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

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Date:   Wed, 4 Apr 2018 06:36:25 +0300
From:   Yury Norov <ynorov@...iumnetworks.com>
To:     Mark Rutland <mark.rutland@....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()

Hi Mark,

Thank you for review.

On Tue, Apr 03, 2018 at 02:48:32PM +0100, Mark Rutland wrote:
> Hi Yury,
> 
> On Sun, Apr 01, 2018 at 02:11:08PM +0300, Yury Norov wrote:
> > +/*
> > + * Flush I-cache if CPU is in extended quiescent state
> > + */
> 
> This comment is misleading. An ISB doesn't touch the I-cache; it forces
> a context synchronization event.
> 
> > +	.macro	isb_if_eqs
> > +#ifndef CONFIG_TINY_RCU
> > +	bl	rcu_is_watching
> > +	tst	w0, #0xff
> > +	b.ne	1f
> 
> The TST+B.NE can be a CBNZ:
> 
> 	bl	rcu_is_watching
> 	cbnz	x0, 1f
> 	isb
> 1:
> 
> > +	/* Pairs with aarch64_insn_patch_text for EQS CPUs. */
> > +	isb
> > +1:
> > +#endif
> > +	.endm
> > +
> >  el0_sync_invalid:
> >  	inv_entry 0, BAD_SYNC
> >  ENDPROC(el0_sync_invalid)
> > @@ -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. But after some
headscratching, I think it's safe. I'll do like you suggested here.

Thanks,
Yury