netdev - RE: [PATCH] net: Fix race condition on receive path.

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Date:	Fri, 05 Mar 2010 15:44:27 -0800
From:	Daniel Walker <dwalker@...eaurora.org>
To:	"Harford, Jim" <c_jharfo@...cinc.com>
Cc:	Stephen Hemminger <shemminger@...tta.com>,
	"David S. Miller" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"Smith, Alan" <agsmith@...cinc.com>
Subject: RE: [PATCH] net: Fix race condition on receive path.

On Fri, 2010-03-05 at 15:41 -0800, Harford, Jim wrote:
> It appears that this patch is no longer necessary.  It was made against 2.6.29, but I see that more recent kernel versions don't have the problem code.  For a more detailed explanation, see below.  All code references are in routine process_backlog(), file net/core/dev.c.
> 
> In kernel version 2.6.27.45, __napi_complete() is invoked BEFORE interrupts are re-enabled.  Thus, the receive queue status is cleaned up before another interrupt (due to a receive packet) can occur.  This is good design.
> 
> In kernel version 2.6.29, git commit ID 303c6a025 inverts this ordering.  Routine napi_complete() is invoked AFTER interrupts are re-enabled.  We observed interrupts taken after interrupts were re-enabled, but before napi_complete cleaned up the receive queue.  This would then shut down the processing of subsequent received packets.
> 
> In kernel versions 2.6.30.10 and later, the sequence of operations is identical to 2.6.27.45, so there is no problem.
> 
> Jim Harford
> Qualcomm Innovation Center

Ok, I guess we can ignore this one then.

Daniel

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