From: "Russell King (Oracle)" <linux@armlinux.org.uk>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>,
x86@kernel.org, Dave Hansen <dave.hansen@intel.com>,
LKML <linux-kernel@vger.kernel.org>,
linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Nicholas Piggin <npiggin@gmail.com>,
linux-arm-kernel@lists.infradead.org,
Will Deacon <will@kernel.org>
Subject: Re: [PATCH 7/8] membarrier: Remove arm (32) support for SYNC_CORE
Date: Wed, 16 Jun 2021 14:22:27 +0100 [thread overview]
Message-ID: <20210616132226.GD22278@shell.armlinux.org.uk> (raw)
In-Reply-To: <YMncQv1uT5QyQ84w@hirez.programming.kicks-ass.net>
On Wed, Jun 16, 2021 at 01:10:58PM +0200, Peter Zijlstra wrote:
> 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.
If it's a problem, then it needs fixing. sys_cacheflush() is used to
implement GCC's __builtin___clear_cache(). I'm not sure who added this
to gcc.
> > 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.
Likely only in places where we care about I/D coherency - as the data
cache is required to be PIPT on these SMP platforms.
--
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!
next prev parent reply other threads:[~2021-06-16 13:22 UTC|newest]
Thread overview: 91+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-16 3:21 [PATCH 0/8] membarrier cleanups Andy Lutomirski
2021-06-16 3:21 ` [PATCH 1/8] membarrier: Document why membarrier() works Andy Lutomirski
2021-06-16 4:00 ` Nicholas Piggin
2021-06-16 7:30 ` Peter Zijlstra
2021-06-17 23:45 ` Andy Lutomirski
2021-06-16 3:21 ` [PATCH 2/8] x86/mm: Handle unlazying membarrier core sync in the arch code Andy Lutomirski
2021-06-16 4:25 ` Nicholas Piggin
2021-06-16 18:31 ` Andy Lutomirski
2021-06-16 17:49 ` Mathieu Desnoyers
2021-06-16 18:31 ` Andy Lutomirski
2021-06-16 3:21 ` [PATCH 3/8] membarrier: Remove membarrier_arch_switch_mm() prototype in core code Andy Lutomirski
2021-06-16 4:26 ` Nicholas Piggin
2021-06-16 17:52 ` Mathieu Desnoyers
2021-06-16 3:21 ` [PATCH 4/8] membarrier: Make the post-switch-mm barrier explicit Andy Lutomirski
2021-06-16 4:19 ` Nicholas Piggin
2021-06-16 7:35 ` Peter Zijlstra
2021-06-16 18:41 ` Andy Lutomirski
2021-06-17 1:37 ` Nicholas Piggin
2021-06-17 2:57 ` Andy Lutomirski
2021-06-17 5:32 ` Andy Lutomirski
2021-06-17 6:51 ` Nicholas Piggin
2021-06-17 23:49 ` Andy Lutomirski
2021-06-19 2:53 ` Nicholas Piggin
2021-06-19 3:20 ` Andy Lutomirski
2021-06-19 4:27 ` Nicholas Piggin
2021-06-17 9:08 ` [RFC][PATCH] sched: Use lightweight hazard pointers to grab lazy mms Peter Zijlstra
2021-06-17 9:10 ` Peter Zijlstra
2021-06-17 10:00 ` Nicholas Piggin
2021-06-17 9:13 ` Peter Zijlstra
2021-06-17 14:06 ` Andy Lutomirski
2021-06-17 9:28 ` Peter Zijlstra
2021-06-17 14:03 ` Andy Lutomirski
2021-06-17 14:10 ` Andy Lutomirski
2021-06-17 15:45 ` Peter Zijlstra
2021-06-18 3:29 ` Paul E. McKenney
2021-06-18 5:04 ` Andy Lutomirski
2021-06-17 15:02 ` [PATCH 4/8] membarrier: Make the post-switch-mm barrier explicit Paul E. McKenney
2021-06-18 0:06 ` Andy Lutomirski
2021-06-18 3:35 ` Paul E. McKenney
2021-06-17 8:45 ` Peter Zijlstra
2021-06-16 3:21 ` [PATCH 5/8] membarrier, kthread: Use _ONCE accessors for task->mm Andy Lutomirski
2021-06-16 4:28 ` Nicholas Piggin
2021-06-16 18:08 ` Mathieu Desnoyers
2021-06-16 18:45 ` Andy Lutomirski
2021-06-16 3:21 ` [PATCH 6/8] powerpc/membarrier: Remove special barrier on mm switch Andy Lutomirski
2021-06-16 4:36 ` Nicholas Piggin
2021-06-16 3:21 ` [PATCH 7/8] membarrier: Remove arm (32) support for SYNC_CORE Andy Lutomirski
2021-06-16 9:28 ` Russell King (Oracle)
2021-06-16 10:16 ` Peter Zijlstra
2021-06-16 10:20 ` Peter Zijlstra
2021-06-16 10:34 ` Russell King (Oracle)
2021-06-16 11:10 ` Peter Zijlstra
2021-06-16 13:22 ` Russell King (Oracle) [this message]
2021-06-16 15:04 ` Catalin Marinas
2021-06-16 15:23 ` Russell King (Oracle)
2021-06-16 15:45 ` Catalin Marinas
2021-06-16 16:00 ` Catalin Marinas
2021-06-16 16:27 ` Russell King (Oracle)
2021-06-17 8:55 ` Krzysztof Hałasa
2021-06-18 12:54 ` Linus Walleij
2021-06-18 13:19 ` Russell King (Oracle)
2021-06-18 13:36 ` Arnd Bergmann
2021-06-17 10:40 ` Mark Rutland
2021-06-17 11:23 ` Russell King (Oracle)
2021-06-17 11:33 ` Mark Rutland
2021-06-17 13:41 ` Andy Lutomirski
2021-06-17 13:51 ` Mark Rutland
2021-06-17 14:00 ` Andy Lutomirski
2021-06-17 14:20 ` Mark Rutland
2021-06-17 15:01 ` Peter Zijlstra
2021-06-17 15:13 ` Peter Zijlstra
2021-06-17 14:16 ` Mathieu Desnoyers
2021-06-17 14:05 ` Peter Zijlstra
2021-06-18 0:07 ` Andy Lutomirski
2021-06-16 3:21 ` [PATCH 8/8] membarrier: Rewrite sync_core_before_usermode() and improve documentation Andy Lutomirski
2021-06-16 4:45 ` Nicholas Piggin
2021-06-16 18:52 ` Andy Lutomirski
2021-06-16 23:48 ` Andy Lutomirski
2021-06-18 15:27 ` Christophe Leroy
2021-06-16 10:20 ` Will Deacon
2021-06-16 23:58 ` Andy Lutomirski
2021-06-17 14:47 ` Mathieu Desnoyers
2021-06-18 0:12 ` Andy Lutomirski
2021-06-18 16:31 ` Mathieu Desnoyers
2021-06-18 19:58 ` Andy Lutomirski
2021-06-18 20:09 ` Mathieu Desnoyers
2021-06-19 6:02 ` Nicholas Piggin
2021-06-19 15:50 ` Andy Lutomirski
2021-06-20 2:10 ` Nicholas Piggin
2021-06-17 15:16 ` Mathieu Desnoyers
2021-06-18 0:13 ` Andy Lutomirski
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20210616132226.GD22278@shell.armlinux.org.uk \
--to=linux@armlinux.org.uk \
--cc=akpm@linux-foundation.org \
--cc=dave.hansen@intel.com \
--cc=linux-arm-kernel@lists.infradead.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=luto@kernel.org \
--cc=mathieu.desnoyers@efficios.com \
--cc=npiggin@gmail.com \
--cc=peterz@infradead.org \
--cc=will@kernel.org \
--cc=x86@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).