linux-kernel - Re: context tracking vs. syscall_trace_leave & do_notify

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Message-ID: <CALCETrX4EfXPMWfMjZ20tmgm_tM1E69uwv4ewingVpsw+=TnjA@mail.gmail.com>
Date:	Fri, 1 May 2015 09:16:40 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Rik van Riel <riel@...hat.com>
Cc:	Fr馘駻ic Weisbecker <fweisbec@...il.com>,
	X86 <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Denys Vlasenko <dvlasenk@...hat.com>
Subject: Re: context tracking vs. syscall_trace_leave & do_notify_resume loop

On Fri, May 1, 2015 at 9:14 AM, Rik van Riel <riel@...hat.com> wrote:
> On 05/01/2015 12:05 PM, Andy Lutomirski wrote:
>> On Fri, May 1, 2015 at 9:00 AM, Rik van Riel <riel@...hat.com> wrote:
>
>>> I suspect we probably only need two possible function
>>> calls at syscall exit time:
>>>
>>> 1) A function that is called with interrupts still
>>>    enabled, testing flags that could be set again
>>>    if something happens (eg. preemption) between
>>>    when the function is called, and we return to
>>>    user space.
>>>
>>> 2) A function that is called after the point of
>>>    no return, with interrupts disabled, which
>>>    does (mostly) small things that only happen
>>>    once.
>>
>> I think we only need one function.  It would be (asm pseudocode):
>>
>> disable irqs;
>> if (slow) {
>>   save extra regs;
>>   call function;
>>   restore extra regs;
>> }
>>
>> return via opportunistic sysret path.
>>
>> I can't see any legitimate reason for the current mess, except that
>> it's no complicated and so poorly documented that everyone's afraid of
>> fixing it.
>
> do_notify_resume() can call do_signal(), which can sleep, after
> which all bets are off on what new flags may have been set.
>
> On the other hand, we have stuff that can run just fine with
> irqs disabled that we really want to call only once.
>
> For that reason, I suspect we need two functions.

C can have loops just as easily as assembly can :)  I still don't see
why we need magic asm code to schedule and deliver signals.  We
certainly need to have valid pt_regs to deliver signals, but that's
easy and much cheaper than it used to be.

--Andy
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