linux-kernel - Re: [PATCH 2/2] smp: introduce kick_active_cpus

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Date:   Wed, 4 Apr 2018 10:08:32 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Yury Norov <ynorov@...iumnetworks.com>
Cc:     Will Deacon <will.deacon@....com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Christopher Lameter <cl@...ux.com>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, kvm-ppc@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] smp: introduce kick_active_cpus_sync()

On Wed, Apr 04, 2018 at 06:36:25AM +0300, Yury Norov wrote:
> On Tue, Apr 03, 2018 at 02:48:32PM +0100, Mark Rutland wrote:
> > On Sun, Apr 01, 2018 at 02:11:08PM +0300, Yury Norov wrote:
> > > @@ -840,8 +861,10 @@ el0_svc:
> > >  	mov	wsc_nr, #__NR_syscalls
> > >  el0_svc_naked:					// compat entry point
> > >  	stp	x0, xscno, [sp, #S_ORIG_X0]	// save the original x0 and syscall number
> > > +	isb_if_eqs
> > >  	enable_dbg_and_irq
> > > -	ct_user_exit 1
> > > +	ct_user_exit
> > 
> > I don't think this is safe. here we issue the ISB *before* exiting a
> > quiesecent state, so I think we can race with another CPU that calls
> > kick_all_active_cpus_sync, e.g.
> > 
> > 	CPU0				CPU1
> > 
> > 	ISB
> > 					patch_some_text()
> > 					kick_all_active_cpus_sync()
> > 	ct_user_exit
> > 
> > 	// not synchronized!
> > 	use_of_patched_text()
> > 
> > ... and therefore the ISB has no effect, which could be disasterous.
> > 
> > I believe we need the ISB *after* we transition into a non-quiescent
> > state, so that we can't possibly miss a context synchronization event.
>  
> I decided to put isb() in entry because there's a chance that there will
> be patched code prior to exiting a quiescent state.

If we do patch entry text, then I think we have no option but to use
kick_all_active_cpus_sync(), or we risk races similar to the above.

> But after some headscratching, I think it's safe. I'll do like you
> suggested here.

Sounds good.

Thanks,
Mark.