Merge tag 'rcu.release.v6.8' of https://github.com/neeraju/linux

Pull RCU updates from Neeraj Upadhyay:

 - Documentation and comment updates

 - RCU torture, locktorture updates that include cleanups; nolibc init
   build support for mips, ppc and rv64; testing of mid stall duration
   scenario and fixing fqs task creation conditions

 - Misc fixes, most notably restricting usage of RCU CPU stall
   notifiers, to confine their usage primarily to debug kernels

 - RCU tasks minor fixes

 - lockdep annotation fix for NMI-safe accesses, callback
   advancing/acceleration cleanup and documentation improvements

* tag 'rcu.release.v6.8' of https://github.com/neeraju/linux:
  rcu: Force quiescent states only for ongoing grace period
  doc: Clarify historical disclaimers in memory-barriers.txt
  doc: Mention address and data dependencies in rcu_dereference.rst
  doc: Clarify RCU Tasks reader/updater checklist
  rculist.h: docs: Fix wrong function summary
  Documentation: RCU: Remove repeated word in comments
  srcu: Use try-lock lockdep annotation for NMI-safe access.
  srcu: Explain why callbacks invocations can't run concurrently
  srcu: No need to advance/accelerate if no callback enqueued
  srcu: Remove superfluous callbacks advancing from srcu_gp_start()
  rcu: Remove unused macros from rcupdate.h
  rcu: Restrict access to RCU CPU stall notifiers
  rcu-tasks: Mark RCU Tasks accesses to current->rcu_tasks_idle_cpu
  rcutorture: Add fqs_holdoff check before fqs_task is created
  rcutorture: Add mid-sized stall to TREE07
  rcutorture: add nolibc init support for mips, ppc and rv64
  locktorture: Increase Hamming distance between call_rcu_chain and rcu_call_chains

Author: torvalds

Documentation/RCU/checklist.rst

@@ -241,15 +241,22 @@ over a rather long period of time, but improvements are always welcome!
 	srcu_struct.  The rules for the expedited RCU grace-period-wait
 	primitives are the same as for their non-expedited counterparts.
 
-	If the updater uses call_rcu_tasks() or synchronize_rcu_tasks(),
-	then the readers must refrain from executing voluntary
-	context switches, that is, from blocking.  If the updater uses
-	call_rcu_tasks_trace() or synchronize_rcu_tasks_trace(), then
-	the corresponding readers must use rcu_read_lock_trace() and
-	rcu_read_unlock_trace().  If an updater uses call_rcu_tasks_rude()
-	or synchronize_rcu_tasks_rude(), then the corresponding readers
-	must use anything that disables preemption, for example,
-	preempt_disable() and preempt_enable().
+	Similarly, it is necessary to correctly use the RCU Tasks flavors:
+
+	a.	If the updater uses synchronize_rcu_tasks() or
+		call_rcu_tasks(), then the readers must refrain from
+		executing voluntary context switches, that is, from
+		blocking.
+
+	b.	If the updater uses call_rcu_tasks_trace()
+		or synchronize_rcu_tasks_trace(), then the
+		corresponding readers must use rcu_read_lock_trace()
+		and rcu_read_unlock_trace().
+
+	c.	If an updater uses call_rcu_tasks_rude() or
+		synchronize_rcu_tasks_rude(), then the corresponding
+		readers must use anything that disables preemption,
+		for example, preempt_disable() and preempt_enable().
 
 	Mixing things up will result in confusion and broken kernels, and
 	has even resulted in an exploitable security issue.  Therefore,

Documentation/RCU/rcu_dereference.rst

@@ -3,13 +3,26 @@
 PROPER CARE AND FEEDING OF RETURN VALUES FROM rcu_dereference()
 ===============================================================
 
-Most of the time, you can use values from rcu_dereference() or one of
-the similar primitives without worries.  Dereferencing (prefix "*"),
-field selection ("->"), assignment ("="), address-of ("&"), addition and
-subtraction of constants, and casts all work quite naturally and safely.
-
-It is nevertheless possible to get into trouble with other operations.
-Follow these rules to keep your RCU code working properly:
+Proper care and feeding of address and data dependencies is critically
+important to correct use of things like RCU.  To this end, the pointers
+returned from the rcu_dereference() family of primitives carry address and
+data dependencies.  These dependencies extend from the rcu_dereference()
+macro's load of the pointer to the later use of that pointer to compute
+either the address of a later memory access (representing an address
+dependency) or the value written by a later memory access (representing
+a data dependency).
+
+Most of the time, these dependencies are preserved, permitting you to
+freely use values from rcu_dereference().  For example, dereferencing
+(prefix "*"), field selection ("->"), assignment ("="), address-of
+("&"), casts, and addition or subtraction of constants all work quite
+naturally and safely.  However, because current compilers do not take
+either address or data dependencies into account it is still possible
+to get into trouble.
+
+Follow these rules to preserve the address and data dependencies emanating
+from your calls to rcu_dereference() and friends, thus keeping your RCU
+readers working properly:
 
 -	You must use one of the rcu_dereference() family of primitives
 	to load an RCU-protected pointer, otherwise CONFIG_PROVE_RCU

Documentation/RCU/torture.rst

@@ -185,7 +185,7 @@ argument.
 Not all changes require that all scenarios be run.  For example, a change
 to Tree SRCU might run only the SRCU-N and SRCU-P scenarios using the
 --configs argument to kvm.sh as follows:  "--configs 'SRCU-N SRCU-P'".
-Large systems can run multiple copies of of the full set of scenarios,
+Large systems can run multiple copies of the full set of scenarios,
 for example, a system with 448 hardware threads can run five instances
 of the full set concurrently.  To make this happen::
 

Documentation/admin-guide/kernel-parameters.txt

@@ -5313,6 +5313,12 @@
 			Dump ftrace buffer after reporting RCU CPU
 			stall warning.
 
+	rcupdate.rcu_cpu_stall_notifiers= [KNL]
+			Provide RCU CPU stall notifiers, but see the
+			warnings in the RCU_CPU_STALL_NOTIFIER Kconfig
+			option's help text.  TL;DR:  You almost certainly
+			do not want rcupdate.rcu_cpu_stall_notifiers.
+
 	rcupdate.rcu_cpu_stall_suppress= [KNL]
 			Suppress RCU CPU stall warning messages.
 

Documentation/memory-barriers.txt

@@ -396,10 +396,11 @@ Memory barriers come in four basic varieties:
 
 
  (2) Address-dependency barriers (historical).
-     [!] This section is marked as HISTORICAL: For more up-to-date
-     information, including how compiler transformations related to pointer
-     comparisons can sometimes cause problems, see
-     Documentation/RCU/rcu_dereference.rst.
+     [!] This section is marked as HISTORICAL: it covers the long-obsolete
+     smp_read_barrier_depends() macro, the semantics of which are now
+     implicit in all marked accesses.  For more up-to-date information,
+     including how compiler transformations can sometimes break address
+     dependencies, see Documentation/RCU/rcu_dereference.rst.
 
      An address-dependency barrier is a weaker form of read barrier.  In the
      case where two loads are performed such that the second depends on the
@@ -560,9 +561,11 @@ There are certain things that the Linux kernel memory barriers do not guarantee:
 
 ADDRESS-DEPENDENCY BARRIERS (HISTORICAL)
 ----------------------------------------
-[!] This section is marked as HISTORICAL: For more up-to-date information,
-including how compiler transformations related to pointer comparisons can
-sometimes cause problems, see Documentation/RCU/rcu_dereference.rst.
+[!] This section is marked as HISTORICAL: it covers the long-obsolete
+smp_read_barrier_depends() macro, the semantics of which are now implicit
+in all marked accesses.  For more up-to-date information, including
+how compiler transformations can sometimes break address dependencies,
+see Documentation/RCU/rcu_dereference.rst.
 
 As of v4.15 of the Linux kernel, an smp_mb() was added to READ_ONCE() for
 DEC Alpha, which means that about the only people who need to pay attention

include/linux/rcu_notifier.h

@@ -13,20 +13,20 @@
 #define RCU_STALL_NOTIFY_NORM	1
 #define RCU_STALL_NOTIFY_EXP	2
 
-#ifdef CONFIG_RCU_STALL_COMMON
+#if defined(CONFIG_RCU_STALL_COMMON) && defined(CONFIG_RCU_CPU_STALL_NOTIFIER)
 
 #include <linux/notifier.h>
 #include <linux/types.h>
 
 int rcu_stall_chain_notifier_register(struct notifier_block *n);
 int rcu_stall_chain_notifier_unregister(struct notifier_block *n);
 
-#else // #ifdef CONFIG_RCU_STALL_COMMON
+#else // #if defined(CONFIG_RCU_STALL_COMMON) && defined(CONFIG_RCU_CPU_STALL_NOTIFIER)
 
 // No RCU CPU stall warnings in Tiny RCU.
 static inline int rcu_stall_chain_notifier_register(struct notifier_block *n) { return -EEXIST; }
 static inline int rcu_stall_chain_notifier_unregister(struct notifier_block *n) { return -ENOENT; }
 
-#endif // #else // #ifdef CONFIG_RCU_STALL_COMMON
+#endif // #else // #if defined(CONFIG_RCU_STALL_COMMON) && defined(CONFIG_RCU_CPU_STALL_NOTIFIER)
 
 #endif /* __LINUX_RCU_NOTIFIER_H */

include/linux/rculist.h

@@ -355,7 +355,7 @@ static inline void list_splice_tail_init_rcu(struct list_head *list,
 })
 
 /**
- * list_next_or_null_rcu - get the first element from a list
+ * list_next_or_null_rcu - get the next element from a list
  * @head:	the head for the list.
  * @ptr:        the list head to take the next element from.
  * @type:       the type of the struct this is embedded in.

include/linux/rcupdate.h

@@ -34,9 +34,6 @@
 
 #define ULONG_CMP_GE(a, b)	(ULONG_MAX / 2 >= (a) - (b))
 #define ULONG_CMP_LT(a, b)	(ULONG_MAX / 2 < (a) - (b))
-#define ulong2long(a)		(*(long *)(&(a)))
-#define USHORT_CMP_GE(a, b)	(USHRT_MAX / 2 >= (unsigned short)((a) - (b)))
-#define USHORT_CMP_LT(a, b)	(USHRT_MAX / 2 < (unsigned short)((a) - (b)))
 
 /* Exported common interfaces */
 void call_rcu(struct rcu_head *head, rcu_callback_t func);
@@ -301,6 +298,11 @@ static inline void rcu_lock_acquire(struct lockdep_map *map)
 	lock_acquire(map, 0, 0, 2, 0, NULL, _THIS_IP_);
 }
 
+static inline void rcu_try_lock_acquire(struct lockdep_map *map)
+{
+	lock_acquire(map, 0, 1, 2, 0, NULL, _THIS_IP_);
+}
+
 static inline void rcu_lock_release(struct lockdep_map *map)
 {
 	lock_release(map, _THIS_IP_);
@@ -315,6 +317,7 @@ int rcu_read_lock_any_held(void);
 #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 
 # define rcu_lock_acquire(a)		do { } while (0)
+# define rcu_try_lock_acquire(a)	do { } while (0)
 # define rcu_lock_release(a)		do { } while (0)
 
 static inline int rcu_read_lock_held(void)

include/linux/srcu.h

@@ -229,7 +229,7 @@ static inline int srcu_read_lock_nmisafe(struct srcu_struct *ssp) __acquires(ssp
 
 	srcu_check_nmi_safety(ssp, true);
 	retval = __srcu_read_lock_nmisafe(ssp);
-	rcu_lock_acquire(&ssp->dep_map);
+	rcu_try_lock_acquire(&ssp->dep_map);
 	return retval;
 }
 

kernel/locking/locktorture.c

@@ -124,7 +124,7 @@ struct call_rcu_chain {
 	struct rcu_head crc_rh;
 	bool crc_stop;
 };
-struct call_rcu_chain *call_rcu_chain;
+struct call_rcu_chain *call_rcu_chain_list;
 
 /* Forward reference. */
 static void lock_torture_cleanup(void);
@@ -1074,12 +1074,12 @@ static int call_rcu_chain_init(void)
 
 	if (call_rcu_chains <= 0)
 		return 0;
-	call_rcu_chain = kcalloc(call_rcu_chains, sizeof(*call_rcu_chain), GFP_KERNEL);
-	if (!call_rcu_chain)
+	call_rcu_chain_list = kcalloc(call_rcu_chains, sizeof(*call_rcu_chain_list), GFP_KERNEL);
+	if (!call_rcu_chain_list)
 		return -ENOMEM;
 	for (i = 0; i < call_rcu_chains; i++) {
-		call_rcu_chain[i].crc_stop = false;
-		call_rcu(&call_rcu_chain[i].crc_rh, call_rcu_chain_cb);
+		call_rcu_chain_list[i].crc_stop = false;
+		call_rcu(&call_rcu_chain_list[i].crc_rh, call_rcu_chain_cb);
 	}
 	return 0;
 }
@@ -1089,13 +1089,13 @@ static void call_rcu_chain_cleanup(void)
 {
 	int i;
 
-	if (!call_rcu_chain)
+	if (!call_rcu_chain_list)
 		return;
 	for (i = 0; i < call_rcu_chains; i++)
-		smp_store_release(&call_rcu_chain[i].crc_stop, true);
+		smp_store_release(&call_rcu_chain_list[i].crc_stop, true);
 	rcu_barrier();
-	kfree(call_rcu_chain);
-	call_rcu_chain = NULL;
+	kfree(call_rcu_chain_list);
+	call_rcu_chain_list = NULL;
 }
 
 static void lock_torture_cleanup(void)