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Subject[PATCH] fix rcu_try_flip_waitack_needed() to prevent grace-period stall
The comment was correct -- need to make the code match the comment.
Without this patch, if a CPU goes dynticks idle (and stays there forever)
in just the right phase of preemptible-RCU grace-period processing,
grace periods stall. The offending sequence of events (courtesy
of Promela/spin, at least after I got the liveness criterion coded
correctly...) is as follows:

o CPU 0 is in dynticks-idle mode. Its dynticks_progress_counter
is (say) 10.

o CPU 0 takes an interrupt, so rcu_irq_enter() increments CPU 0's
dynticks_progress_counter to 11.

o CPU 1 is doing RCU grace-period processing in rcu_try_flip_idle(),
sees rcu_pending(), so invokes dyntick_save_progress_counter(),
which in turn takes a snapshot of CPU 0's dynticks_progress_counter
into CPU 0's rcu_dyntick_snapshot -- now set to 11. CPU 1 then
updates the RCU grace-period state to rcu_try_flip_waitack().

o CPU 0 returns from its interrupt, so rcu_irq_exit() increments
CPU 0's dynticks_progress_counter to 12.

o CPU 1 later invokes rcu_try_flip_waitack(), which notices that
CPU 0 has not yet responded, and hence in turn invokes
rcu_try_flip_waitack_needed(). This function examines the
state of CPU 0's dynticks_progress_counter and rcu_dyntick_snapshot
variables, which it copies to curr (== 12) and snap (== 11),
respectively.

Because curr!=snap, the first condition fails.

Because curr-snap is only 1 and snap is odd, the second
condition fails.

rcu_try_flip_waitack_needed() therefore incorrectly concludes
that it must wait for CPU 0 to explicitly acknowledge the
counter flip.

o CPU 0 remains forever in dynticks-idle mode, never taking
any more hardware interrupts or any NMIs, and never running
any more tasks. (Of course, -something- will usually eventually
happen, which might be why we haven't seen this one in the
wild. Still should be fixed!)

Therefore the grace period never ends. Fix is to make the code match
the comment, as shown below. With this fix, the above scenario
would be satisfied with curr being even, and allow the grace period
to proceed.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
rcupreempt.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff -urpNa -X dontdiff linux-2.6.25-rc6/kernel/rcupreempt.c linux-2.6.25-rc6-rcunohz-if/kernel/rcupreempt.c
--- linux-2.6.25-rc6/kernel/rcupreempt.c 2008-03-16 17:45:17.000000000 -0700
+++ linux-2.6.25-rc6-rcunohz-if/kernel/rcupreempt.c 2008-03-18 20:27:47.000000000 -0700
@@ -569,7 +569,7 @@ rcu_try_flip_waitack_needed(int cpu)
* that this CPU already acknowledged the counter.
*/

- if ((curr - snap) > 2 || (snap & 0x1) == 0)
+ if ((curr - snap) > 2 || (curr & 0x1) == 0)
return 0;

/* We need this CPU to explicitly acknowledge the counter flip. */

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