Re: [PATCH 7/8] membarrier: Remove arm (32) support for SYNC_CORE

Peter Zijlstra
2021-06-16T11:12:42Z

On Wed, Jun 16, 2021 at 11:34:46AM +0100, Russell King (Oracle) wrote:
> On Wed, Jun 16, 2021 at 12:20:06PM +0200, Peter Zijlstra wrote:
> > On Wed, Jun 16, 2021 at 12:16:27PM +0200, Peter Zijlstra wrote:
> > > On Tue, Jun 15, 2021 at 08:21:12PM -0700, Andy Lutomirski wrote:
> > > > On arm32, the only way to safely flush icache from usermode is to call
> > > > cacheflush(2). This also handles any required pipeline flushes, so
> > > > membarrier's SYNC_CORE feature is useless on arm. Remove it.
> > >
> > > So SYNC_CORE is there to help an architecture that needs to do something
> > > per CPU. If I$ invalidation is broadcast and I$ invalidation also
> > > triggers the flush of any uarch caches derived from it (if there are
> > > any).
> >
> > Incomplete sentence there: + then we don't need SYNC_CORE.
> >
> > > Now arm_syscall() NR(cacheflush) seems to do flush_icache_user_range(),
> > > which, if I read things right, end up in arch/arm/mm/*.S, but that
> > > doesn't consider cache_ops_need_broadcast().
> > >
> > > Will suggests that perhaps ARM 11MPCore might need this due to their I$
> > > flush maybe not being broadcast
>
> If it leaves other cores with incoherent I cache, then that's already
> a problem for SMP cores, since there could be no guarantee that the
> modifications made by one core will be visible to some other core that
> ends up running that code - and there is little option for userspace to
> work around that except by pinning the thread making the modifications
> and subsequently executing the code to a core.

That's where SYNC_CORE can help. Or you make sys_cacheflush() do a
system wide IPI.

> The same is also true of flush_icache_range() - which is used when
> loading a kernel module. In the case Will is referring to, these alias
> to the same code.

Yes, cache_ops_need_broadcast() seems to be missing in more places.