Re: [RFC][PATCH 4/6] SRCU free VMAs

[Peter Zijlstra]

On Mon, Oct 20, 2014 at 04:41:45PM -0700, Linus Torvalds wrote:
> On Mon, Oct 20, 2014 at 2:56 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > Manage the VMAs with SRCU such that we can do a lockless VMA lookup.
> 
> Can you explain why srcu, and not plain regular rcu?
> 
> Especially as you then *note* some of the problems srcu can have.
> Making it regular rcu would also seem to make it possible to make the
> seqlock be just a seqcount, no?

Ah, the reason I did the seqlock is because the read side will spin-wait
for &1 to go away. If the write side is preemptible that's horrid. I
used seqlock because that takes a lock (and thus disables preemption) on
the write side, but I could equally have done:

	preempt_disable();
	write_seqcount_begin();

	...

	write_seqcount_end();
	preempt_disable();

Since the lock is indeed superfluous, we're already fully serialized by
mmap_sem in this path.

Using regular RCU isn't sufficient, because of PREEMPT_RCU.
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