linux-kernel - Re: [PATCH v8 01/11] cpuidle/poll_state: poll via smp_cond_load

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <Zw6dZ7HxvcHJaDgm@arm.com>
Date: Tue, 15 Oct 2024 17:50:47 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: "Christoph Lameter (Ampere)" <cl@...two.org>
Cc: Ankur Arora <ankur.a.arora@...cle.com>, linux-pm@...r.kernel.org,
	kvm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, will@...nel.org, tglx@...utronix.de,
	mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
	x86@...nel.org, hpa@...or.com, pbonzini@...hat.com,
	wanpengli@...cent.com, vkuznets@...hat.com, rafael@...nel.org,
	daniel.lezcano@...aro.org, peterz@...radead.org, arnd@...db.de,
	lenb@...nel.org, mark.rutland@....com, harisokn@...zon.com,
	mtosatti@...hat.com, sudeep.holla@....com,
	misono.tomohiro@...itsu.com, maobibo@...ngson.cn,
	joao.m.martins@...cle.com, boris.ostrovsky@...cle.com,
	konrad.wilk@...cle.com
Subject: Re: [PATCH v8 01/11] cpuidle/poll_state: poll via
 smp_cond_load_relaxed()

On Tue, Oct 15, 2024 at 09:42:56AM -0700, Christoph Lameter (Ampere) wrote:
> On Tue, 15 Oct 2024, Catalin Marinas wrote:
> > > +			unsigned int loop_count = 0;
> > >  			if (local_clock_noinstr() - time_start > limit) {
> > >  				dev->poll_time_limit = true;
> > >  				break;
> > >  			}
> > > +
> > > +			smp_cond_load_relaxed(&current_thread_info()->flags,
> > > +					      VAL & _TIF_NEED_RESCHED ||
> > > +					      loop_count++ >= POLL_IDLE_RELAX_COUNT);
> >
> > The above is not guaranteed to make progress if _TIF_NEED_RESCHED is
> > never set. With the event stream enabled on arm64, the WFE will
> > eventually be woken up, loop_count incremented and the condition would
> > become true. However, the smp_cond_load_relaxed() semantics require that
> > a different agent updates the variable being waited on, not the waiting
> > CPU updating it itself. Also note that the event stream can be disabled
> > on arm64 on the kernel command line.
> 
> Setting of need_resched() from another processor involves sending an IPI
> after that was set. I dont think we need to smp_cond_load_relaxed since
> the IPI will cause an event. For ARM a WFE would be sufficient.

I'm not worried about the need_resched() case, even without an IPI it
would still work.

The loop_count++ side of the condition is supposed to timeout in the
absence of a need_resched() event. You can't do an smp_cond_load_*() on
a variable that's only updated by the waiting CPU. Nothing guarantees to
wake it up to update the variable (the event stream on arm64, yes, but
that's generic code).

-- 
Catalin