linux-kernel - Re: [PATCH v2] vmalloc: use rcu list iterator to reduce vmap_area

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Date:	Tue, 10 Jun 2014 22:43:39 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Zhang Yanfei <zhangyanfei.yes@...il.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Andi Kleen <andi@...stfloor.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Richard Yao <ryao@...too.org>
Subject: Re: [PATCH v2] vmalloc: use rcu list iterator to reduce
 vmap_area_lock contention

On Tue, 2014-06-10 at 23:32 -0400, Peter Hurley wrote:

> While rcu list traversal over the vmap_area_list is safe, this may
> arrive at different results than the spinlocked version. The rcu list
> traversal version will not be a 'snapshot' of a single, valid instant
> of the entire vmap_area_list, but rather a potential amalgam of
> different list states.
> 
> This is because the vmap_area_list can continue to change during
> list traversal.


As soon as we exit from get_vmalloc_info(), information can be obsolete
anyway, especially if we held a spinlock for the whole list traversal.

So using the spinlock is certainly not protecting anything in this
regard.



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