Message-ID: <41A8BFB8.3000804@colorfullife.com>
Date: Sat, 27 Nov 2004 18:56:08 +0100
From: Manfred Spraul <manfred@colorfullife.com>
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To: Oleg Nesterov <oleg@tv-sign.ru>
CC: linux-kernel@vger.kernel.org, Dipankar Sarma <dipankar@in.ibm.com>,
       Andrew Morton <akpm@osdl.org>
Subject: Re: PATCH? rcu: eliminate rcu_ctrlblk.lock
References: <41A8A57A.DD2338BB@tv-sign.ru>
In-Reply-To: <41A8A57A.DD2338BB@tv-sign.ru>
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Oleg Nesterov wrote:

>Hello.
>
>I am trying to understand the rcu implementetion.
>
>I can't understand why rcu_ctrlblk.seqcount is needed.
>It seems to me it can be replaced by a couple of barriers.
>
>  
>

Your patch would add one new corner case:

start: next_pending==1. rcp->cur == 11.
cpu 1: rcu_start_back sets next_pending to 0.
cpu 2: rdp->batch = rcp->cur + 1 [i.e. wait for end of period 12]
cpu 2: notices next_pending == 0, tries to acquire the spinlock [blocks]
cpu 1: rcp->cur++ [i.e. start period 12]
cpu 1: releases the spinlock
cpu 2: gets the spinlock, sets next_pending to 1 and exits.

Now next_pending is 1 [i.e. at the end of grace period 12 grace period 
13 is automatically started], although noone has callbacks waiting for 
period 13.

This is not fatal: the combination is rare, so perhaps we could tolerate 
the race. But on the other hand the sequence locks are outside of the 
hot paths and not much slower than a smp_rmb().

Dipankar - what do you think?

--
    Manfred
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