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Date:   Mon, 24 Jan 2022 17:44:03 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     mingo@redhat.com, tglx@linutronix.de, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
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Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        linux-api@vger.kernel.org, x86@kernel.org, pjt@google.com,
        posk@google.com, avagin@google.com, jannh@google.com,
        tdelisle@uwaterloo.ca, mark.rutland@arm.com, posk@posk.io
Subject: Re: [RFC][PATCH v2 5/5] sched: User Mode Concurency Groups
Message-ID: <Ye7XU666240YyRBH@hirez.programming.kicks-ass.net>
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On Mon, Jan 24, 2022 at 03:29:56PM +0100, Peter Zijlstra wrote:
> On Fri, Jan 21, 2022 at 04:18:46PM +0100, Peter Zijlstra wrote:
> > Something like this, still yuck though. Also still need to write me a
> > test for this.
> > 

> > --- a/kernel/sched/umcg.c
> > +++ b/kernel/sched/umcg.c
> > @@ -232,6 +232,8 @@ static int umcg_update_state(struct task
> >  /* Called from syscall enter path and exceptions that can schedule */
> >  void umcg_sys_enter(struct pt_regs *regs, long syscall)
> >  {
> > +	current->umcg_timeout = 0;
> > +
> >  	/* avoid recursion vs our own syscalls */
> >  	if (syscall == __NR_umcg_wait ||
> >  	    syscall == __NR_umcg_ctl)
> > @@ -519,6 +521,7 @@ void umcg_notify_resume(struct pt_regs *
> >  	struct umcg_task __user *self = tsk->umcg_task;
> >  	bool worker = tsk->flags & PF_UMCG_WORKER;
> >  	u32 state;
> > +	int ret;
> >  
> >  	/* avoid recursion vs schedule() */
> >  	if (worker)
> > @@ -554,12 +557,17 @@ void umcg_notify_resume(struct pt_regs *
> >  		umcg_unpin_pages();
> >  	}
> >  
> > -	switch (umcg_wait(0)) {
> > +	ret = umcg_wait(tsk->umcg_timeout);
> 
> Oh how I hate signals... this can get scribbled by a syscall/fault from
> sigcontext :/
> 
> Maybe I can recover the timo argument from the original syscall
> pt_regs.. let me try.

Urgh, recursive hell... If the signal does *anything* that tickles
notify-resume it'll find RUNNABLE and go wait there --- ad infinitum.

I need to go cook dinner, I'll prod more at this later