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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.