linux-kernel - Re: [PATCH 3/4]percpu_counter: fix code for 32bit systems

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Date:	Wed, 13 Apr 2011 11:03:01 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"cl@...ux.com" <cl@...ux.com>, "tj@...nel.org" <tj@...nel.org>
Subject: Re: [PATCH 3/4]percpu_counter: fix code for 32bit systems

On Wed, 2011-04-13 at 10:47 +0800, Eric Dumazet wrote:
> Le mercredi 13 avril 2011 à 10:41 +0800, Shaohua Li a écrit :
> > On Wed, 2011-04-13 at 10:32 +0800, Eric Dumazet wrote:
> > > Le mercredi 13 avril 2011 à 09:01 +0800, Shaohua Li a écrit :
> > > > On Tue, 2011-04-12 at 17:03 +0800, Eric Dumazet wrote:
> > > > > 
> > > > > Hmm... did you test this with LOCKDEP on ?
> > > > > 
> > > > > You add a possible deadlock here.
> > > > > 
> > > > > Hint : Some percpu_counter are used from irq context.
> > > > there are some places we didn't disable interrupt, for example
> > > > percpu_counter_add. So the API isn't irq safe to me.
> > > > 
> > > 
> > > So what ? Callers must disable IRQ before calling percpu_counter_add(),
> > > and they actually do in network stack. Please check again,
> > > tcp_sockets_allocated for example.
> > Did you check other code? for example, __vm_enough_memory() doesn't
> > disable IRQ before calling percpu_counter_add().
> > 
> 
> Did you read my mails ?
> 
> I said : fix the buggy parts
that's the difference. Why the parts are buggy? what I said is the
interface is never IRQ safe.

>  dont add new bugs or slow down parts that
> are OK.
> 
> 
> > > > > This interface assumes caller take the appropriate locking.
> > > > no comments say this, and some places we don't hold locking.
> > > > for example, meminfo_proc_show.
> > > > 
> > > 
> > > This doesnt answer my question about LOCKDEP ;)
> > > 
> > > Just fix the few callers that might need a fix, since this is the only
> > > way to deal with potential problems without adding performance penalty
> > > (for stable trees)
> > I mean the interface doesn't assume caller should take locking. Since
> > there isn't locking taking, we should make the interface itself correct,
> > instead of fixing caller.
> > 
> 
> No _please_
> 
> Q: Is spin_lock() irq safe ?
> A: No
> 
> Q: Should we make it irq safe ?
> A: just use spin_lock_... variants
I can do this, but please give a reason. If network code is the only
place requiring disable irq, why not network code do it?

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