linux-kernel - Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by WRITE

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Message-ID: <20160523210455.7rycskjo7n5rw73k@linux-uzut.site>
Date:	Mon, 23 May 2016 14:04:55 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Jason Low <jason.low2@....com>,
	Peter Hurley <peter@...leysoftware.com>, jason.low2@...com,
	Waiman Long <waiman.long@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Dave Chinner <david@...morbit.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by
 WRITE_ONCE

On Mon, 23 May 2016, Paul E. McKenney wrote:

>But rcu_read_lock() does not exclude updates, which is one reason why
>pointer reads use rcu_dereference() rather than normal assignments.

Yes, I was referring to readers. With updates, otoh, are done holding a number of locks.

>
>So I do not believe that rcu_read_lock() is helping you in this case.
>
>That said, it is a bit hard to imagine the compiler tearing a load from
>an int...

Oh, right, good point.

Thanks,
Davidlohr