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From: Peter Zijlstra <peterz@infradead.org>
To: Byungchul Park <byungchul.park@lge.com>
Cc: mingo@kernel.org, tglx@linutronix.de, walken@google.com,
	boqun.feng@gmail.com, kirill@shutemov.name,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	iamjoonsoo.kim@lge.com, akpm@linux-foundation.org,
	npiggin@gmail.com, kernel-team@lge.com
Subject: Re: [PATCH v5 06/13] lockdep: Implement crossrelease feature
Date: Wed, 1 Mar 2017 11:43:28 +0100	[thread overview]
Message-ID: <20170301104328.GD6515@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <20170301072128.GH11663@X58A-UD3R>

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.

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.

I would say this needs to be done for all 'work-queue' like things, and
there are quite a few outside of the obvious ones, smpboot threads and
many other kthreads fall into this category.

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.

> > 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?

Does the above make things clear?

  reply	other threads:[~2017-03-01 10:43 UTC|newest]

Thread overview: 63+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-01-18 13:17 [PATCH v5 00/13] lockdep: Implement crossrelease feature Byungchul Park
2017-01-18 13:17 ` [PATCH v5 01/13] lockdep: Refactor lookup_chain_cache() Byungchul Park
2017-01-19  9:16   ` Boqun Feng
2017-01-19  9:52     ` Byungchul Park
2017-01-26  7:53     ` Byungchul Park
2017-01-18 13:17 ` [PATCH v5 02/13] lockdep: Fix wrong condition to print bug msgs for MAX_LOCKDEP_CHAIN_HLOCKS Byungchul Park
2017-01-18 13:17 ` [PATCH v5 03/13] lockdep: Add a function building a chain between two classes Byungchul Park
2017-01-18 13:17 ` [PATCH v5 04/13] lockdep: Refactor save_trace() Byungchul Park
2017-01-18 13:17 ` [PATCH v5 05/13] lockdep: Pass a callback arg to check_prev_add() to handle stack_trace Byungchul Park
2017-01-26  7:43   ` Byungchul Park
2017-01-18 13:17 ` [PATCH v5 06/13] lockdep: Implement crossrelease feature Byungchul Park
2017-02-28 12:26   ` Peter Zijlstra
2017-02-28 12:45   ` Peter Zijlstra
2017-02-28 12:49     ` Peter Zijlstra
2017-03-01  6:20       ` Byungchul Park
2017-02-28 13:05   ` Peter Zijlstra
2017-02-28 13:28     ` Byungchul Park
2017-02-28 13:35       ` Peter Zijlstra
2017-02-28 14:00         ` Byungchul Park
2017-02-28 13:10   ` Peter Zijlstra
2017-02-28 13:24     ` Byungchul Park
2017-02-28 18:29       ` Peter Zijlstra
2017-03-01  4:40         ` Byungchul Park
2017-03-01 10:45           ` Peter Zijlstra
2017-03-01 12:10             ` Byungchul Park
2017-02-28 13:40   ` Peter Zijlstra
2017-03-01  5:43     ` Byungchul Park
2017-03-01 12:28       ` Peter Zijlstra
2017-03-02 13:40         ` Peter Zijlstra
2017-03-03  0:17           ` Byungchul Park
2017-03-03  8:14             ` Peter Zijlstra
2017-03-03  9:13               ` Peter Zijlstra
2017-03-03  9:32                 ` Peter Zijlstra
2017-03-05  3:33                 ` Byungchul Park
2017-08-10 12:18                 ` [tip:locking/core] locking/lockdep: Avoid creating redundant links tip-bot for Peter Zijlstra
2017-03-05  3:08               ` [PATCH v5 06/13] lockdep: Implement crossrelease feature Byungchul Park
2017-03-07 11:42                 ` Peter Zijlstra
2017-03-03  0:39           ` Byungchul Park
2017-02-28 15:49   ` Peter Zijlstra
2017-03-01  5:17     ` Byungchul Park
2017-03-01  6:18       ` Byungchul Park
2017-03-02  2:52       ` Byungchul Park
2017-02-28 18:15   ` Peter Zijlstra
2017-03-01  7:21     ` Byungchul Park
2017-03-01 10:43       ` Peter Zijlstra [this message]
2017-03-01 12:27         ` Byungchul Park
2017-03-02  4:20     ` Matthew Wilcox
2017-03-02  4:45       ` byungchul.park
2017-03-02 14:39         ` Matthew Wilcox
2017-03-02 23:50           ` Byungchul Park
2017-03-05  8:01             ` Byungchul Park
2017-03-14  7:36     ` Byungchul Park
2017-03-02 13:41   ` Peter Zijlstra
2017-03-02 23:43     ` Byungchul Park
2017-01-18 13:17 ` [PATCH v5 07/13] lockdep: Make print_circular_bug() aware of crossrelease Byungchul Park
2017-01-18 13:17 ` [PATCH v5 08/13] lockdep: Apply crossrelease to completions Byungchul Park
2017-01-18 13:17 ` [PATCH v5 09/13] pagemap.h: Remove trailing white space Byungchul Park
2017-01-18 13:17 ` [PATCH v5 10/13] lockdep: Apply crossrelease to PG_locked locks Byungchul Park
2017-01-18 13:17 ` [PATCH v5 11/13] lockdep: Apply lock_acquire(release) on __Set(__Clear)PageLocked Byungchul Park
2017-01-18 13:17 ` [PATCH v5 12/13] lockdep: Move data of CONFIG_LOCKDEP_PAGELOCK from page to page_ext Byungchul Park
2017-01-18 13:17 ` [PATCH v5 13/13] lockdep: Crossrelease feature documentation Byungchul Park
2017-01-20  9:08   ` [REVISED DOCUMENT] " Byungchul Park
2017-02-20  8:38 ` [PATCH v5 00/13] lockdep: Implement crossrelease feature Byungchul Park

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