3 files changed, 582 insertions, 0 deletions

diff --git a/Documentation/RCU/00-INDEX b/Documentation/RCU/00-INDEX
index 0f2a8d081681..9bb62f7b89c3 100644
--- a/Documentation/RCU/00-INDEX
+++ b/Documentation/RCU/00-INDEX
@@ -18,6 +18,8 @@ RTFP.txt
         - List of RCU papers (bibliography) going back to 1980.
 torture.txt
         - RCU Torture Test Operation (CONFIG_RCU_TORTURE_TEST)
+trace.txt
+        - CONFIG_RCU_TRACE debugfs files and formats
 UP.txt
         - RCU on Uniprocessor Systems
 whatisRCU.txt
diff --git a/Documentation/RCU/rculist_nulls.txt b/Documentation/RCU/rculist_nulls.txt
new file mode 100644
index 000000000000..239f542d48ba
--- /dev/null
+++ b/Documentation/RCU/rculist_nulls.txt
@@ -0,0 +1,167 @@
+Using hlist_nulls to protect read-mostly linked lists and
+objects using SLAB_DESTROY_BY_RCU allocations.
+
+Please read the basics in Documentation/RCU/listRCU.txt
+
+Using special makers (called 'nulls') is a convenient way
+to solve following problem :
+
+A typical RCU linked list managing objects which are
+allocated with SLAB_DESTROY_BY_RCU kmem_cache can
+use following algos :
+
+1) Lookup algo
+--------------
+rcu_read_lock()
+begin:
+obj = lockless_lookup(key);
+if (obj) {
+  if (!try_get_ref(obj)) // might fail for free objects
+    goto begin;
+  /*
+   * Because a writer could delete object, and a writer could
+   * reuse these object before the RCU grace period, we
+   * must check key after geting the reference on object
+   */
+  if (obj->key != key) { // not the object we expected
+     put_ref(obj);
+     goto begin;
+   }
+}
+rcu_read_unlock();
+
+Beware that lockless_lookup(key) cannot use traditional hlist_for_each_entry_rcu()
+but a version with an additional memory barrier (smp_rmb())
+
+lockless_lookup(key)
+{
+   struct hlist_node *node, *next;
+   for (pos = rcu_dereference((head)->first);
+          pos && ({ next = pos->next; smp_rmb(); prefetch(next); 1; }) &&
+          ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1; });
+          pos = rcu_dereference(next))
+      if (obj->key == key)
+         return obj;
+   return NULL;
+
+And note the traditional hlist_for_each_entry_rcu() misses this smp_rmb() :
+
+   struct hlist_node *node;
+   for (pos = rcu_dereference((head)->first);
+                pos && ({ prefetch(pos->next); 1; }) &&
+                ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1; });
+                pos = rcu_dereference(pos->next))
+      if (obj->key == key)
+         return obj;
+   return NULL;
+}
+
+Quoting Corey Minyard :
+
+"If the object is moved from one list to another list in-between the
+ time the hash is calculated and the next field is accessed, and the
+ object has moved to the end of a new list, the traversal will not
+ complete properly on the list it should have, since the object will
+ be on the end of the new list and there's not a way to tell it's on a
+ new list and restart the list traversal.  I think that this can be
+ solved by pre-fetching the "next" field (with proper barriers) before
+ checking the key."
+
+2) Insert algo :
+----------------
+
+We need to make sure a reader cannot read the new 'obj->obj_next' value
+and previous value of 'obj->key'. Or else, an item could be deleted
+from a chain, and inserted into another chain. If new chain was empty
+before the move, 'next' pointer is NULL, and lockless reader can
+not detect it missed following items in original chain.
+
+/*
+ * Please note that new inserts are done at the head of list,
+ * not in the middle or end.
+ */
+obj = kmem_cache_alloc(...);
+lock_chain(); // typically a spin_lock()
+obj->key = key;
+atomic_inc(&obj->refcnt);
+/*
+ * we need to make sure obj->key is updated before obj->next
+ */
+smp_wmb();
+hlist_add_head_rcu(&obj->obj_node, list);
+unlock_chain(); // typically a spin_unlock()
+
+
+3) Remove algo
+--------------
+Nothing special here, we can use a standard RCU hlist deletion.
+But thanks to SLAB_DESTROY_BY_RCU, beware a deleted object can be reused
+very very fast (before the end of RCU grace period)
+
+if (put_last_reference_on(obj) {
+   lock_chain(); // typically a spin_lock()
+   hlist_del_init_rcu(&obj->obj_node);
+   unlock_chain(); // typically a spin_unlock()
+   kmem_cache_free(cachep, obj);
+}
+
+
+
+--------------------------------------------------------------------------
+With hlist_nulls we can avoid extra smp_rmb() in lockless_lookup()
+and extra smp_wmb() in insert function.
+
+For example, if we choose to store the slot number as the 'nulls'
+end-of-list marker for each slot of the hash table, we can detect
+a race (some writer did a delete and/or a move of an object
+to another chain) checking the final 'nulls' value if
+the lookup met the end of chain. If final 'nulls' value
+is not the slot number, then we must restart the lookup at
+the begining. If the object was moved to same chain,
+then the reader doesnt care : It might eventually
+scan the list again without harm.
+
+
+1) lookup algo
+
+ head = &table[slot];
+ rcu_read_lock();
+begin:
+ hlist_nulls_for_each_entry_rcu(obj, node, head, member) {
+   if (obj->key == key) {
+      if (!try_get_ref(obj)) // might fail for free objects
+         goto begin;
+      if (obj->key != key) { // not the object we expected
+         put_ref(obj);
+         goto begin;
+      }
+  goto out;
+ }
+/*
+ * if the nulls value we got at the end of this lookup is
+ * not the expected one, we must restart lookup.
+ * We probably met an item that was moved to another chain.
+ */
+ if (get_nulls_value(node) != slot)
+   goto begin;
+ obj = NULL;
+
+out:
+ rcu_read_unlock();
+
+2) Insert function :
+--------------------
+
+/*
+ * Please note that new inserts are done at the head of list,
+ * not in the middle or end.
+ */
+obj = kmem_cache_alloc(cachep);
+lock_chain(); // typically a spin_lock()
+obj->key = key;
+atomic_set(&obj->refcnt, 1);
+/*
+ * insert obj in RCU way (readers might be traversing chain)
+ */
+hlist_nulls_add_head_rcu(&obj->obj_node, list);
+unlock_chain(); // typically a spin_unlock()
diff --git a/Documentation/RCU/trace.txt b/Documentation/RCU/trace.txt
new file mode 100644
index 000000000000..068848240a8b
--- /dev/null
+++ b/Documentation/RCU/trace.txt
@@ -0,0 +1,413 @@
+CONFIG_RCU_TRACE debugfs Files and Formats
+
+
+The rcupreempt and rcutree implementations of RCU provide debugfs trace
+output that summarizes counters and state.  This information is useful for
+debugging RCU itself, and can sometimes also help to debug abuses of RCU.
+Note that the rcuclassic implementation of RCU does not provide debugfs
+trace output.
+
+The following sections describe the debugfs files and formats for
+preemptable RCU (rcupreempt) and hierarchical RCU (rcutree).
+
+
+Preemptable RCU debugfs Files and Formats
+
+This implementation of RCU provides three debugfs files under the
+top-level directory RCU: rcu/rcuctrs (which displays the per-CPU
+counters used by preemptable RCU) rcu/rcugp (which displays grace-period
+counters), and rcu/rcustats (which internal counters for debugging RCU).
+
+The output of "cat rcu/rcuctrs" looks as follows:
+
+CPU last cur F M
+  0    5  -5 0 0
+  1   -1   0 0 0
+  2    0   1 0 0
+  3    0   1 0 0
+  4    0   1 0 0
+  5    0   1 0 0
+  6    0   2 0 0
+  7    0  -1 0 0
+  8    0   1 0 0
+ggp = 26226, state = waitzero
+
+The per-CPU fields are as follows:
+
+o       "CPU" gives the CPU number.  Offline CPUs are not displayed.
+
+o       "last" gives the value of the counter that is being decremented
+        for the current grace period phase.  In the example above,
+        the counters sum to 4, indicating that there are still four
+        RCU read-side critical sections still running that started
+        before the last counter flip.
+
+o       "cur" gives the value of the counter that is currently being
+        both incremented (by rcu_read_lock()) and decremented (by
+        rcu_read_unlock()).  In the example above, the counters sum to
+        1, indicating that there is only one RCU read-side critical section
+        still running that started after the last counter flip.
+
+o       "F" indicates whether RCU is waiting for this CPU to acknowledge
+        a counter flip.  In the above example, RCU is not waiting on any,
+        which is consistent with the state being "waitzero" rather than
+        "waitack".
+
+o       "M" indicates whether RCU is waiting for this CPU to execute a
+        memory barrier.  In the above example, RCU is not waiting on any,
+        which is consistent with the state being "waitzero" rather than
+        "waitmb".
+
+o       "ggp" is the global grace-period counter.
+
+o       "state" is the RCU state, which can be one of the following:
+
+        o       "idle": there is no grace period in progress.
+
+        o       "waitack": RCU just incremented the global grace-period
+                counter, which has the effect of reversing the roles of
+                the "last" and "cur" counters above, and is waiting for
+                all the CPUs to acknowledge the flip.  Once the flip has
+                been acknowledged, CPUs will no longer be incrementing
+                what are now the "last" counters, so that their sum will
+                decrease monotonically down to zero.
+
+        o       "waitzero": RCU is waiting for the sum of the "last" counters
+                to decrease to zero.
+
+        o       "waitmb": RCU is waiting for each CPU to execute a memory
+                barrier, which ensures that instructions from a given CPU's
+                last RCU read-side critical section cannot be reordered
+                with instructions following the memory-barrier instruction.
+
+The output of "cat rcu/rcugp" looks as follows:
+
+oldggp=48870  newggp=48873
+
+Note that reading from this file provokes a synchronize_rcu().  The
+"oldggp" value is that of "ggp" from rcu/rcuctrs above, taken before
+executing the synchronize_rcu(), and the "newggp" value is also the
+"ggp" value, but taken after the synchronize_rcu() command returns.
+
+
+The output of "cat rcu/rcugp" looks as follows:
+
+na=1337955 nl=40 wa=1337915 wl=44 da=1337871 dl=0 dr=1337871 di=1337871
+1=50989 e1=6138 i1=49722 ie1=82 g1=49640 a1=315203 ae1=265563 a2=49640
+z1=1401244 ze1=1351605 z2=49639 m1=5661253 me1=5611614 m2=49639
+
+These are counters tracking internal preemptable-RCU events, however,
+some of them may be useful for debugging algorithms using RCU.  In
+particular, the "nl", "wl", and "dl" values track the number of RCU
+callbacks in various states.  The fields are as follows:
+
+o       "na" is the total number of RCU callbacks that have been enqueued
+        since boot.
+
+o       "nl" is the number of RCU callbacks waiting for the previous
+        grace period to end so that they can start waiting on the next
+        grace period.
+
+o       "wa" is the total number of RCU callbacks that have started waiting
+        for a grace period since boot.  "na" should be roughly equal to
+        "nl" plus "wa".
+
+o       "wl" is the number of RCU callbacks currently waiting for their
+        grace period to end.
+
+o       "da" is the total number of RCU callbacks whose grace periods
+        have completed since boot.  "wa" should be roughly equal to
+        "wl" plus "da".
+
+o       "dr" is the total number of RCU callbacks that have been removed
+        from the list of callbacks ready to invoke.  "dr" should be roughly
+        equal to "da".
+
+o       "di" is the total number of RCU callbacks that have been invoked
+        since boot.  "di" should be roughly equal to "da", though some
+        early versions of preemptable RCU had a bug so that only the
+        last CPU's count of invocations was displayed, rather than the
+        sum of all CPU's counts.
+
+o       "1" is the number of calls to rcu_try_flip().  This should be
+        roughly equal to the sum of "e1", "i1", "a1", "z1", and "m1"
+        described below.  In other words, the number of times that
+        the state machine is visited should be equal to the sum of the
+        number of times that each state is visited plus the number of
+        times that the state-machine lock acquisition failed.
+
+o       "e1" is the number of times that rcu_try_flip() was unable to
+        acquire the fliplock.
+
+o       "i1" is the number of calls to rcu_try_flip_idle().
+
+o       "ie1" is the number of times rcu_try_flip_idle() exited early
+        due to the calling CPU having no work for RCU.
+
+o       "g1" is the number of times that rcu_try_flip_idle() decided
+        to start a new grace period.  "i1" should be roughly equal to
+        "ie1" plus "g1".
+
+o       "a1" is the number of calls to rcu_try_flip_waitack().
+
+o       "ae1" is the number of times that rcu_try_flip_waitack() found
+        that at least one CPU had not yet acknowledge the new grace period
+        (AKA "counter flip").
+
+o       "a2" is the number of time rcu_try_flip_waitack() found that
+        all CPUs had acknowledged.  "a1" should be roughly equal to
+        "ae1" plus "a2".  (This particular output was collected on
+        a 128-CPU machine, hence the smaller-than-usual fraction of
+        calls to rcu_try_flip_waitack() finding all CPUs having already
+        acknowledged.)
+
+o       "z1" is the number of calls to rcu_try_flip_waitzero().
+
+o       "ze1" is the number of times that rcu_try_flip_waitzero() found
+        that not all of the old RCU read-side critical sections had
+        completed.
+
+o       "z2" is the number of times that rcu_try_flip_waitzero() finds
+        the sum of the counters equal to zero, in other words, that
+        all of the old RCU read-side critical sections had completed.
+        The value of "z1" should be roughly equal to "ze1" plus
+        "z2".
+
+o       "m1" is the number of calls to rcu_try_flip_waitmb().
+
+o       "me1" is the number of times that rcu_try_flip_waitmb() finds
+        that at least one CPU has not yet executed a memory barrier.
+
+o       "m2" is the number of times that rcu_try_flip_waitmb() finds that
+        all CPUs have executed a memory barrier.
+
+
+Hierarchical RCU debugfs Files and Formats
+
+This implementation of RCU provides three debugfs files under the
+top-level directory RCU: rcu/rcudata (which displays fields in struct
+rcu_data), rcu/rcugp (which displays grace-period counters), and
+rcu/rcuhier (which displays the struct rcu_node hierarchy).
+
+The output of "cat rcu/rcudata" looks as follows:
+
+rcu:
+  0 c=4011 g=4012 pq=1 pqc=4011 qp=0 rpfq=1 rp=3c2a dt=23301/73 dn=2 df=1882 of=0 ri=2126 ql=2 b=10
+  1 c=4011 g=4012 pq=1 pqc=4011 qp=0 rpfq=3 rp=39a6 dt=78073/1 dn=2 df=1402 of=0 ri=1875 ql=46 b=10
+  2 c=4010 g=4010 pq=1 pqc=4010 qp=0 rpfq=-5 rp=1d12 dt=16646/0 dn=2 df=3140 of=0 ri=2080 ql=0 b=10
+  3 c=4012 g=4013 pq=1 pqc=4012 qp=1 rpfq=3 rp=2b50 dt=21159/1 dn=2 df=2230 of=0 ri=1923 ql=72 b=10
+  4 c=4012 g=4013 pq=1 pqc=4012 qp=1 rpfq=3 rp=1644 dt=5783/1 dn=2 df=3348 of=0 ri=2805 ql=7 b=10
+  5 c=4012 g=4013 pq=0 pqc=4011 qp=1 rpfq=3 rp=1aac dt=5879/1 dn=2 df=3140 of=0 ri=2066 ql=10 b=10
+  6 c=4012 g=4013 pq=1 pqc=4012 qp=1 rpfq=3 rp=ed8 dt=5847/1 dn=2 df=3797 of=0 ri=1266 ql=10 b=10
+  7 c=4012 g=4013 pq=1 pqc=4012 qp=1 rpfq=3 rp=1fa2 dt=6199/1 dn=2 df=2795 of=0 ri=2162 ql=28 b=10
+rcu_bh:
+  0 c=-268 g=-268 pq=1 pqc=-268 qp=0 rpfq=-145 rp=21d6 dt=23301/73 dn=2 df=0 of=0 ri=0 ql=0 b=10
+  1 c=-268 g=-268 pq=1 pqc=-268 qp=1 rpfq=-170 rp=20ce dt=78073/1 dn=2 df=26 of=0 ri=5 ql=0 b=10
+  2 c=-268 g=-268 pq=1 pqc=-268 qp=1 rpfq=-83 rp=fbd dt=16646/0 dn=2 df=28 of=0 ri=4 ql=0 b=10
+  3 c=-268 g=-268 pq=1 pqc=-268 qp=0 rpfq=-105 rp=178c dt=21159/1 dn=2 df=28 of=0 ri=2 ql=0 b=10
+  4 c=-268 g=-268 pq=1 pqc=-268 qp=1 rpfq=-30 rp=b54 dt=5783/1 dn=2 df=32 of=0 ri=0 ql=0 b=10
+  5 c=-268 g=-268 pq=1 pqc=-268 qp=1 rpfq=-29 rp=df5 dt=5879/1 dn=2 df=30 of=0 ri=3 ql=0 b=10
+  6 c=-268 g=-268 pq=1 pqc=-268 qp=1 rpfq=-28 rp=788 dt=5847/1 dn=2 df=32 of=0 ri=0 ql=0 b=10
+  7 c=-268 g=-268 pq=1 pqc=-268 qp=1 rpfq=-53 rp=1098 dt=6199/1 dn=2 df=30 of=0 ri=3 ql=0 b=10
+
+The first section lists the rcu_data structures for rcu, the second for
+rcu_bh.  Each section has one line per CPU, or eight for this 8-CPU system.
+The fields are as follows:
+
+o       The number at the beginning of each line is the CPU number.
+        CPUs numbers followed by an exclamation mark are offline,
+        but have been online at least once since boot.  There will be
+        no output for CPUs that have never been online, which can be
+        a good thing in the surprisingly common case where NR_CPUS is
+        substantially larger than the number of actual CPUs.
+
+o       "c" is the count of grace periods that this CPU believes have
+        completed.  CPUs in dynticks idle mode may lag quite a ways
+        behind, for example, CPU 4 under "rcu" above, which has slept
+        through the past 25 RCU grace periods.  It is not unusual to
+        see CPUs lagging by thousands of grace periods.
+
+o       "g" is the count of grace periods that this CPU believes have
+        started.  Again, CPUs in dynticks idle mode may lag behind.
+        If the "c" and "g" values are equal, this CPU has already
+        reported a quiescent state for the last RCU grace period that
+        it is aware of, otherwise, the CPU believes that it owes RCU a
+        quiescent state.
+
+o       "pq" indicates that this CPU has passed through a quiescent state
+        for the current grace period.  It is possible for "pq" to be
+        "1" and "c" different than "g", which indicates that although
+        the CPU has passed through a quiescent state, either (1) this
+        CPU has not yet reported that fact, (2) some other CPU has not
+        yet reported for this grace period, or (3) both.
+
+o       "pqc" indicates which grace period the last-observed quiescent
+        state for this CPU corresponds to.  This is important for handling
+        the race between CPU 0 reporting an extended dynticks-idle
+        quiescent state for CPU 1 and CPU 1 suddenly waking up and
+        reporting its own quiescent state.  If CPU 1 was the last CPU
+        for the current grace period, then the CPU that loses this race
+        will attempt to incorrectly mark CPU 1 as having checked in for
+        the next grace period!
+
+o       "qp" indicates that RCU still expects a quiescent state from
+        this CPU.
+
+o       "rpfq" is the number of rcu_pending() calls on this CPU required
+        to induce this CPU to invoke force_quiescent_state().
+
+o       "rp" is low-order four hex digits of the count of how many times
+        rcu_pending() has been invoked on this CPU.
+
+o       "dt" is the current value of the dyntick counter that is incremented
+        when entering or leaving dynticks idle state, either by the
+        scheduler or by irq.  The number after the "/" is the interrupt
+        nesting depth when in dyntick-idle state, or one greater than
+        the interrupt-nesting depth otherwise.
+
+        This field is displayed only for CONFIG_NO_HZ kernels.
+
+o       "dn" is the current value of the dyntick counter that is incremented
+        when entering or leaving dynticks idle state via NMI.  If both
+        the "dt" and "dn" values are even, then this CPU is in dynticks
+        idle mode and may be ignored by RCU.  If either of these two
+        counters is odd, then RCU must be alert to the possibility of
+        an RCU read-side critical section running on this CPU.
+
+        This field is displayed only for CONFIG_NO_HZ kernels.
+
+o       "df" is the number of times that some other CPU has forced a
+        quiescent state on behalf of this CPU due to this CPU being in
+        dynticks-idle state.
+
+        This field is displayed only for CONFIG_NO_HZ kernels.
+
+o       "of" is the number of times that some other CPU has forced a
+        quiescent state on behalf of this CPU due to this CPU being
+        offline.  In a perfect world, this might neve happen, but it
+        turns out that offlining and onlining a CPU can take several grace
+        periods, and so there is likely to be an extended period of time
+        when RCU believes that the CPU is online when it really is not.
+        Please note that erring in the other direction (RCU believing a
+        CPU is offline when it is really alive and kicking) is a fatal
+        error, so it makes sense to err conservatively.
+
+o       "ri" is the number of times that RCU has seen fit to send a
+        reschedule IPI to this CPU in order to get it to report a
+        quiescent state.
+
+o       "ql" is the number of RCU callbacks currently residing on
+        this CPU.  This is the total number of callbacks, regardless
+        of what state they are in (new, waiting for grace period to
+        start, waiting for grace period to end, ready to invoke).
+
+o       "b" is the batch limit for this CPU.  If more than this number
+        of RCU callbacks is ready to invoke, then the remainder will
+        be deferred.
+
+
+The output of "cat rcu/rcugp" looks as follows:
+
+rcu: completed=33062  gpnum=33063
+rcu_bh: completed=464  gpnum=464
+
+Again, this output is for both "rcu" and "rcu_bh".  The fields are
+taken from the rcu_state structure, and are as follows:
+
+o       "completed" is the number of grace periods that have completed.
+        It is comparable to the "c" field from rcu/rcudata in that a
+        CPU whose "c" field matches the value of "completed" is aware
+        that the corresponding RCU grace period has completed.
+
+o       "gpnum" is the number of grace periods that have started.  It is
+        comparable to the "g" field from rcu/rcudata in that a CPU
+        whose "g" field matches the value of "gpnum" is aware that the
+        corresponding RCU grace period has started.
+
+        If these two fields are equal (as they are for "rcu_bh" above),
+        then there is no grace period in progress, in other words, RCU
+        is idle.  On the other hand, if the two fields differ (as they
+        do for "rcu" above), then an RCU grace period is in progress.
+
+
+The output of "cat rcu/rcuhier" looks as follows, with very long lines:
+
+c=6902 g=6903 s=2 jfq=3 j=72c7 nfqs=13142/nfqsng=0(13142) fqlh=6
+1/1 0:127 ^0    
+3/3 0:35 ^0    0/0 36:71 ^1    0/0 72:107 ^2    0/0 108:127 ^3    
+3/3f 0:5 ^0    2/3 6:11 ^1    0/0 12:17 ^2    0/0 18:23 ^3    0/0 24:29 ^4    0/0 30:35 ^5    0/0 36:41 ^0    0/0 42:47 ^1    0/0 48:53 ^2    0/0 54:59 ^3    0/0 60:65 ^4    0/0 66:71 ^5    0/0 72:77 ^0    0/0 78:83 ^1    0/0 84:89 ^2    0/0 90:95 ^3    0/0 96:101 ^4    0/0 102:107 ^5    0/0 108:113 ^0    0/0 114:119 ^1    0/0 120:125 ^2    0/0 126:127 ^3    
+rcu_bh:
+c=-226 g=-226 s=1 jfq=-5701 j=72c7 nfqs=88/nfqsng=0(88) fqlh=0
+0/1 0:127 ^0    
+0/3 0:35 ^0    0/0 36:71 ^1    0/0 72:107 ^2    0/0 108:127 ^3    
+0/3f 0:5 ^0    0/3 6:11 ^1    0/0 12:17 ^2    0/0 18:23 ^3    0/0 24:29 ^4    0/0 30:35 ^5    0/0 36:41 ^0    0/0 42:47 ^1    0/0 48:53 ^2    0/0 54:59 ^3    0/0 60:65 ^4    0/0 66:71 ^5    0/0 72:77 ^0    0/0 78:83 ^1    0/0 84:89 ^2    0/0 90:95 ^3    0/0 96:101 ^4    0/0 102:107 ^5    0/0 108:113 ^0    0/0 114:119 ^1    0/0 120:125 ^2    0/0 126:127 ^3
+
+This is once again split into "rcu" and "rcu_bh" portions.  The fields are
+as follows:
+
+o       "c" is exactly the same as "completed" under rcu/rcugp.
+
+o       "g" is exactly the same as "gpnum" under rcu/rcugp.
+
+o       "s" is the "signaled" state that drives force_quiescent_state()'s
+        state machine.
+
+o       "jfq" is the number of jiffies remaining for this grace period
+        before force_quiescent_state() is invoked to help push things
+        along.  Note that CPUs in dyntick-idle mode thoughout the grace
+        period will not report on their own, but rather must be check by
+        some other CPU via force_quiescent_state().
+
+o       "j" is the low-order four hex digits of the jiffies counter.
+        Yes, Paul did run into a number of problems that turned out to
+        be due to the jiffies counter no longer counting.  Why do you ask?
+
+o       "nfqs" is the number of calls to force_quiescent_state() since
+        boot.
+
+o       "nfqsng" is the number of useless calls to force_quiescent_state(),
+        where there wasn't actually a grace period active.  This can
+        happen due to races.  The number in parentheses is the difference
+        between "nfqs" and "nfqsng", or the number of times that
+        force_quiescent_state() actually did some real work.
+
+o       "fqlh" is the number of calls to force_quiescent_state() that
+        exited immediately (without even being counted in nfqs above)
+        due to contention on ->fqslock.
+
+o       Each element of the form "1/1 0:127 ^0" represents one struct
+        rcu_node.  Each line represents one level of the hierarchy, from
+        root to leaves.  It is best to think of the rcu_data structures
+        as forming yet another level after the leaves.  Note that there
+        might be either one, two, or three levels of rcu_node structures,
+        depending on the relationship between CONFIG_RCU_FANOUT and
+        CONFIG_NR_CPUS.
+        
+        o       The numbers separated by the "/" are the qsmask followed
+                by the qsmaskinit.  The qsmask will have one bit
+                set for each entity in the next lower level that
+                has not yet checked in for the current grace period.
+                The qsmaskinit will have one bit for each entity that is
+                currently expected to check in during each grace period.
+                The value of qsmaskinit is assigned to that of qsmask
+                at the beginning of each grace period.
+
+                For example, for "rcu", the qsmask of the first entry
+                of the lowest level is 0x14, meaning that we are still
+                waiting for CPUs 2 and 4 to check in for the current
+                grace period.
+
+        o       The numbers separated by the ":" are the range of CPUs
+                served by this struct rcu_node.  This can be helpful
+                in working out how the hierarchy is wired together.
+
+                For example, the first entry at the lowest level shows
+                "0:5", indicating that it covers CPUs 0 through 5.
+
+        o       The number after the "^" indicates the bit in the
+                next higher level rcu_node structure that this
+                rcu_node structure corresponds to.
+
+                For example, the first entry at the lowest level shows
+                "^0", indicating that it corresponds to bit zero in
+                the first entry at the middle level.