linux-kernel - Re: suspicious RCU usage warnings in 3.3.0

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Message-ID: <alpine.SOC.1.00.1204131444150.2736@math.ut.ee>
Date:	Fri, 13 Apr 2012 14:55:12 +0300 (EEST)
From:	mroos@...ux.ee
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	David Miller <davem@...emloft.net>, shemminger@...tta.com,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	netdev@...r.kernel.org
Subject: Re: suspicious RCU usage warnings in 3.3.0

> sparc64: Eliminate obsolete __handle_softirq() function
> 
> The invocation of softirq is now handled by irq_exit(), so there is no
> need for sparc64 to invoke it on the trap-return path.  In fact, doing so
> is a bug because if the trap occurred in the idle loop, this invocation
> can result in lockdep-RCU failures.  The problem is that RCU ignores idle
> CPUs, and the sparc64 trap-return path to the softirq handlers fails to
> tell RCU that the CPU must be considered non-idle while those handlers
> are executing.  This means that RCU is ignoring any RCU read-side critical
> sections in those handlers, which in turn means that RCU-protected data
> can be yanked out from under those read-side critical sections.
> 
> The shiny new lockdep-RCU ability to detect RCU read-side critical sections
> that RCU is ignoring located this problem.
> 
> The fix is straightforward: Make sparc64 stop manually invoking the
> softirq handlers.
> 
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

It works for me on Sun Fire V100 - no more RCU warnings under ping 
flood.

Tested-by: Meelis Roos <mroos@...ux.ee>

-- 
Meelis Roos (mroos@...ux.ee)
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