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Message-ID: <87v9088a5q.ffs@tglx>
Date: Wed, 01 Dec 2021 19:14:41 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Mark Rutland <mark.rutland@....com>
Cc: Steven Rostedt <rostedt@...dmis.org>,
Nicolas Saenz Julienne <nsaenzju@...hat.com>,
linux-kernel <linux-kernel@...r.kernel.org>,
linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
rcu@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
paulmck@...nel.org, mtosatti <mtosatti@...hat.com>,
frederic <frederic@...nel.org>, Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH] Documentation: Fill the gaps about entry/noinstr
constraints
Mark,
On Wed, Dec 01 2021 at 10:56, Mark Rutland wrote:
> On Tue, Nov 30, 2021 at 11:31:30PM +0100, Thomas Gleixner wrote:
>> ---
>> Documentation/core-api/entry.rst | 268 +++++++++++++++++++++++++++++++++++++++
>> Documentation/core-api/index.rst | 8 +
>> kernel/entry/common.c | 1
>
> I think the change to kernel/entry/common.c got included by accident?
That's what I get from doing such things 30 minutes before midnight...
>> +
>> +Syscall entry exit code starts obviously in low level architecture specific
>
> As a small nit, can we remove the "obviously"? It's certainly obvious to you
> and me, but it doesn't meaningfully affect the sentence either way.
Indeed.
>> +assembly code and calls out into C-code after establishing low level
>> +architecture specific state and stack frames. This low level code must not
>> +be instrumented. A typical syscall handling function invoked from low level
>> +assembly code looks like this::
>> +
>> + noinstr void do_syscall(struct pt_regs \*regs, int nr)
> ^^
>
> Is `\*` necessary here? ... and/or should this be an explicit code block (which
> IIUC doesn't require this esacping), e.g.
>
> .. code-block:: c
Right. Let me try that.
> noinstr void do_syscall(struct pt_regs *regs, int nr)
>> +
>> +If the interrupt is raised while the CPU executes in kernel space the entry
>> +and exit handling is slightly different. RCU state is only updated when the
>> +interrupt was raised in context of the idle task because that's the only
>
> Since we have an idle task for each cpu, perhaps either:
>
> s/the idle task/an idle task/
> s/the idle task/the CPU's idle task/
Yes, that's more precise
>> +Note, that the update of the preemption counter has to be the first
>> +operation on enter and the last operation on exit. The reason is that both
>> +lockdep and RCU rely on in_nmi() returning true in this case. The
>> +preemption count modification in the NMI entry/exit case can obviously not
>> +be traced.
>
> Could we say "must not" instead of "can not", e.g.
>
> The preemption count modification in the NMI entry/exit must not be traced.
>
> That way it's clearly a requirement, rather than a limitation.
Yes.
>> +Architecture specific code looks like this::
>> +
>> + noinstr void do_nmi(struct pt_regs \*regs)
>> + {
>> + arch_nmi_enter(regs);
>> + state = irqentry_nmi_enter(regs);
>> +
>> + instrumentation_begin();
>> +
>> + invoke_nmi_handler(regs);
>> +
>> + instrumentation_end();
>> + irqentry_nmi_exit(regs);
>> + }
>
> To keep the begin/end and enter/exit calls visually balanced, should the
> instrumentation_end() call have trailing a line space, e.g.
Yup.
Thanks,
tglx
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