linux-kernel - Re: [patch 00/13] preempt: Make preempt count unconditional

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Message-ID: <20200916204359.GB29330@paulmck-ThinkPad-P72>
Date:   Wed, 16 Sep 2020 13:43:59 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Daniel Vetter <daniel@...ll.ch>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ard Biesheuvel <ardb@...nel.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Valentin Schneider <valentin.schneider@....com>,
        Richard Henderson <rth@...ddle.net>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        alpha <linux-alpha@...r.kernel.org>,
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        linux-um <linux-um@...ts.infradead.org>,
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        Shuah Khan <shuah@...nel.org>, rcu@...r.kernel.org,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>
Subject: Re: [patch 00/13] preempt: Make preempt count unconditional

On Wed, Sep 16, 2020 at 11:32:00AM -0700, Linus Torvalds wrote:
> On Wed, Sep 16, 2020 at 8:29 AM Paul E. McKenney <paulmck@...nel.org> wrote:
> >
> > All fair, but some of us need to write code that must handle being
> > invoked from a wide variety of contexts.
> 
> Note that I think that core functionality is different from random drivers.
> 
> Of course core code can (and will) look at things like
> 
>         if (in_interrupt())
>             .. schedule work asynchronously ..
> 
> because core code ends up being called from odd places, and code like
> that is expected to have understanding of the rules it plays with.
> 
> But something like RCU is a very different beast from some "walk the
> scatter-gather list" code.
> 
> RCU does its work in the background, and works with lots of different
> things. And it's so core and used everywhere that it knows about these
> things. I mean, we literally have special code explicitly to let RCU
> know "we entered kernel context now".
> 
> But something like a driver list walking thing should not be doing
> different things behind peoples back depending on whether they hold
> spinlocks or not. It should either just work regardless, or there
> should be a flag (or special interface) for the "you're being called
> in a crtitical region".
> 
> Because dynamically changing behavior really is very confusing.

Whew!  I feel much better now.  ;-)

							Thanx, Paul