netdev - Re: [RFC PATCH 1/9] ipvs network name space aware

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Date:	Thu, 21 Oct 2010 10:58:24 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Hans Schillstrom <hans.schillstrom@...csson.com>,
	Daniel Lezcano <daniel.lezcano@...e.fr>,
	"lvs-devel@...r.kernel.org" <lvs-devel@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"netfilter-devel@...r.kernel.org" <netfilter-devel@...r.kernel.org>,
	"horms@...ge.net.au" <horms@...ge.net.au>, "ja@....bg" <ja@....bg>,
	"wensong@...ux-vs.org" <wensong@...ux-vs.org>
Subject: Re: [RFC PATCH 1/9] ipvs network name space aware


> You said that there were a lot of "stepi" commands to get through
> rcu_read_lock() on x86_64.  This is quite surprising, especially if you
> built with CONFIG_RCU_TREE.  Even if you built with CONFIG_PREEMPT_RCU_TREE,
> you should only see something like the following from rcu_read_lock():
> 
> 000000b7 <__rcu_read_lock>:
>       b7:	55                   	push   %ebp
>       b8:	64 a1 00 00 00 00    	mov    %fs:0x0,%eax
>       be:	ff 80 80 01 00 00    	incl   0x180(%eax)
>       c4:	89 e5                	mov    %esp,%ebp
>       c6:	5d                   	pop    %ebp
>       c7:	c3                   	ret    
> 
> Unless you have some sort of debugging options turned on.  Or unless
> six instructions counts for "quite many" stepi commands.  ;-)
> 

Paul, this should be inlined, dont you think ?

Also, I dont understand why we use ACCESS_ONCE() in rcu_read_lock()

ACCESS_ONCE(current->rcu_read_lock_nesting)++;

Apparently, some compilers are a bit noisy here.

mov    0x1b0(%rdx),%eax
inc    %eax
mov    %eax,0x1b0(%rdx)

instead of :

incl   0x1b0(%rax)

So if the ACCESS_ONCE() is needed, we might add a comment, because it's
not obvious ;)



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