Message-ID: <1402465419.3645.453.camel@edumazet-glaptop2.roam.corp.google.com>
Subject: Re: [PATCH v2] vmalloc: use rcu list iterator to reduce
 vmap_area_lock contention
From: Eric Dumazet <eric.dumazet@gmail.com>
To: Peter Hurley <peter@hurleysoftware.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Zhang Yanfei <zhangyanfei.yes@gmail.com>,
        Johannes Weiner <hannes@cmpxchg.org>, Andi Kleen <andi@firstfloor.org>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Richard Yao <ryao@gentoo.org>
Date: Tue, 10 Jun 2014 22:43:39 -0700
In-Reply-To: <5397CDC3.1050809@hurleysoftware.com>
References: <1402453146-10057-1-git-send-email-iamjoonsoo.kim@lge.com>
	 <5397CDC3.1050809@hurleysoftware.com>
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Sender: linux-kernel-owner@vger.kernel.org

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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