Re: [RFC PATCH 2/2] Revert "csky: Add support for restartable sequence"

From: Guo Ren <guoren@kernel.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	paulmck <paulmck@kernel.org>, Boqun Feng <boqun.feng@gmail.com>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	"linux-csky@vger.kernel.org" <linux-csky@vger.kernel.org>
Subject: Re: [RFC PATCH 2/2] Revert "csky: Add support for restartable sequence"
Date: Sat, 10 Sep 2022 10:35:33 +0800	[thread overview]
Message-ID: <CAJF2gTRbn-JiUMiX_s52geJSiTs7qYV3oEkW1+NC8Vwp1YAKvg@mail.gmail.com> (raw)
In-Reply-To: <cbd08ae8-4558-60e3-17e4-988e625984a1@efficios.com>

On Sat, Sep 10, 2022 at 4:01 AM Mathieu Desnoyers
<mathieu.desnoyers@efficios.com> wrote:
>
> On 2021-07-26 12:28, Mathieu Desnoyers wrote:
> > ----- On Jul 26, 2021, at 12:10 PM, Guo Ren guoren@kernel.org wrote:
> >
> >> Hi Mathieu,
> >>
> >> Sorry for forgetting to CC you in the last patch, and that patch has
> >> been merged into master which has the problem of syscall restart.
> >>
> >> I still want to keep rseq feature for csky, and implement the
> >> RSEQ_SKIP_FASTPATH for self-test, it that okay?
> >
> > No, the RSEQ_SKIP_FASTPATH is the one special-case of test build which
> > skips building rseq critical sections entirely. This leaves out any
> > relevant testing of rseq per-se. With what we have in the upstream
> > selftests, I expect this test configuration to abort at runtime because
> > no slow-path fallbacks are available when the fastpath is disabled.
> >
> > The asm glue to test rseq user-space really needs to be implemented
> > for any useful testing to be done here.
> >
> > Unless that asm glue is contributed, none of the rseq logic is actually
> > tested on that architecture.
> >
> > Considering the extremely-hard-to-debug nature of races with a broken
> > rseq kernel implementation, proper testing coverage is paramount, so I
> > still recommend the revert unless the selftests user-space asm glue is
> > contributed for C-Sky.
>
> Hi Guo,
>
> A friendly ping after 1.5 year about the fact that the selftests code is
> still missing for the csky architecture. So there is no way to validate
> that the kernel pieces that were merged actually work on that architecture.
>
> What is the timeline for contribution of the missing bits needed to enable
> those tests ?
I would send the 1st version of patchset before 2022/10/30, or just
revert it from csky.

1. a patch for fixup the bug:

diff --git a/arch/csky/kernel/signal.c b/arch/csky/kernel/signal.c
index b7b3685283d7..434dcf2e8e37 100644
--- a/arch/csky/kernel/signal.c
+++ b/arch/csky/kernel/signal.c
@@ -179,8 +179,6 @@ static void handle_signal(struct ksignal *ksig,
struct pt_regs *regs)
        sigset_t *oldset = sigmask_to_save();
        int ret;

-       rseq_signal_deliver(ksig, regs);
-
        /* Are we from a system call? */
        if (in_syscall(regs)) {
                /* Avoid additional syscall restarting via ret_from_exception */
@@ -206,6 +204,8 @@ static void handle_signal(struct ksignal *ksig,
struct pt_regs *regs)
                }
        }

+       rseq_signal_deliver(ksig, regs);
+
        /* Set up the stack frame */
        ret = setup_rt_frame(ksig, oldset, regs);

2. Add tools/testing/selftests/rseq/rseq-csky* codes.



>
> Here are the required header files:
>
> tools/testing/selftests/rseq/rseq-csky.h
> tools/testing/selftests/rseq/rseq-csky-thread-pointer.h (only if __builtin_thread_pointer is not implemented by gcc)
>
> I'm open to suggestions from other rseq co-maintainers on how to best address
> this situation.
>
> Thanks,
>
> Mathieu
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com



-- 
Best Regards
 Guo Ren
