[PATCH 2/2] arm64: factor work_pending state machine to C

Mark Rutland
2016-01-06T13:43:14Z

On Tue, Jan 05, 2016 at 05:33:35PM +0000, Mark Rutland 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 mirror the existing behaviour as closely as possible
> while using the usual C control flow primitives. There should be no
> functional change as a result of this patch.

I realised there is a problem with this for kernel built with
TRACE_IRQFLAGS, as local_irq_{enable,disable}() will verify that the IRQ
state is as expected.

In ret_fast_syscall we disable irqs behind the back of the tracer, so
when we get into do_notify_resume we'll get a splat.

In the non-syscall cases we do not disable interrupts first, so we can't
balance things in do_notify_resume.

We can either add a trace_hardirqs_off call to ret_fast_syscall, or we
can use raw_local_irq_{disable,enable}. The latter would match the
current behaviour (and is a nicer diff). Once the syscall path is moved
to C it would be possible to use the non-raw variants all-over.

Catalin, are you happy with using the raw accessors in do_notify_resume,
or would you prefer using trace_hardirqs_off?

Thanks,
Mark.

> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Chris Metcalf <cmetcalf@ezchip.com>
> Cc: Will Deacon <will.deacon@arm.com>
> ---
> arch/arm64/kernel/entry.S | 24 +++---------------------
> arch/arm64/kernel/signal.c | 36 ++++++++++++++++++++++++++----------
> 2 files changed, 29 insertions(+), 31 deletions(-)
>
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index 6b30ab1..41f5dfc 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -612,35 +612,17 @@ ret_fast_syscall:
> ldr x1, [tsk, #TI_FLAGS] // re-check for syscall tracing
> and x2, x1, #_TIF_SYSCALL_WORK
> cbnz x2, ret_fast_syscall_trace
> - and x2, x1, #_TIF_WORK_MASK
> - cbnz x2, work_pending
> - enable_step_tsk x1, x2
> - kernel_exit 0
> + b ret_to_user
> ret_fast_syscall_trace:
> enable_irq // enable interrupts
> b __sys_trace_return_skipped // we already saved x0
>
> /*
> - * 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:
> - bl schedule
> -
> -/*
> * "slow" syscall return path.
> */
> ret_to_user:
> - disable_irq // disable interrupts
> - ldr x1, [tsk, #TI_FLAGS]
> - and x2, x1, #_TIF_WORK_MASK
> - cbnz x2, work_pending
> + bl do_notify_resume
> + ldr x1, [tsk, #TI_FLAGS] // re-check for single-step
> enable_step_tsk x1, x2
> kernel_exit 0
> ENDPROC(ret_to_user)
> diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
> index e18c48c..3a6c60b 100644
> --- a/arch/arm64/kernel/signal.c
> +++ b/arch/arm64/kernel/signal.c
> @@ -399,18 +399,34 @@ static void do_signal(struct pt_regs *regs)
> restore_saved_sigmask();
> }
>
> -asmlinkage void do_notify_resume(struct pt_regs *regs,
> - unsigned int thread_flags)
> +asmlinkage void do_notify_resume(void)
> {
> - if (thread_flags & _TIF_SIGPENDING)
> - do_signal(regs);
> + struct pt_regs *regs = task_pt_regs(current);
> + unsigned long thread_flags;
>
> - if (thread_flags & _TIF_NOTIFY_RESUME) {
> - clear_thread_flag(TIF_NOTIFY_RESUME);
> - tracehook_notify_resume(regs);
> - }
> + for (;;) {
> + local_irq_disable();

This should be raw_local_irq_disable()...

> +
> + thread_flags = READ_ONCE(current_thread_info()->flags);
> + if (!(thread_flags & _TIF_WORK_MASK))
> + break;
> +
> + if (thread_flags & _TIF_NEED_RESCHED) {
> + schedule();
> + continue;
> + }
>
> - if (thread_flags & _TIF_FOREIGN_FPSTATE)
> - fpsimd_restore_current_state();
> + local_irq_enable();

... likewise, raw_local_irq_enable() here.

>
> + if (thread_flags & _TIF_SIGPENDING)
> + do_signal(regs);
> +
> + 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();
> + }
> }
> --
> 1.9.1
>