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rcu: Document SRCU dead-CPU capabilities, emphasize read-side limits
The current documentation did not help someone grepping for SRCU to learn that disabling preemption is not a replacement for srcu_read_lock(), so upgrade the documentation to bring this out, not just for SRCU, but also for RCU-bh. Also document the fact that SRCU readers are respected on CPUs executing in user mode, idle CPUs, and even on offline CPUs. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org> Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
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@ -310,6 +310,12 @@ over a rather long period of time, but improvements are always welcome!
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code under the influence of preempt_disable(), you instead
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need to use synchronize_irq() or synchronize_sched().
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This same limitation also applies to synchronize_rcu_bh()
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and synchronize_srcu(), as well as to the asynchronous and
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expedited forms of the three primitives, namely call_rcu(),
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call_rcu_bh(), call_srcu(), synchronize_rcu_expedited(),
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synchronize_rcu_bh_expedited(), and synchronize_srcu_expedited().
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12. Any lock acquired by an RCU callback must be acquired elsewhere
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with softirq disabled, e.g., via spin_lock_irqsave(),
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spin_lock_bh(), etc. Failing to disable irq on a given
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@ -873,7 +873,7 @@ d. Do you need to treat NMI handlers, hardirq handlers,
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and code segments with preemption disabled (whether
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via preempt_disable(), local_irq_save(), local_bh_disable(),
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or some other mechanism) as if they were explicit RCU readers?
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If so, you need RCU-sched.
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If so, RCU-sched is the only choice that will work for you.
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e. Do you need RCU grace periods to complete even in the face
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of softirq monopolization of one or more of the CPUs? For
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@ -884,7 +884,12 @@ f. Is your workload too update-intensive for normal use of
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RCU, but inappropriate for other synchronization mechanisms?
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If so, consider SLAB_DESTROY_BY_RCU. But please be careful!
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g. Otherwise, use RCU.
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g. Do you need read-side critical sections that are respected
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even though they are in the middle of the idle loop, during
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user-mode execution, or on an offlined CPU? If so, SRCU is the
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only choice that will work for you.
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h. Otherwise, use RCU.
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Of course, this all assumes that you have determined that RCU is in fact
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the right tool for your job.
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