Messages in this thread | | | Date | Wed, 28 Feb 2007 00:46:01 +0300 | From | Oleg Nesterov <> | Subject | Re: [PATCH] adapt page_lock_anon_vma() to PREEMPT_RCU |
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On 02/27, Andrew Morton wrote: > > > On Sun, 25 Feb 2007 23:06:21 +0300 Oleg Nesterov <oleg@tv-sign.ru> wrote: > > page_lock_anon_vma() uses spin_lock() to block RCU. This doesn't work with > > PREEMPT_RCU, we have to do rcu_read_lock() explicitely. Otherwise, it is > > theoretically possible that slab returns anon_vma's memory to the system > > before we do spin_unlock(&anon_vma->lock). > > > > ... > > > > +static void page_unlock_anon_vma(struct anon_vma *anon_vma) > > +{ > > + spin_unlock(&anon_vma->lock); > > + rcu_read_unlock(); > > } > > It's a bit sad doing a double preempt_disable() for non-PREEMPT_RCU builds.
Actually, we don't in this case. This patch in essence moves "preempt_enable" from "lock" to "unlock" side. Zero impact for non-PREEMPT_RCU builds, except .text grows a bit.
Before this patch, page_lock_anon_vma() does preempt_enable() before return, but this can't help because ->preempt_count was incremented by spin_lock().
> Perhaps we would benefit from a new rcu_read_lock_preempt_rcu() which is a > no-op if !PREEMPT_RCU.
I also thought about things like
rcu_read_lock_when_we_know_that_preemption_disabled() rcu_read_lock_when_we_know_that_irqs_disabled()
which are noops when !PREEMPT_RCU.
Oleg.
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