From: Peter Zijlstra <peterz@infradead.org>
To: Byungchul Park <max.byungchul.park@gmail.com>
Cc: Byungchul Park <byungchul.park@lge.com>,
Ingo Molnar <mingo@kernel.org>,
tglx@linutronix.de, Michel Lespinasse <walken@google.com>,
boqun.feng@gmail.com, kirill@shutemov.name,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
linux-mm@kvack.org, iamjoonsoo.kim@lge.com,
akpm@linux-foundation.org, npiggin@gmail.com
Subject: Re: [PATCH v3 07/15] lockdep: Implement crossrelease feature
Date: Wed, 14 Sep 2016 10:11:17 +0200 [thread overview]
Message-ID: <20160914081117.GK5008@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <CANrsvROL43uYXsU7-kmFbHFgiKARBXYHNeqL71V9GxGzBYEdNA@mail.gmail.com>
On Wed, Sep 14, 2016 at 11:27:22AM +0900, Byungchul Park wrote:
> > Well, there is, its just not trivially observable. We must be able to
> > acquire a in order to complete b, therefore there is a dependency.
>
> No. We cannot say there is a dependency unconditionally. There can
> be a dependency or not.
>
> L a L a
> U a
> ~~~~~~~~~ what if serialized by something?
Well, there's no serialization in the example, so no what if.
> W b C b
>
> If something we don't recognize serializes locks, which ensures
> 'W b' happens after 'L a , U a' in the other context, then there's
> no dependency here.
Its not there.
> We should say 'b depends on a' in only case that the sequence
> 'W b and then L a and then C b, where last two ops are in same
> context' _actually_ happened at least once. Otherwise, it might
> add a false dependency.
>
> It's same as how original lockdep works with typical locks. It adds
> a dependency only when a lock is actually hit.
But since these threads are independently scheduled there is no point in
transferring the point in time thread A does W to thread B. There is no
relation there.
B could have already executed the complete or it could not yet have
started execution at all or anything in between, entirely random.
> > What does that mean? Any why? This is a random point in time without
> > actual meaning.
>
> It's not random point. We have to consider meaningful sequences among
> those which are globally observable. That's why we need to serialize
> those locks.
Serialize how? there is no serialization.
> For example,
>
> W b
> L a
> U a
> C b
>
> Once this sequence is observable globally, we can say 'It's possible to
> run in this sequence. Is this sequence problematic or not?'.
>
> L a
> U a
> W b
> C b
>
> If only this sequence can be observable, we should not assume
> this sequence can be changed. However once the former sequence
> happens, it has a possibility to hit the same sequence again later.
> So we can check deadlock possibility with the sequence,
>
> _not randomly_.
I still don't get it.
> We need to connect between the crosslock and the first lock among
> locks having been acquired since the crosslock was held.
Which can be _any_ lock in the history of that thread. It could be
rq->lock from getting the thread scheduled.
> Others will be
> connected each other by original lockdep.
>
> By the way, does my document miss this description? If so, sorry.
> I will check and update it.
I couldn't find anything useful, but then I could not understand most of
what was written, and I tried hard :-(
next prev parent reply other threads:[~2016-09-14 8:11 UTC|newest]
Thread overview: 49+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-09-13 9:44 [PATCH v3 00/15] lockdep: Implement crossrelease feature Byungchul Park
2016-09-13 9:45 ` [PATCH v3 01/15] x86/dumpstack: Optimize save_stack_trace Byungchul Park
2016-09-13 13:18 ` Josh Poimboeuf
2016-09-13 14:54 ` Byungchul Park
2016-09-13 9:45 ` [PATCH v3 02/15] x86/dumpstack: Add save_stack_trace()_fast() Byungchul Park
2016-09-13 13:20 ` Josh Poimboeuf
2016-09-13 9:45 ` [PATCH v3 03/15] lockdep: Refactor lookup_chain_cache() Byungchul Park
2016-09-15 15:33 ` Nilay Vaish
2016-09-19 3:05 ` Byungchul Park
2016-09-19 16:36 ` Nilay Vaish
2016-09-20 2:00 ` Byungchul Park
2016-09-13 9:45 ` [PATCH v3 04/15] lockdep: Add a function building a chain between two classes Byungchul Park
2016-09-13 9:45 ` [PATCH v3 05/15] lockdep: Make check_prev_add can use a separate stack_trace Byungchul Park
2016-09-13 9:45 ` [PATCH v3 06/15] lockdep: Make save_trace can skip stack tracing of the current Byungchul Park
2016-09-13 9:45 ` [PATCH v3 07/15] lockdep: Implement crossrelease feature Byungchul Park
2016-09-13 10:05 ` Peter Zijlstra
2016-09-13 12:09 ` Peter Zijlstra
2016-09-13 15:14 ` Byungchul Park
2016-09-13 15:05 ` Peter Zijlstra
2016-09-13 17:12 ` Byungchul Park
2016-09-13 19:38 ` Peter Zijlstra
2016-09-13 21:42 ` Peter Zijlstra
2016-09-14 1:01 ` Byungchul Park
2016-09-14 2:27 ` Byungchul Park
2016-09-14 4:49 ` Byungchul Park
2016-09-14 8:11 ` Peter Zijlstra [this message]
2016-09-19 2:41 ` Byungchul Park
2016-09-19 8:50 ` Peter Zijlstra
2016-09-20 5:50 ` Byungchul Park
2016-09-20 6:26 ` Byungchul Park
2016-09-21 1:37 ` Byungchul Park
2016-09-22 2:57 ` Byungchul Park
2016-09-13 9:45 ` [PATCH v3 08/15] lockdep: Make crossrelease use save_stack_trace_fast() Byungchul Park
2016-09-13 9:45 ` [PATCH v3 09/15] lockdep: Make print_circular_bug() crosslock-aware Byungchul Park
2016-09-13 9:45 ` [PATCH v3 10/15] lockdep: Apply crossrelease to completion operation Byungchul Park
2016-09-13 9:45 ` [PATCH v3 11/15] pagemap.h: Remove trailing white space Byungchul Park
2016-09-13 9:45 ` [PATCH v3 12/15] lockdep: Apply crossrelease to PG_locked lock Byungchul Park
2016-09-13 9:45 ` [PATCH v3 13/15] lockdep: Apply lock_acquire(release) on __Set(__Clear)PageLocked Byungchul Park
2016-09-13 9:45 ` [PATCH v3 14/15] lockdep: Move data used in CONFIG_LOCKDEP_PAGELOCK from page to page_ext Byungchul Park
2016-09-13 9:45 ` [PATCH v3 15/15] lockdep: Crossrelease feature documentation Byungchul Park
2016-09-15 17:25 ` Nilay Vaish
2016-09-19 2:59 ` Byungchul Park
2016-09-16 15:47 ` Nilay Vaish
2016-09-19 3:00 ` Byungchul Park
2016-09-20 5:00 ` Byungchul Park
2016-09-13 9:58 ` [FYI] Output of 'cat /proc/lockdep' after applying crossrelease Byungchul Park
2016-11-02 5:42 ` [REVISED DOC on v3] Crossrelease Lockdep Byungchul Park
2016-11-03 8:18 ` Byungchul Park
2016-11-08 2:54 ` Byungchul Park
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