The patch titled rcu: fix rcu_try_flip_waitack_needed() to prevent grace-period stall has been removed from the -mm tree. Its filename was rcu-fix-rcu_try_flip_waitack_needed-to-prevent-grace-period-stall.patch This patch was dropped because it was merged into mainline or a subsystem tree The current -mm tree may be found at http://userweb.kernel.org/~akpm/mmotm/ ------------------------------------------------------ Subject: rcu: fix rcu_try_flip_waitack_needed() to prevent grace-period stall From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx> Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> Cc: Ingo Molnar <mingo@xxxxxxx> Cc: Josh Triplett <josh@xxxxxxxxxx> Cc: Dipankar Sarma <dipankar@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- kernel/rcupreempt.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff -puN kernel/rcupreempt.c~rcu-fix-rcu_try_flip_waitack_needed-to-prevent-grace-period-stall kernel/rcupreempt.c --- a/kernel/rcupreempt.c~rcu-fix-rcu_try_flip_waitack_needed-to-prevent-grace-period-stall +++ a/kernel/rcupreempt.c @@ -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. */ _ Patches currently in -mm which might be from paulmck@xxxxxxxxxxxxxxxxxx are origin.patch linux-next.patch add-rcu_assign_index-if-ever-needed.patch rcu-fix-rcu_try_flip_waitack_needed-to-prevent-grace-period-stall.patch add-warn_on_secs-macro.patch use-warn_on_secs-in-rcupreempth.patch -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html