linux-kernel - Re: PREEMPT_RCU breaks anon

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Message-ID: <Pine.LNX.4.64.0702242204220.4370@blonde.wat.veritas.com>
Date:	Sat, 24 Feb 2007 22:10:57 +0000 (GMT)
From:	Hugh Dickins <hugh@...itas.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	Oleg Nesterov <oleg@...sign.ru>, dipankar@...ibm.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: PREEMPT_RCU breaks anon_vma locking ?

On Fri, 23 Feb 2007, Paul E. McKenney wrote:
> 
> This look like a valid fix to me, at least as long as the lock is never
> dropped in the meantime (e.g., to do I/O).  If the lock -is- dropped in
> the meantime, then presumably whatever is done to keep the page from
> vanishing should allow an rcu_read_unlock() to be placed after each
> spin_unlock(&...->lock) and an rcu_read_lock() to be placed before each
> spin_lock(&...->lock).

Thankfully no complications of that kind, page_lock_anon_vma is static
to mm/rmap.c, and only used to hold the spin lock while examining page
tables of the vmas in the list, never a need to drop that lock at all.
(Until the day when someone reports such a long list that we start to
worry about the latency.)

Hugh
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