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Message-ID: <20120412045428.GB2497@linux.vnet.ibm.com> Date: Wed, 11 Apr 2012 21:54:28 -0700 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: David Miller <davem@...emloft.net> Cc: shemminger@...tta.com, mroos@...ux.ee, linux-kernel@...r.kernel.org, netdev@...r.kernel.org Subject: Re: suspicious RCU usage warnings in 3.3.0 On Wed, Apr 11, 2012 at 09:08:43PM -0400, David Miller wrote: > From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> > Date: Wed, 11 Apr 2012 17:45:07 -0700 > > > If I am confused about the simple function call, and if control is really > > passing via an interrupt or exception, then rcu_irq_enter() should be > > called on entry to the interrupt or exception and rcu_irq_exit() should > > be called on exit. > > Hmm, it seems the convention changed such that platforms aren't > supposed to invoke do_softirq() from their trap return trap any more. > It's handled completely by irq_exit(). > > When did that start happening? :-) Heh! It appears that git doesn't go back far enough for me to find the answer to that question. ;-) > Anyways I bet that's the problem, sparc64 invokes do_softirq() in it's > trap return path if softirqs are pending, and that doesn't do any > of the RCU frobbing you mention. The following untested patch that probably does not even build is offered up for your amusement. I don't know enough about SPARC's needs for alignment, handling of branch-delay slots, and so on for this to have any chance of working, but hey! ;-) Thanx, Paul ------------------------------------------------------------------------ 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> --- rtrap_64.S | 7 ------- 1 file changed, 7 deletions(-) diff --git a/arch/sparc/kernel/rtrap_64.S b/arch/sparc/kernel/rtrap_64.S index 77f1b95..9171fc2 100644 --- a/arch/sparc/kernel/rtrap_64.S +++ b/arch/sparc/kernel/rtrap_64.S @@ -20,11 +20,6 @@ .text .align 32 -__handle_softirq: - call do_softirq - nop - ba,a,pt %xcc, __handle_softirq_continue - nop __handle_preemption: call schedule wrpr %g0, RTRAP_PSTATE, %pstate @@ -89,9 +84,7 @@ rtrap: cmp %l1, 0 /* mm/ultra.S:xcall_report_regs KNOWS about this load. */ - bne,pn %icc, __handle_softirq ldx [%sp + PTREGS_OFF + PT_V9_TSTATE], %l1 -__handle_softirq_continue: rtrap_xcall: sethi %hi(0xf << 20), %l4 and %l1, %l4, %l4 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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