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

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Date:   Wed, 1 Mar 2017 21:27:23 +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 Wed, Mar 01, 2017 at 11:43:28AM +0100, Peter Zijlstra wrote:
> On Wed, Mar 01, 2017 at 04:21:28PM +0900, Byungchul Park wrote:
> > 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.
> 
> The effect of changing work_id is that history disappears, yes? That is,
> by changing it, all our hist_locks don't match the context anymore and
> therefore we have no history.

Right. Now I understood your words.

> This is a useful operation.
> 
> You would want to do this at points where you know there will not be any
> dependencies on prior action, and typically at the same points we want
> to not be holding any locks.
> 
> Hence my term: 'bottom of a context', referring to an empty (held) lock
> stack.

Right.

> I would say this needs to be done for all 'work-queue' like things, and

Of course.

> there are quite a few outside of the obvious ones, smpboot threads and
> many other kthreads fall into this category.

Where can I check those?

> Similarly the return to userspace point that I already mentioned.
> 
> I would propose something like:
> 
> 	lockdep_assert_empty();
> 
> Or something similar, which would verify the lock stack is indeed empty
> and wipe our entire hist_lock buffer when cross-release is enabled.

Right. I should do that.

> > > 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.

Let me think more. It looks not a simple problem.

> > 
> > Sorry. Could you explain it more?
> 
> Does the above make things clear?

Perfect. Thank you very much.