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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2008-05-12 15:21:06 -0400
committerIngo Molnar <mingo@elte.hu>2008-05-19 04:03:38 -0400
commitd7c0651390b6a03ad53f99faec0ba88109d7191d (patch)
tree9021a1e5cf6f66e296a8871ffbdf376a5c94d9e8 /kernel
parent10aa9d2cf9878757b003023d33ff90a37aa3044b (diff)
rcu: 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> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Josh Triplett <josh@kernel.org> Cc: Dipankar Sarma <dipankar@in.ibm.com> Cc: <stable@kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Ingo Molnar <mingo@elte.hu>
Diffstat (limited to 'kernel')
-rw-r--r--kernel/rcupreempt.c2
1 files changed, 1 insertions, 1 deletions
diff --git a/kernel/rcupreempt.c b/kernel/rcupreempt.c
index aaa7976bd85f..27d0748d8d32 100644
--- a/kernel/rcupreempt.c
+++ b/kernel/rcupreempt.c
@@ -597,7 +597,7 @@ rcu_try_flip_waitack_needed(int cpu)
597 * that this CPU already acknowledged the counter. 597 * that this CPU already acknowledged the counter.
598 */ 598 */
599 599
600 if ((curr - snap) > 2 || (snap & 0x1) == 0) 600 if ((curr - snap) > 2 || (curr & 0x1) == 0)
601 return 0; 601 return 0;
602 602
603 /* We need this CPU to explicitly acknowledge the counter flip. */ 603 /* We need this CPU to explicitly acknowledge the counter flip. */