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+Using RCU's CPU Stall Detector
+
+The CONFIG_RCU_CPU_STALL_DETECTOR kernel config parameter enables
+RCU's CPU stall detector, which detects conditions that unduly delay
+RCU grace periods. The stall detector's idea of what constitutes
+"unduly delayed" is controlled by a pair of C preprocessor macros:
+
+RCU_SECONDS_TILL_STALL_CHECK
+
+ This macro defines the period of time that RCU will wait from
+ the beginning of a grace period until it issues an RCU CPU
+ stall warning. It is normally ten seconds.
+
+RCU_SECONDS_TILL_STALL_RECHECK
+
+ This macro defines the period of time that RCU will wait after
+ issuing a stall warning until it issues another stall warning.
+ It is normally set to thirty seconds.
+
+RCU_STALL_RAT_DELAY
+
+ The CPU stall detector tries to make the offending CPU rat on itself,
+ as this often gives better-quality stack traces. However, if
+ the offending CPU does not detect its own stall in the number
+ of jiffies specified by RCU_STALL_RAT_DELAY, then other CPUs will
+ complain. This is normally set to two jiffies.
+
+The following problems can result in an RCU CPU stall warning:
+
+o A CPU looping in an RCU read-side critical section.
+
+o A CPU looping with interrupts disabled.
+
+o A CPU looping with preemption disabled.
+
+o For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the kernel
+ without invoking schedule().
+
+o A bug in the RCU implementation.
+
+o A hardware failure. This is quite unlikely, but has occurred
+ at least once in a former life. A CPU failed in a running system,
+ becoming unresponsive, but not causing an immediate crash.
+ This resulted in a series of RCU CPU stall warnings, eventually
+ leading the realization that the CPU had failed.
+
+The RCU, RCU-sched, and RCU-bh implementations have CPU stall warning.
+SRCU does not do so directly, but its calls to synchronize_sched() will
+result in RCU-sched detecting any CPU stalls that might be occurring.
+
+To diagnose the cause of the stall, inspect the stack traces. The offending
+function will usually be near the top of the stack. If you have a series
+of stall warnings from a single extended stall, comparing the stack traces
+can often help determine where the stall is occurring, which will usually
+be in the function nearest the top of the stack that stays the same from
+trace to trace.
+
+RCU bugs can often be debugged with the help of CONFIG_RCU_TRACE.