netdev - Re: [PATCH] SCTP: fix race between sctp_bind_addr_free() and sctp_bind_addr

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Date:	Wed, 18 May 2011 10:29:18 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jacek Luczak <difrost.kernel@...il.com>
Cc:	netdev@...r.kernel.org, Vlad Yasevich <vladislav.yasevich@...com>
Subject: Re: [PATCH] SCTP: fix race between sctp_bind_addr_free() and
 sctp_bind_addr_conflict()

Le mercredi 18 mai 2011 à 10:06 +0200, Jacek Luczak a écrit :
> 2011/5/18 Eric Dumazet <eric.dumazet@...il.com>:

> > If you're removing items from this list, you must be a writer here, with
> > exclusive access. So rcu_read_lock()/rcu_read_unlock() is not necessary.
> 
> I could agree to some extend ... but strict RCU section IMO is needed here.
> I can check this if the issue exists.
> 

I can tell you for sure rcu_read_lock() is not needed here. Its only
showing confusion from code's author.

Please read Documentation/RCU/listRCU.txt for concise explanations,
line 117.


> > Therefore, I guess following code is better :
> >
> > list_for_each_entry(addr, &bp->address_list, list) {
> >        list_del_rcu(&addr->list);
> >        call_rcu(&addr->rcu, sctp_local_addr_free);
> >        SCTP_DBG_OBJCNT_DEC(addr);
> > }
> >
> > Then, why dont you fix sctp_bind_addr_clean() instead ?
> >
> > if 'struct sctp_sockaddr_entry' is recu protected, then all frees should
> > be protected as well.
> 
> The _clean() as claimed by Vlad is called many times from various places
> in code and this could give a overhead. I guess Vlad would need to comment.

I guess a full review of this code is needed. You'll have to prove
sctp_bind_addr_clean() is always called after one RCU grace period if
you want to leave it as is.

You cant get RCU for free, some rules must be followed or you risk
crashes.



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