linux-kernel - Re: [PATCH v5 06/13] lockdep: Implement crossrelease feature

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Message-ID: <20170301072128.GH11663@X58A-UD3R>
Date:   Wed, 1 Mar 2017 16:21:28 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     <mingo@...nel.org>, <tglx@...utronix.de>, <walken@...gle.com>,
        <boqun.feng@...il.com>, <kirill@...temov.name>,
        <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        <iamjoonsoo.kim@....com>, <akpm@...ux-foundation.org>,
        <npiggin@...il.com>, <kernel-team@....com>
Subject: Re: [PATCH v5 06/13] lockdep: Implement crossrelease feature

On Tue, Feb 28, 2017 at 07:15:47PM +0100, Peter Zijlstra wrote:
> On Wed, Jan 18, 2017 at 10:17:32PM +0900, Byungchul Park wrote:
> > +	/*
> > +	 * Each work of workqueue might run in a different context,
> > +	 * thanks to concurrency support of workqueue. So we have to
> > +	 * distinguish each work to avoid false positive.
> > +	 *
> > +	 * TODO: We can also add dependencies between two acquisitions
> > +	 * of different work_id, if they don't cause a sleep so make
> > +	 * the worker stalled.
> > +	 */
> > +	unsigned int		work_id;
> 
> > +/*
> > + * Crossrelease needs to distinguish each work of workqueues.
> > + * Caller is supposed to be a worker.
> > + */
> > +void crossrelease_work_start(void)
> > +{
> > +	if (current->xhlocks)
> > +		current->work_id++;
> > +}
> 
> So what you're trying to do with that 'work_id' thing is basically wipe
> the entire history when we're at the bottom of a context.

Sorry, but I do not understand what you are trying to say.

What I was trying to do with the 'work_id' is to distinguish between
different works, which will be used to check if history locks were held
in the same context as a release one.

> Which is a useful operation, but should arguably also be done on the
> return to userspace path. Any historical lock from before the current
> syscall is irrelevant.

Sorry. Could you explain it more?

> 
> (And we should not be returning to userspace with locks held anyway --
> lockdep already has a check for that).

Yes right. We should not be returning to userspace without reporting it
in that case.