1 files changed, 31 insertions, 10 deletions

diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.txt
index 44c6dcc93d6d..4e959208f736 100644
--- a/Documentation/RCU/stallwarn.txt
+++ b/Documentation/RCU/stallwarn.txt
@@ -1,22 +1,25 @@
 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 set of C preprocessor macros:
+The rcu_cpu_stall_suppress module parameter enables RCU's CPU stall
+detector, which detects conditions that unduly delay RCU grace periods.
+This module parameter enables CPU stall detection by default, but
+may be overridden via boot-time parameter or at runtime via sysfs.
+The stall detector's idea of what constitutes "unduly delayed" is
+controlled by a set of kernel configuration variables and cpp macros:
 
-RCU_SECONDS_TILL_STALL_CHECK
+CONFIG_RCU_CPU_STALL_TIMEOUT
 
-        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.  This time period is normally ten seconds.
+        This kernel configuration parameter defines the period of time
+        that RCU will wait from the beginning of a grace period until it
+        issues an RCU CPU stall warning.  This time period 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
-        for the same stall.  This time period is normally set to thirty
-        seconds.
+        for the same stall.  This time period is normally set to three
+        times the check interval plus thirty seconds.
 
 RCU_STALL_RAT_DELAY
 
@@ -80,6 +83,24 @@ o	A CPU looping with bottom halves disabled.  This condition can
 o       For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the kernel
         without invoking schedule().
 
+o       A CPU-bound real-time task in a CONFIG_PREEMPT kernel, which might
+        happen to preempt a low-priority task in the middle of an RCU
+        read-side critical section.   This is especially damaging if
+        that low-priority task is not permitted to run on any other CPU,
+        in which case the next RCU grace period can never complete, which
+        will eventually cause the system to run out of memory and hang.
+        While the system is in the process of running itself out of
+        memory, you might see stall-warning messages.
+
+o       A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that
+        is running at a higher priority than the RCU softirq threads.
+        This will prevent RCU callbacks from ever being invoked,
+        and in a CONFIG_TREE_PREEMPT_RCU kernel will further prevent
+        RCU grace periods from ever completing.  Either way, the
+        system will eventually run out of memory and hang.  In the
+        CONFIG_TREE_PREEMPT_RCU case, you might see stall-warning
+        messages.
+
 o       A bug in the RCU implementation.
 
 o       A hardware failure.  This is quite unlikely, but has occurred