linux-kernel - Re: [PATCH v0.9.1 3/6] sched/umcg: implement UMCG syscalls

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Date:   Sat, 27 Nov 2021 02:16:43 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>,
        Peter Oskolkov <posk@...k.io>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-api@...r.kernel.org, Paul Turner <pjt@...gle.com>,
        Ben Segall <bsegall@...gle.com>,
        Peter Oskolkov <posk@...gle.com>,
        Andrei Vagin <avagin@...gle.com>, Jann Horn <jannh@...gle.com>,
        Thierry Delisle <tdelisle@...terloo.ca>
Subject: Re: [PATCH v0.9.1 3/6] sched/umcg: implement UMCG syscalls

On Fri, Nov 26 2021 at 23:16, Peter Zijlstra wrote:
> On Fri, Nov 26, 2021 at 06:09:10PM +0100, Peter Zijlstra wrote:
>>  
>> -	if (unlikely(ti_work & EXIT_TO_USER_MODE_WORK))
>> +	if (unlikely(ti_work & (EXIT_TO_USER_MODE_WORK | _TIF_UMCG)))
>>  		ti_work = exit_to_user_mode_loop(regs, ti_work);
>>  
>>  	arch_exit_to_user_mode_prepare(regs, ti_work);
>
> Thomas, since you're looking at this. I'm not quite sure I got this
> right. The intent is that when _TIF_UMCG is set (and it is never cleared
> until the task unregisters) it is called at least once.

Right.

> The thinking is that if umcg_wait() gets interrupted, we'll drop out,
> handle the signal and then resume the wait, which can obviously happen
> any number of times.

Right.

> It's just that I'm never quite sure where signal crud happens; I'm
> assuming handle_signal_work() simply mucks about with regs (sets sp and
> ip etc.. to the signal stack) and drops out of kernel mode, and on
> re-entry we do this whole merry cycle once again. But I never actually
> dug that deep.

Yes. It sets up the signal frame and once the loop is left because there
are no more TIF flags to handle it drops back to user space into the
signal handler. That returns to the kernel via sys_[rt_]sigreturn()
which undoes the regs damage either by restoring the previous state or
fiddling it to restart the syscall instead of dropping back to user
space.

So yes, this should work, but I hate the sticky nature of TIF_UMCG. I
have no real good idea how to avoid that yet, but let me think about it
some more.

Thanks,

        tglx