netdev - Re: netif_poll_enable() & barrier

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Message-Id: <20061228.210956.85409699.davem@davemloft.net>
Date:	Thu, 28 Dec 2006 21:09:56 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	benh@....ibm.com
Cc:	netdev@...r.kernel.org
Subject: Re: netif_poll_enable() & barrier

From: Benjamin Herrenschmidt <benh@....ibm.com>
Date: Wed, 20 Dec 2006 14:44:12 +1100

> I stumbled accross what might be a bug on out of order architecture:
> 
> netif_poll_enable() only does a clear_bit(). However,
> netif_poll_disable/enable pairs are often used as simili-spinlocks.
> 
> (netif_poll_enable() has pretty much spin_lock semantics except that it
> schedules instead of looping).
> 
> Thus, shouldn't netif_poll_disable() do an smp_wmb(); before clearing
> the bit to make sure that any stores done within the poll-disabled
> section are properly visible to the rest of the system before clearing
> the bit ?

Although I couldn't find a problematic case with any current
in-tree drivers, it's better to be safe than sorry :-)

So I'll add a smp_mb__before_clear_bit() to netif_poll_enable() :)
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