Messages in this thread Patch in this message |  | | | From | "Paul E. McKenney" <> | | Subject | [PATCH RFC tip/core/rcu 10/28] rcu: Disable preemption in rcu_is_cpu_idle() | | Date | Wed, 2 Nov 2011 13:30:31 -0700 |
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From: Paul E. McKenney <paul.mckenney@linaro.org>
Because rcu_is_cpu_idle() is to be used to check for extended quiescent states in RCU-preempt read-side critical sections, it cannot assume that preemption is disabled. And preemption must be disabled when accessing the dyntick-idle state, because otherwise the following sequence of events could occur:
1. Task A on CPU 1 enters rcu_is_cpu_idle() and picks up the pointer to CPU 1's per-CPU variables.
2. Task B preempts Task A and starts running on CPU 1.
3. Task A migrates to CPU 2.
4. Task B blocks, leaving CPU 1 idle.
5. Task A continues execution on CPU 2, accessing CPU 1's dyntick-idle information using the pointer fetched in step 1 above, and finds that CPU 1 is idle.
6. Task A therefore incorrectly concludes that it is executing in an extended quiescent state, possibly issuing a spurious splat.
Therefore, this commit disables preemption within the rcu_is_cpu_idle() function.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> --- kernel/rcutree.c | 10 +++++++--- 1 files changed, 7 insertions(+), 3 deletions(-) diff --git a/kernel/rcutree.c b/kernel/rcutree.c index abb5167..c097394 100644 --- a/kernel/rcutree.c +++ b/kernel/rcutree.c @@ -556,12 +556,16 @@ void rcu_nmi_exit(void) * rcu_is_cpu_idle - see if RCU thinks that the current CPU is idle * * If the current CPU is in its idle loop and is neither in an interrupt - * or NMI handler, return true. The caller must have at least disabled - * preemption. + * or NMI handler, return true. */ int rcu_is_cpu_idle(void) { - return (atomic_read(&__get_cpu_var(rcu_dynticks).dynticks) & 0x1) == 0; + int ret; + + preempt_disable(); + ret = (atomic_read(&__get_cpu_var(rcu_dynticks).dynticks) & 0x1) == 0; + preempt_enable(); + return ret; } #endif /* #ifdef CONFIG_PROVE_RCU */ -- 1.7.3.2
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