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

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Message-ID: <20141023110346.GP21513@worktop.programming.kicks-ass.net>
Date:	Thu, 23 Oct 2014 13:03:46 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	torvalds@...ux-foundation.org, paulmck@...ux.vnet.ibm.com,
	tglx@...utronix.de, akpm@...ux-foundation.org, riel@...hat.com,
	mgorman@...e.de, oleg@...hat.com, mingo@...hat.com,
	minchan@...nel.org, kamezawa.hiroyu@...fujitsu.com,
	viro@...iv.linux.org.uk, dave@...olabs.net,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC][PATCH 4/6] SRCU free VMAs

On Thu, Oct 23, 2014 at 06:14:45PM +0800, Lai Jiangshan wrote:
> 
> >  
> > +struct vm_area_struct *find_vma_srcu(struct mm_struct *mm, unsigned long addr)
> > +{
> > +	struct vm_area_struct *vma;
> > +	unsigned int seq;
> > +
> > +	WARN_ON_ONCE(!srcu_read_lock_held(&vma_srcu));
> > +
> > +	do {
> > +		seq = read_seqbegin(&mm->mm_seq);
> > +		vma = __find_vma(mm, addr);
> 
> will the __find_vma() loops for ever due to the rotations in the RBtree?

No, a rotation takes a tree and generates a tree, furthermore the
rotation has a fairly strict fwd progress guarantee seeing how its now
done with preemption disabled.

Therefore, even if we're in a node that's being rotated up, we can only
'loop' for as long as it takes for the new pointer stores to become
visible on our CPU.

Thus we have a tree descent termination guarantee.
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