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From: Boqun Feng <boqun.feng@gmail.com>
To: Byungchul Park <byungchul.park@lge.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Byungchul Park <max.byungchul.park@gmail.com>,
	Ingo Molnar <mingo@kernel.org>, Tejun Heo <tj@kernel.org>,
	david@fromorbit.com, Johannes Berg <johannes@sipsolutions.net>,
	oleg@redhat.com,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	kernel-team@lge.com
Subject: Re: [PATCH 4/4] lockdep: Fix workqueue crossrelease annotation
Date: Wed, 6 Sep 2017 08:42:11 +0800	[thread overview]
Message-ID: <20170906004211.GT11771@tardis> (raw)
In-Reply-To: <20170905235235.GZ3240@X58A-UD3R>

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On Wed, Sep 06, 2017 at 08:52:35AM +0900, Byungchul Park wrote:
> On Tue, Sep 05, 2017 at 03:46:43PM +0200, Peter Zijlstra wrote:
> > On Tue, Sep 05, 2017 at 07:58:38PM +0900, Byungchul Park wrote:
> > > On Tue, Sep 05, 2017 at 07:31:44PM +0900, Byungchul Park wrote:
> > > > Recursive-read and the hint I proposed(a.k.a. might) should be used for
> > > > their different specific applications. Both meaning and constraints of
> > > > them are totally different.
> > > > 
> > > > Using a right function semantically is more important than making it
> > > > just work, as you know. Wrong?
> > 
> > > Of course, in the following cases, the results are same:
> > > 
> > >    recursive-read(A) -> recursive-read(A), is like nothing, and also
> > >    might(A)          -> might(A)         , is like nothing.
> > > 
> > >    recursive-read(A) -> lock(A), end in a deadlock, and also
> > >    might(A)          -> lock(A), end in a deadlock.
> > 
> > And these are exactly the cases we need.
> > 
> > > Futhermore, recursive-read-might() can be used if needed, since their
> > > semantics are orthogonal so they can be used in mixed forms.
> > > 
> > > I really hope you accept the new semantics... I think current workqueue
> > > code exactly needs the semantics.
> > 
> > I really don't want to introduce this extra state if we don't have to.
> 
> OK. If the workqueue is only user of the weird lockdep annotations, then
> it might be better to defer introducing the extra state until needed.
> 
> But, the 'might' thing I introduced would be necessary if more users
> want to report deadlocks at the time for crosslocks with speculative
> acquisitions like the workqueue does, since the recursive-read thing
> would generate false dependencies much more than we want, while the

What do you mean by "false dependencies"? AFAICT, recursive-read could
have dependencies to the following cross commit, for example:

	A(a)
			ARR(a)
			RRR(a)
	WFC(X)
			C(X)

This is a deadlock, no?

In my upcoming v2 for recursive-read support, I'm going to make this
detectable. But please note as crossrelease doesn't have any selftests
as normal lockdep stuffs, I may miss something subtle.

Regards,
Boqun

> 'might' thing generate them just as many as we want.
> 

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  reply	other threads:[~2017-09-06  0:41 UTC|newest]

Thread overview: 61+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-08-23 11:58 [PATCH 0/4] workqueue and lockdep stuffs Peter Zijlstra
2017-08-23 11:58 ` [PATCH 1/4] workqueue: Use TASK_IDLE Peter Zijlstra
2017-08-23 13:31   ` Tejun Heo
2017-08-23 11:58 ` [PATCH 2/4] lockdep/selftests: Add mixed read-write ABBA tests Peter Zijlstra
2017-08-23 11:58 ` [PATCH 3/4] workqueue/lockdep: Fix flush_work() annotation Peter Zijlstra
2017-08-23 11:58 ` [PATCH 4/4] lockdep: Fix workqueue crossrelease annotation Peter Zijlstra
2017-08-24  2:18   ` Byungchul Park
2017-08-24 14:02     ` Peter Zijlstra
2017-08-25  1:11       ` Byungchul Park
2017-08-29  8:59         ` Peter Zijlstra
2017-08-29 14:23           ` [tip:locking/core] locking/lockdep: Untangle xhlock history save/restore from task independence tip-bot for Peter Zijlstra
2017-08-29 16:02           ` [PATCH 4/4] lockdep: Fix workqueue crossrelease annotation Byungchul Park
2017-08-29 18:47             ` Peter Zijlstra
2017-08-30  2:09           ` Byungchul Park
2017-08-30  7:41             ` Byungchul Park
2017-08-30  8:53               ` Peter Zijlstra
2017-08-30  9:01                 ` Byungchul Park
2017-08-30  9:12                   ` Peter Zijlstra
2017-08-30  9:14                     ` Peter Zijlstra
2017-08-30  9:35                       ` Byungchul Park
2017-08-30  9:24                     ` Byungchul Park
2017-08-30 11:25                       ` Byungchul Park
2017-08-30 12:49                         ` Byungchul Park
2017-08-31  7:26                         ` Byungchul Park
2017-08-31  8:04                         ` Peter Zijlstra
2017-08-31  8:15                           ` Byungchul Park
2017-08-31  8:34                             ` Peter Zijlstra
2017-09-01  2:05                               ` Byungchul Park
2017-09-01  9:47                                 ` Peter Zijlstra
2017-09-01 10:16                                   ` Byungchul Park
2017-09-01 12:09                                     ` 박병철/선임연구원/SW Platform(연)AOT팀(byungchul.park@lge.com)
2017-09-01 12:38                                     ` Peter Zijlstra
2017-09-01 13:51                                       ` Byungchul Park
2017-09-01 16:38                                         ` Peter Zijlstra
2017-09-04  1:30                                           ` Byungchul Park
2017-09-04  2:08                                             ` Byungchul Park
2017-09-04 11:42                                             ` Peter Zijlstra
2017-09-05  0:38                                               ` Byungchul Park
2017-09-05  7:08                                                 ` Peter Zijlstra
2017-09-05  7:19                                                   ` Peter Zijlstra
2017-09-05  8:57                                                     ` Byungchul Park
2017-09-05  9:36                                                       ` Peter Zijlstra
2017-09-05 10:31                                                         ` Byungchul Park
2017-09-05 10:52                                                           ` Peter Zijlstra
2017-09-05 11:24                                                             ` Byungchul Park
2017-09-05 10:58                                                           ` Byungchul Park
2017-09-05 13:46                                                             ` Peter Zijlstra
2017-09-05 23:52                                                               ` Byungchul Park
2017-09-06  0:42                                                                 ` Boqun Feng [this message]
2017-09-06  1:32                                                                   ` Byungchul Park
2017-09-06 23:59                                                                     ` Byungchul Park
2017-09-07  0:11                                                                     ` Byungchul Park
2017-09-06  0:48                                                               ` Byungchul Park
2017-09-05  8:30                                                   ` Byungchul Park
2017-08-31  8:07                       ` Peter Zijlstra
2017-08-25  4:39       ` Byungchul Park
2017-08-29  6:46   ` Byungchul Park
2017-08-29  9:01     ` Peter Zijlstra
2017-08-29 16:12       ` Byungchul Park
2017-08-23 13:32 ` [PATCH 0/4] workqueue and lockdep stuffs Tejun Heo
2017-08-23 13:45   ` Peter Zijlstra

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