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SubjectRe: [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.

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