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SubjectRe: suspicious RCU usage warnings in 3.3.0
On Fri, Apr 13, 2012 at 02:55:12PM +0300, mroos@linux.ee wrote:
> > 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@linux.vnet.ibm.com>
>
> It works for me on Sun Fire V100 - no more RCU warnings under ping
> flood.
>
> Tested-by: Meelis Roos <mroos@linux.ee>

OK, if this thing is going to actually work, I guess I need to update
the changelog to give credit where it is due, please see below.

My main concern about my patch is my removal of this line:

bne,pn %icc, __handle_softirq

It is quite possible that this should instead change to look as follows:

bne,pn %icc, __handle_preemption

This code is under #ifndef CONFIG_SMP, so Meelis's testing would not
reach it.

Anyway, patch with updated changelog below.

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.

Reported-by: Meelis Roos <mroos@linux.ee>
Suggested-by: David Miller <davem@davemloft.net>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Meelis Roos <mroos@linux.ee>

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


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