Merge branch 'rcu/next' of git://git.kernel.org/pub/scm/linux/kernel/…

…git/paulmck/linux-rcu into core/rcu

Conflicts:
	arch/x86/kernel/ptrace.c

Pull the latest RCU tree from Paul E. McKenney:

"       The major features of this series are:

  1.	A first version of no-callbacks CPUs.  This version prohibits
  	offlining CPU 0, but only when enabled via CONFIG_RCU_NOCB_CPU=y.
  	Relaxing this constraint is in progress, but not yet ready
  	for prime time.  These commits were posted to LKML at
  	https://lkml.org/lkml/2012/10/30/724, and are at branch rcu/nocb.

  2.	Changes to SRCU that allows statically initialized srcu_struct
  	structures.  These commits were posted to LKML at
  	https://lkml.org/lkml/2012/10/30/296, and are at branch rcu/srcu.

  3.	Restructuring of RCU's debugfs output.  These commits were posted
  	to LKML at https://lkml.org/lkml/2012/10/30/341, and are at
  	branch rcu/tracing.

  4.	Additional CPU-hotplug/RCU improvements, posted to LKML at
  	https://lkml.org/lkml/2012/10/30/327, and are at branch rcu/hotplug.
  	Note that the commit eliminating __stop_machine() was judged to
  	be too-high of risk, so is deferred to 3.9.

  5.	Changes to RCU's idle interface, most notably a new module
  	parameter that redirects normal grace-period operations to
  	their expedited equivalents.  These were posted to LKML at
  	https://lkml.org/lkml/2012/10/30/739, and are at branch rcu/idle.

  6.	Additional diagnostics for RCU's CPU stall warning facility,
  	posted to LKML at https://lkml.org/lkml/2012/10/30/315, and
  	are at branch rcu/stall.  The most notable change reduces the
  	default RCU CPU stall-warning time from 60 seconds to 21 seconds,
  	so that it once again happens sooner than the softlockup timeout.

  7.	Documentation updates, which were posted to LKML at
  	https://lkml.org/lkml/2012/10/30/280, and are at branch rcu/doc.
  	A couple of late-breaking changes were posted at
  	https://lkml.org/lkml/2012/11/16/634 and
  	https://lkml.org/lkml/2012/11/16/547.

  8.	Miscellaneous fixes, which were posted to LKML at
  	https://lkml.org/lkml/2012/10/30/309, along with a late-breaking
  	change posted at Fri, 16 Nov 2012 11:26:25 -0800 with message-ID
  	<[email protected]>, but which lkml.org
  	seems to have missed.  These are at branch rcu/fixes.

  9.	Finally, a fix for an lockdep-RCU splat was posted to LKML
  	at https://lkml.org/lkml/2012/11/7/486.  This is at rcu/next. "

Signed-off-by: Ingo Molnar <[email protected]>

Documentation/RCU/RTFP.txt

@@ -186,7 +186,7 @@ Bibtex Entries
 
 @article{Kung80
 ,author="H. T. Kung and Q. Lehman"
-,title="Concurrent Maintenance of Binary Search Trees"
+,title="Concurrent Manipulation of Binary Search Trees"
 ,Year="1980"
 ,Month="September"
 ,journal="ACM Transactions on Database Systems"

Documentation/RCU/checklist.txt

@@ -271,15 +271,14 @@ over a rather long period of time, but improvements are always welcome!
 	The same cautions apply to call_rcu_bh() and call_rcu_sched().
 
 9.	All RCU list-traversal primitives, which include
-	rcu_dereference(), list_for_each_entry_rcu(),
-	list_for_each_continue_rcu(), and list_for_each_safe_rcu(),
-	must be either within an RCU read-side critical section or
-	must be protected by appropriate update-side locks.  RCU
-	read-side critical sections are delimited by rcu_read_lock()
-	and rcu_read_unlock(), or by similar primitives such as
-	rcu_read_lock_bh() and rcu_read_unlock_bh(), in which case
-	the matching rcu_dereference() primitive must be used in order
-	to keep lockdep happy, in this case, rcu_dereference_bh().
+	rcu_dereference(), list_for_each_entry_rcu(), and
+	list_for_each_safe_rcu(), must be either within an RCU read-side
+	critical section or must be protected by appropriate update-side
+	locks.	RCU read-side critical sections are delimited by
+	rcu_read_lock() and rcu_read_unlock(), or by similar primitives
+	such as rcu_read_lock_bh() and rcu_read_unlock_bh(), in which
+	case the matching rcu_dereference() primitive must be used in
+	order to keep lockdep happy, in this case, rcu_dereference_bh().
 
 	The reason that it is permissible to use RCU list-traversal
 	primitives when the update-side lock is held is that doing so

Documentation/RCU/listRCU.txt

@@ -205,7 +205,7 @@ RCU ("read-copy update") its name.  The RCU code is as follows:
 				audit_copy_rule(&ne->rule, &e->rule);
 				ne->rule.action = newaction;
 				ne->rule.file_count = newfield_count;
-				list_replace_rcu(e, ne);
+				list_replace_rcu(&e->list, &ne->list);
 				call_rcu(&e->rcu, audit_free_rule);
 				return 0;
 			}

Documentation/RCU/rcuref.txt

@@ -20,7 +20,7 @@ release_referenced()			delete()
 {					{
     ...					    write_lock(&list_lock);
     atomic_dec(&el->rc, relfunc)	    ...
-    ...					    delete_element
+    ...					    remove_element
 }					    write_unlock(&list_lock);
  					    ...
 					    if (atomic_dec_and_test(&el->rc))
@@ -52,7 +52,7 @@ release_referenced()			delete()
 {					{
     ...					    spin_lock(&list_lock);
     if (atomic_dec_and_test(&el->rc))       ...
-        call_rcu(&el->head, el_free);       delete_element
+        call_rcu(&el->head, el_free);       remove_element
     ...                                     spin_unlock(&list_lock);
 } 					    ...
 					    if (atomic_dec_and_test(&el->rc))
@@ -64,3 +64,60 @@ Sometimes, a reference to the element needs to be obtained in the
 update (write) stream.  In such cases, atomic_inc_not_zero() might be
 overkill, since we hold the update-side spinlock.  One might instead
 use atomic_inc() in such cases.
+
+It is not always convenient to deal with "FAIL" in the
+search_and_reference() code path.  In such cases, the
+atomic_dec_and_test() may be moved from delete() to el_free()
+as follows:
+
+1.					2.
+add()					search_and_reference()
+{					{
+    alloc_object			    rcu_read_lock();
+    ...					    search_for_element
+    atomic_set(&el->rc, 1);		    atomic_inc(&el->rc);
+    spin_lock(&list_lock);		    ...
+
+    add_element				    rcu_read_unlock();
+    ...					}
+    spin_unlock(&list_lock);		4.
+}					delete()
+3.					{
+release_referenced()			    spin_lock(&list_lock);
+{					    ...
+    ...					    remove_element
+    if (atomic_dec_and_test(&el->rc))       spin_unlock(&list_lock);
+        kfree(el);			    ...
+    ...                                     call_rcu(&el->head, el_free);
+} 					    ...
+5.					}
+void el_free(struct rcu_head *rhp)
+{
+    release_referenced();
+}
+
+The key point is that the initial reference added by add() is not removed
+until after a grace period has elapsed following removal.  This means that
+search_and_reference() cannot find this element, which means that the value
+of el->rc cannot increase.  Thus, once it reaches zero, there are no
+readers that can or ever will be able to reference the element.  The
+element can therefore safely be freed.  This in turn guarantees that if
+any reader finds the element, that reader may safely acquire a reference
+without checking the value of the reference counter.
+
+In cases where delete() can sleep, synchronize_rcu() can be called from
+delete(), so that el_free() can be subsumed into delete as follows:
+
+4.
+delete()
+{
+    spin_lock(&list_lock);
+    ...
+    remove_element
+    spin_unlock(&list_lock);
+    ...
+    synchronize_rcu();
+    if (atomic_dec_and_test(&el->rc))
+    	kfree(el);
+    ...
+}