linux-kernel - Re: [PATCH 4/4] lockdep: Fix workqueue crossrelease annotation

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Date:   Tue, 5 Sep 2017 19:58:38 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Byungchul Park <max.byungchul.park@...il.com>,
        Ingo Molnar <mingo@...nel.org>, Tejun Heo <tj@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>, david@...morbit.com,
        Johannes Berg <johannes@...solutions.net>, oleg@...hat.com,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kernel-team@....com
Subject: Re: [PATCH 4/4] lockdep: Fix workqueue crossrelease annotation

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?

For example, _semantically_:

   lock(A) -> recursive-read(A), end in a deadlock, while
   lock(A) -> might(A)         , is like nothing.

   recursive-read(A) -> might(A), is like nothing, while
   might(A) -> recursive-read(A), end in a deadlock.

   And so on...

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.

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.