linux-kernel - Re: [RFC][PATCH 2/3] locking,entry: #PF vs TRACE

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Date:   Mon, 10 Aug 2020 13:57:43 +0200
From:   peterz@...radead.org
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     tglx@...utronix.de, mingo@...nel.org, will@...nel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org, elver@...gle.com,
        paulmck@...nel.org, rjw@...ysocki.net
Subject: Re: [RFC][PATCH 2/3] locking,entry: #PF vs TRACE_IRQFLAGS

On Fri, Aug 07, 2020 at 04:21:48PM -0400, Steven Rostedt wrote:
> On Fri, 07 Aug 2020 21:23:38 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > Much of the complexity in irqenter_{enter,exit}() is due to #PF being
> > the sole exception that can schedule from kernel context.
> > 
> > One additional wrinkle with #PF is that it is non-maskable, it can
> > happen _anywhere_. Due to this, and the wonders of tracing, we can get
> > the 'normal' NMI nesting vs TRACE_IRQFLAGS:
> > 
> > 	local_irq_disable()
> > 	  raw_local_irq_disable();
> > 	  trace_hardirqs_off();
> > 
> > 	local_irq_enable();
> 
> Do you mean to have that ';' there? That is, it the below is called
> from local_irq_enable(), right? A ';' means that local_irq_enable()
> is completed.

Indeed, it's just really hard not to type ';' at the end :-)

> 
> > 	  trace_hardirqs_on();
> > 	  <#PF>
> > 	    trace_hardirqs_off()
> > 	    ...
> > 	    if (!regs_irqs_disabled(regs)
> 
> regs has it disabled, so this is false, right?

Yup, I'll add: // false, after it to clarify.

> > 	      trace_hardirqs_on();
> > 	  </#PF>
> 
> I missed the '/' in the above. At first I thought this was another page
> fault :-/
> 
> > 	  // WHOOPS -- lockdep thinks IRQs are disabled again!
> > 	  raw_local_irqs_enable();
> > 
> > Rework irqenter_{enter,exit}() to save/restore the software state.
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> >  include/linux/entry-common.h |    1 
> >  kernel/entry/common.c        |   52 ++++++++++++++++++++-----------------------
> >  2 files changed, 26 insertions(+), 27 deletions(-)
> > 
> > --- a/include/linux/entry-common.h
> > +++ b/include/linux/entry-common.h
> > @@ -310,6 +310,7 @@ void irqentry_exit_to_user_mode(struct p
> >  #ifndef irqentry_state
> >  typedef struct irqentry_state {
> >  	bool	exit_rcu;
> > +	bool	irqs_enabled;
> 
> Instead of passing a structure around, should we look at converting
> "irqentry_state" into a flags field?

Probably, on x86_64-linux sizeof(_Bool) == 1, so it's two bytes and that
fits perfectly fine in a normal return value, but yeah, this is common
code now and we can't rely on sizeof(_Bool) being sane.