Re: [PATCH] Avoid calling down_read and down_write during startup
From: Alan Stern
Date: Fri Feb 24 2006 - 12:56:59 EST
On Fri, 24 Feb 2006, Benjamin LaHaise wrote:
> On Fri, Feb 24, 2006 at 11:44:23AM -0500, Alan Stern wrote:
> > In that case you should be worried not about acquiring and releasing the
> > rwsem at the beginning and end of blocking_notifier_call_chain; you should
> > be worried about all the RCU serialization in the core
> > notifier_call_chain routine.
> RCU doesn't synchronize readers.
It does on architectures where smp_read_barrier_depends() expands into
something nontrivial. Maybe that doesn't include any of the machines
you're interested in.
> > The atomic chains are a different matter. The ones that don't run in NMI
> > context could use an rw-spinlock for protection, allowing them also to
> > avoid memory barriers while going through the list. The notifier chains
> > that do run in NMI don't have this luxury. Fortunately I don't think
> > there are very many of them.
> A read lock is a memory barrier. That's why I'm opposed to using non-rcu
> style locking for them.
But RCU-style locking can't be used in situations where the reader may
block. So it's not possible to use it with blocking notifier chains.
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