netdev - RE: [PATCH] rebalance locks by converting write_lock_bh towrite_lock+local_bh

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Date:	Mon, 25 Nov 2013 09:40:08 -0000
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Nicholas Mc Guire" <der.herr@...r.at>,
	"David Miller" <davem@...emloft.net>
Cc:	<kuznet@....inr.ac.ru>, <eric.dumazet@...il.com>,
	<roque@...fc.ul.pt>, <peterz@...radead.org>,
	<netdev@...r.kernel.org>
Subject: RE: [PATCH] rebalance locks by converting write_lock_bh towrite_lock+local_bh_disable

> in one of the cases I do not understand the intent behind the split:
> in net/core/sock.c:lock_sock_fast
> 
> 	spin_lock_bh(&sk->sk_lock.slock);
> 	...
>         spin_unlock(&sk->sk_lock.slock);
>         /*
>          * The sk_lock has mutex_lock() semantics here:
>          */
>         mutex_acquire(&sk->sk_lock.dep_map, 0, 0, _RET_IP_);
>         local_bh_enable();
> 
>  I think that
> 
> 	spin_lock_bh(&sk->sk_lock.slock);
> 	...
>         /*
>          * The sk_lock has mutex_lock() semantics here:
>          */
>         mutex_acquire(&sk->sk_lock.dep_map, 0, 0, _RET_IP_);
>         spin_unlock_bh(&sk->sk_lock.slock);
> 
>  should be equivalent ?

You've added a lock ordering that wasn't there before.
Also I suspect that mutex_acquire() might be allowed to sleep,
whereas you shouldn't sleep with a spin lock held.

	David



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