Re: [PATCH v4 07/15] lockdep: Implement crossrelease feature

From: Boqun Feng <boqun.feng@gmail.com>
To: Byungchul Park <byungchul.park@lge.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	mingo@kernel.org, tglx@linutronix.de, walken@google.com,
	kirill@shutemov.name, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org, iamjoonsoo.kim@lge.com,
	akpm@linux-foundation.org, npiggin@gmail.com
Subject: Re: [PATCH v4 07/15] lockdep: Implement crossrelease feature
Date: Tue, 17 Jan 2017 14:24:08 +0800	[thread overview]
Message-ID: <20170117062408.GE15084@tardis.cn.ibm.com> (raw)
In-Reply-To: <20170117023341.GG3326@X58A-UD3R>

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On Tue, Jan 17, 2017 at 11:33:41AM +0900, Byungchul Park wrote:
> On Mon, Jan 16, 2017 at 04:13:19PM +0100, Peter Zijlstra wrote:
> > On Fri, Dec 09, 2016 at 02:12:03PM +0900, Byungchul Park wrote:
> > > +	/*
> > > +	 * We assign class_idx here redundantly even though following
> > > +	 * memcpy will cover it, in order to ensure a rcu reader can
> > > +	 * access the class_idx atomically without lock.
> > > +	 *
> > > +	 * Here we assume setting a word-sized variable is atomic.
> > 
> > which one, where?
> 
> I meant xlock_class(xlock) in check_add_plock().
> 
> I was not sure about the following two.
> 
> 1. Is it ordered between following a and b?
>    a. memcpy -> list_add_tail_rcu
>    b. list_for_each_entry_rcu -> load class_idx (xlock_class)
>    I assumed that it's not ordered.
> 2. Does memcpy guarantee atomic store for each word?
>    I assumed that it doesn't.
> 
> But I think I was wrong.. The first might be ordered. I will remove
> the following redundant statement. It'd be orderd, right?
> 

Yes, a and b are ordered, IOW, they could be paired, meaning when we
got the item in a list_for_each_entry_rcu() loop, all memory operations
before the corresponding list_add_tail_rcu() should be observed by us.

Regards,
Boqun

> > 
> > > +	 */
> > > +	xlock->hlock.class_idx = hlock->class_idx;
> > > +	gen_id = (unsigned int)atomic_inc_return(&cross_gen_id);
> > > +	WRITE_ONCE(xlock->gen_id, gen_id);
> > > +	memcpy(&xlock->hlock, hlock, sizeof(struct held_lock));
> > > +	INIT_LIST_HEAD(&xlock->xlock_entry);
> > > +	list_add_tail_rcu(&xlock->xlock_entry, &xlocks_head);
> > 

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