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

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Message-ID: <2070284546.6496.1627316900287.JavaMail.zimbra@efficios.com>
Date:   Mon, 26 Jul 2021 12:28:20 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Guo Ren <guoren@...nel.org>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-csky@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] Revert "csky: Add support for restartable
 sequence"

----- On Jul 26, 2021, at 12:10 PM, Guo Ren guoren@...nel.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.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com