From: Minchan Kim <minchan@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Nadav Amit <namit@vmware.com>, Ingo Molnar <mingo@kernel.org>,
Stephen Rothwell <sfr@canb.auug.org.au>,
Andrew Morton <akpm@linux-foundation.org>,
Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@elte.hu>,
"H. Peter Anvin" <hpa@zytor.com>,
Linux-Next Mailing List <linux-next@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Linus <torvalds@linux-foundation.org>
Subject: Re: linux-next: manual merge of the akpm-current tree with the tip tree
Date: Wed, 16 Aug 2017 13:14:18 +0900 [thread overview]
Message-ID: <20170816041418.GB24294@blaptop> (raw)
In-Reply-To: <20170814195723.GO6524@worktop.programming.kicks-ass.net>
On Mon, Aug 14, 2017 at 09:57:23PM +0200, Peter Zijlstra wrote:
> On Mon, Aug 14, 2017 at 05:38:39PM +0900, Minchan Kim wrote:
> > memory-barrier.txt always scares me. I have read it for a while
> > and IIUC, it seems semantic of spin_unlock(&same_pte) would be
> > enough without some memory-barrier inside mm_tlb_flush_nested.
>
> Indeed, see the email I just send. Its both spin_lock() and
> spin_unlock() that we care about.
>
> Aside from the semi permeable barrier of these primitives, RCpc ensures
> these orderings only work against the _same_ lock variable.
>
> Let me try and explain the ordering for PPC (which is by far the worst
> we have in this regard):
>
>
> spin_lock(lock)
> {
> while (test_and_set(lock))
> cpu_relax();
> lwsync();
> }
>
>
> spin_unlock(lock)
> {
> lwsync();
> clear(lock);
> }
>
> Now LWSYNC has fairly 'simple' semantics, but with fairly horrible
> ramifications. Consider LWSYNC to provide _local_ TSO ordering, this
> means that it allows 'stores reordered after loads'.
>
> For the spin_lock() that implies that all load/store's inside the lock
> do indeed stay in, but the ACQUIRE is only on the LOAD of the
> test_and_set(). That is, the actual _set_ can leak in. After all it can
> re-order stores after load (inside the lock).
>
> For unlock it again means all load/store's prior stay prior, and the
> RELEASE is on the store clearing the lock state (nothing surprising
> here).
>
> Now the _local_ part, the main take-away is that these orderings are
> strictly CPU local. What makes the spinlock work across CPUs (as we'd
> very much expect it to) is the address dependency on the lock variable.
>
> In order for the spin_lock() to succeed, it must observe the clear. Its
> this link that crosses between the CPUs and builds the ordering. But
> only the two CPUs agree on this order. A third CPU not involved in
> this transaction can disagree on the order of events.
The detail explanation in your previous reply makes me comfortable
from scary memory-barrier.txt but this reply makes me scared again. ;-)
Thanks for the kind clarification, Peter!
next prev parent reply other threads:[~2017-08-16 4:14 UTC|newest]
Thread overview: 112+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-08-11 7:53 linux-next: manual merge of the akpm-current tree with the tip tree Stephen Rothwell
2017-08-11 9:34 ` Peter Zijlstra
2017-08-11 10:48 ` Peter Zijlstra
2017-08-11 11:45 ` Stephen Rothwell
2017-08-11 11:56 ` Ingo Molnar
2017-08-11 12:17 ` Peter Zijlstra
2017-08-11 12:44 ` Ingo Molnar
2017-08-11 13:49 ` Stephen Rothwell
2017-08-11 14:04 ` Peter Zijlstra
2017-08-13 6:06 ` Nadav Amit
2017-08-13 12:50 ` Peter Zijlstra
2017-08-14 3:16 ` Minchan Kim
2017-08-14 5:07 ` Nadav Amit
2017-08-14 5:23 ` Minchan Kim
2017-08-14 8:38 ` Minchan Kim
2017-08-14 19:57 ` Peter Zijlstra
2017-08-16 4:14 ` Minchan Kim [this message]
2017-08-14 19:38 ` Peter Zijlstra
2017-08-15 7:51 ` Nadav Amit
2017-08-14 3:09 ` Minchan Kim
2017-08-14 18:54 ` Peter Zijlstra
-- strict thread matches above, loose matches on Subject: below --
2022-02-16 5:38 Stephen Rothwell
2021-10-07 6:27 Stephen Rothwell
2021-03-22 6:12 Stephen Rothwell
2020-12-11 8:56 Stephen Rothwell
2020-12-11 12:47 ` Jason Gunthorpe
2020-11-27 7:48 Stephen Rothwell
2020-11-27 7:39 Stephen Rothwell
2020-11-27 11:54 ` Andy Shevchenko
2020-11-30 9:27 ` Thomas Gleixner
2020-11-23 8:05 Stephen Rothwell
2020-11-09 6:00 Stephen Rothwell
2020-10-13 6:59 Stephen Rothwell
2020-07-17 10:19 Stephen Rothwell
2020-05-29 11:05 Stephen Rothwell
2020-05-29 10:18 Stephen Rothwell
2020-05-29 10:05 Stephen Rothwell
2020-05-29 9:58 Stephen Rothwell
2020-05-25 11:04 Stephen Rothwell
2020-05-26 4:41 ` Singh, Balbir
2020-06-03 4:43 ` Stephen Rothwell
2020-05-19 16:18 Stephen Rothwell
2020-03-25 7:48 Stephen Rothwell
2020-03-19 6:42 Stephen Rothwell
2020-01-20 6:37 Stephen Rothwell
2020-01-20 6:30 Stephen Rothwell
2019-10-31 5:43 Stephen Rothwell
2019-06-24 10:24 Stephen Rothwell
2019-05-01 11:10 Stephen Rothwell
2019-01-31 4:31 Stephen Rothwell
2018-08-20 4:32 Stephen Rothwell
2018-08-20 19:52 ` Andrew Morton
2018-03-23 5:59 Stephen Rothwell
2017-12-18 5:04 Stephen Rothwell
2017-11-10 4:33 Stephen Rothwell
2017-11-02 7:19 Stephen Rothwell
2017-08-22 6:57 Stephen Rothwell
2017-08-23 6:39 ` Vlastimil Babka
2017-04-12 6:46 Stephen Rothwell
2017-04-12 20:53 ` Vlastimil Babka
2017-04-20 2:17 ` NeilBrown
2017-03-24 5:25 Stephen Rothwell
2017-02-17 4:40 Stephen Rothwell
2016-11-14 6:08 Stephen Rothwell
2016-07-29 4:14 Stephen Rothwell
2016-06-15 5:23 Stephen Rothwell
2016-06-18 19:39 ` Manfred Spraul
2016-04-29 6:12 Stephen Rothwell
2016-04-29 6:26 ` Ingo Molnar
2016-03-02 5:40 Stephen Rothwell
2016-02-26 5:07 Stephen Rothwell
2016-02-26 21:35 ` Andrew Morton
2016-02-19 4:09 Stephen Rothwell
2016-02-19 15:26 ` Ard Biesheuvel
2015-12-07 8:06 Stephen Rothwell
2015-10-02 4:21 Stephen Rothwell
2015-07-28 6:00 Stephen Rothwell
2015-07-29 17:12 ` Andrea Arcangeli
2015-07-29 17:47 ` Andy Lutomirski
2015-07-29 18:46 ` Thomas Gleixner
2015-07-30 15:38 ` Andrea Arcangeli
2015-07-29 23:06 ` Stephen Rothwell
2015-07-29 23:07 ` Thomas Gleixner
2015-09-07 23:35 ` Stephen Rothwell
2015-09-08 18:11 ` Linus Torvalds
2015-09-08 22:56 ` Stephen Rothwell
2015-09-08 23:03 ` Linus Torvalds
2015-09-08 23:21 ` Andrew Morton
2015-09-16 6:58 ` Geert Uytterhoeven
2015-06-04 12:07 Stephen Rothwell
2015-04-08 8:28 Stephen Rothwell
2015-04-08 8:25 Stephen Rothwell
2014-03-17 9:31 Stephen Rothwell
2014-03-17 9:36 ` Peter Zijlstra
2014-03-19 23:27 ` Andrew Morton
2014-01-14 4:53 Stephen Rothwell
2014-01-14 5:04 ` Davidlohr Bueso
2014-01-14 12:51 ` Peter Zijlstra
2014-01-14 13:17 ` Geert Uytterhoeven
2014-01-14 13:33 ` Peter Zijlstra
2014-01-14 16:19 ` H. Peter Anvin
2014-01-14 15:15 ` H. Peter Anvin
2014-01-14 15:20 ` Geert Uytterhoeven
2014-01-14 15:41 ` Peter Zijlstra
2014-01-14 15:48 ` H. Peter Anvin
2014-01-07 6:00 Stephen Rothwell
2014-01-07 6:34 ` Tang Chen
2013-11-08 7:48 Stephen Rothwell
2013-11-08 18:58 ` Josh Triplett
2013-11-08 23:20 ` Stephen Rothwell
2013-11-09 0:19 ` Josh Triplett
2013-10-30 6:40 Stephen Rothwell
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=20170816041418.GB24294@blaptop \
--to=minchan@kernel.org \
--cc=akpm@linux-foundation.org \
--cc=hpa@zytor.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-next@vger.kernel.org \
--cc=mingo@elte.hu \
--cc=mingo@kernel.org \
--cc=namit@vmware.com \
--cc=peterz@infradead.org \
--cc=sfr@canb.auug.org.au \
--cc=tglx@linutronix.de \
--cc=torvalds@linux-foundation.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).