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rcu: Disable preemption in rcu_is_cpu_idle()
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> Reviewed-by: Josh Triplett <josh@joshtriplett.org>
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1 changed files with 7 additions and 3 deletions
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@ -556,12 +556,16 @@ void rcu_nmi_exit(void)
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* rcu_is_cpu_idle - see if RCU thinks that the current CPU is idle
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*
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* If the current CPU is in its idle loop and is neither in an interrupt
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* or NMI handler, return true. The caller must have at least disabled
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* preemption.
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* or NMI handler, return true.
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*/
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int rcu_is_cpu_idle(void)
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{
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return (atomic_read(&__get_cpu_var(rcu_dynticks).dynticks) & 0x1) == 0;
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int ret;
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preempt_disable();
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ret = (atomic_read(&__get_cpu_var(rcu_dynticks).dynticks) & 0x1) == 0;
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preempt_enable();
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return ret;
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}
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#endif /* #ifdef CONFIG_PROVE_RCU */
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