C control flow primitives. As local_irq_{disable,enable} may
> be instrumented, we balance exception entry (where we will almost most
> likely enable IRQs) with a call to trace_hardirqs_on just before the
> return to userspace.

[...]

> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index 1f7f5a2b61bf..966d0d4308f2 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -674,18 +674,13 @@ ret_fast_syscall_trace:
>   * Ok, we need to do extra processing, enter the slow path.
>   */
>  work_pending:
> -	tbnz	x1, #TIF_NEED_RESCHED, work_resched
> -	/* TIF_SIGPENDING, TIF_NOTIFY_RESUME or TIF_FOREIGN_FPSTATE case */
>  	mov	x0, sp				// 'regs'
> -	enable_irq				// enable interrupts for do_notify_resume()
>  	bl	do_notify_resume
> -	b	ret_to_user
> -work_resched:
>  #ifdef CONFIG_TRACE_IRQFLAGS
> -	bl	trace_hardirqs_off		// the IRQs are off here, inform the tracing code
> +	bl	trace_hardirqs_on		// enabled while in userspace

This doesn't look right to me. We only get here after running
do_notify_resume, which returns with interrupts disabled.

Do we not instead need to inform the tracing code that interrupts are
disabled prior to calling do_notify_resume?

> diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
> index e18c48cb6db1..3432e14b7d6e 100644
> --- a/arch/arm64/kernel/signal.c
> +++ b/arch/arm64/kernel/signal.c
> @@ -402,15 +402,29 @@ static void do_signal(struct pt_regs *regs)
>  asmlinkage void do_notify_resume(struct pt_regs *regs,
>  				 unsigned int thread_flags)
>  {
> -	if (thread_flags & _TIF_SIGPENDING)
> -		do_signal(regs);
> +	while (true) {
>  
> -	if (thread_flags & _TIF_NOTIFY_RESUME) {
> -		clear_thread_flag(TIF_NOTIFY_RESUME);
> -		tracehook_notify_resume(regs);
> -	}
> +		if (thread_flags & _TIF_NEED_RESCHED) {
> +			schedule();
> +		} else {
> +			local_irq_enable();
> +
> +			if (thread_flags & _TIF_SIGPENDING)
> +				do_signal(regs);
>  
> -	if (thread_flags & _TIF_FOREIGN_FPSTATE)
> -		fpsimd_restore_current_state();
> +			if (thread_flags & _TIF_NOTIFY_RESUME) {
> +				clear_thread_flag(TIF_NOTIFY_RESUME);
> +				tracehook_notify_resume(regs);
> +			}
> +
> +			if (thread_flags & _TIF_FOREIGN_FPSTATE)
> +				fpsimd_restore_current_state();
> +		}
>  
> +		local_irq_disable();
> +
> +		thread_flags = READ_ONCE(current_thread_info()->flags);
> +		if (!(thread_flags & _TIF_WORK_MASK))
> +			break;
> +	}

This might be easier to read as a do { ... } while.

Will