Merge tag 'rcu.release.v6.13' of git://git.kernel.org/pub/scm/linux/kernel/git/rcu/linux

Pull RCU updates from Frederic Weisbecker:
 "SRCU:

   - Introduction of the new SRCU-lite flavour with a new pair of
     srcu_read_[un]lock_lite() APIs. In practice the read side using
     this flavour becomes lighter by removing a full memory barrier on
     LOCK and a full memory barrier on UNLOCK. This comes at the expense
     of a higher latency write side with two (in the best case of a
     snaphot of unused read-sides) or more RCU grace periods on the
     update side which now assumes by itself the whole full ordering
     guarantee against the LOCK/UNLOCK counters on both indexes, along
     with the accesses performed inside.

     Uretprobes is a known potential user.

     Note this doesn't replace the default normal flavour of SRCU which
     still behaves the same as usual.

   - Add testing of SRCU-lite through rcutorture and rcuscale

   - Various cleanups on the way.

  Fixes:

   - Allow short-circuiting RCU-TASKS-RUDE grace periods on
     architectures that have sane noinstr boundaries forbidding tracing
     on low-level idle and kernel entry code. RCU-TASKS is enough on
     such configurations because it involves an RCU grace period that
     waits for all idle tasks to either schedule out voluntarily or
     enter into RCU unwatched noinstr code.

   - Allow and test start_poll_synchronize_rcu() with IRQs disabled.

   - Mention rcuog kthreads in relevant documentation and Kconfig help

   - Various fixes and consolidations

  rcutorture:

   - Add --no-affinity on tools to leave the affinity setting of guests
     up to the user.

   - Add guest_os_delay parameter to rcuscale for better warm-up
     control.

   - Fix and improve some rcuscale error handling.

   - Various cleanups and fixes

  stall:

   - Remove dead code

   - Stop dumping tasks if a stalled grace period eventually ended
     midway as that only produces confusing output.

   - Optimize detection of stalling CPUs and avoid useless node locking
     otherwise.

  NOCB:

   - Fix rcu_barrier() hang due to a race against callbacks
     deoffloading. This is not yet used, except by rcutorture, and waits
     for its promised cpusets interface.

   - Remove leftover function declaration"

* tag 'rcu.release.v6.13' of git://git.kernel.org/pub/scm/linux/kernel/git/rcu/linux: (42 commits)
  rcuscale: Remove redundant WARN_ON_ONCE() splat
  rcuscale: Do a proper cleanup if kfree_scale_init() fails
  srcu: Unconditionally record srcu_read_lock_lite() in ->srcu_reader_flavor
  srcu: Check for srcu_read_lock_lite() across all CPUs
  srcu: Remove smp_mb() from srcu_read_unlock_lite()
  rcutorture: Avoid printing cpu=-1 for no-fault RCU boost failure
  rcuscale: Add guest_os_delay module parameter
  refscale: Correct affinity check
  torture: Add --no-affinity parameter to kvm.sh
  rcu/nocb: Fix missed RCU barrier on deoffloading
  rcu/kvfree: Fix data-race in __mod_timer / kvfree_call_rcu
  rcu/srcutiny: don't return before reenabling preemption
  rcu-tasks: Remove open-coded one-byte cmpxchg() emulation
  doc: Remove kernel-parameters.txt entry for rcutorture.read_exit
  rcutorture: Test start-poll primitives with interrupts disabled
  rcu: Permit start_poll_synchronize_rcu*() with interrupts disabled
  rcu: Allow short-circuiting of synchronize_rcu_tasks_rude()
  doc: Add rcuog kthreads to kernel-per-CPU-kthreads.rst
  rcu: Add rcuog kthreads to RCU_NOCB_CPU help text
  rcu: Use the BITS_PER_LONG macro
  ...

5 files changed, 138 insertions, 26 deletions

diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index 0ee270b3f5ed..fe42315f667f 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -165,7 +165,6 @@ static inline bool rcu_inkernel_boot_has_ended(void) { return true; }
 static inline bool rcu_is_watching(void) { return true; }
 static inline void rcu_momentary_eqs(void) { }
 static inline void kfree_rcu_scheduler_running(void) { }
-static inline bool rcu_gp_might_be_stalled(void) { return false; }
 
 /* Avoid RCU read-side critical sections leaking across. */
 static inline void rcu_all_qs(void) { barrier(); }
diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index 90a684f94776..27d86d912781 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -40,7 +40,6 @@ void kvfree_rcu_barrier(void);
 void rcu_barrier(void);
 void rcu_momentary_eqs(void);
 void kfree_rcu_scheduler_running(void);
-bool rcu_gp_might_be_stalled(void);
 
 struct rcu_gp_oldstate {
 	unsigned long rgos_norm;
diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index 835bbb2d1f88..08339eb8a01c 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -56,6 +56,13 @@ void call_srcu(struct srcu_struct *ssp, struct rcu_head *head,
 void cleanup_srcu_struct(struct srcu_struct *ssp);
 int __srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp);
 void __srcu_read_unlock(struct srcu_struct *ssp, int idx) __releases(ssp);
+#ifdef CONFIG_TINY_SRCU
+#define __srcu_read_lock_lite __srcu_read_lock
+#define __srcu_read_unlock_lite __srcu_read_unlock
+#else // #ifdef CONFIG_TINY_SRCU
+int __srcu_read_lock_lite(struct srcu_struct *ssp) __acquires(ssp);
+void __srcu_read_unlock_lite(struct srcu_struct *ssp, int idx) __releases(ssp);
+#endif // #else // #ifdef CONFIG_TINY_SRCU
 void synchronize_srcu(struct srcu_struct *ssp);
 
 #define SRCU_GET_STATE_COMPLETED 0x1
@@ -176,17 +183,6 @@ static inline int srcu_read_lock_held(const struct srcu_struct *ssp)
 
 #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 
-#define SRCU_NMI_UNKNOWN	0x0
-#define SRCU_NMI_UNSAFE		0x1
-#define SRCU_NMI_SAFE		0x2
-
-#if defined(CONFIG_PROVE_RCU) && defined(CONFIG_TREE_SRCU)
-void srcu_check_nmi_safety(struct srcu_struct *ssp, bool nmi_safe);
-#else
-static inline void srcu_check_nmi_safety(struct srcu_struct *ssp,
-					 bool nmi_safe) { }
-#endif
-
 
 /**
  * srcu_dereference_check - fetch SRCU-protected pointer for later dereferencing
@@ -236,33 +232,67 @@ static inline void srcu_check_nmi_safety(struct srcu_struct *ssp,
  * a mutex that is held elsewhere while calling synchronize_srcu() or
  * synchronize_srcu_expedited().
  *
- * Note that srcu_read_lock() and the matching srcu_read_unlock() must
- * occur in the same context, for example, it is illegal to invoke
- * srcu_read_unlock() in an irq handler if the matching srcu_read_lock()
- * was invoked in process context.
+ * The return value from srcu_read_lock() must be passed unaltered
+ * to the matching srcu_read_unlock().  Note that srcu_read_lock() and
+ * the matching srcu_read_unlock() must occur in the same context, for
+ * example, it is illegal to invoke srcu_read_unlock() in an irq handler
+ * if the matching srcu_read_lock() was invoked in process context.  Or,
+ * for that matter to invoke srcu_read_unlock() from one task and the
+ * matching srcu_read_lock() from another.
  */
 static inline int srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp)
 {
 	int retval;
 
-	srcu_check_nmi_safety(ssp, false);
+	srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_NORMAL);
 	retval = __srcu_read_lock(ssp);
 	srcu_lock_acquire(&ssp->dep_map);
 	return retval;
 }
 
 /**
+ * srcu_read_lock_lite - register a new reader for an SRCU-protected structure.
+ * @ssp: srcu_struct in which to register the new reader.
+ *
+ * Enter an SRCU read-side critical section, but for a light-weight
+ * smp_mb()-free reader.  See srcu_read_lock() for more information.
+ *
+ * If srcu_read_lock_lite() is ever used on an srcu_struct structure,
+ * then none of the other flavors may be used, whether before, during,
+ * or after.  Note that grace-period auto-expediting is disabled for _lite
+ * srcu_struct structures because auto-expedited grace periods invoke
+ * synchronize_rcu_expedited(), IPIs and all.
+ *
+ * Note that srcu_read_lock_lite() can be invoked only from those contexts
+ * where RCU is watching, that is, from contexts where it would be legal
+ * to invoke rcu_read_lock().  Otherwise, lockdep will complain.
+ */
+static inline int srcu_read_lock_lite(struct srcu_struct *ssp) __acquires(ssp)
+{
+	int retval;
+
+	srcu_check_read_flavor_lite(ssp);
+	retval = __srcu_read_lock_lite(ssp);
+	rcu_try_lock_acquire(&ssp->dep_map);
+	return retval;
+}
+
+/**
  * srcu_read_lock_nmisafe - register a new reader for an SRCU-protected structure.
  * @ssp: srcu_struct in which to register the new reader.
  *
  * Enter an SRCU read-side critical section, but in an NMI-safe manner.
  * See srcu_read_lock() for more information.
+ *
+ * If srcu_read_lock_nmisafe() is ever used on an srcu_struct structure,
+ * then none of the other flavors may be used, whether before, during,
+ * or after.
  */
 static inline int srcu_read_lock_nmisafe(struct srcu_struct *ssp) __acquires(ssp)
 {
 	int retval;
 
-	srcu_check_nmi_safety(ssp, true);
+	srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_NMI);
 	retval = __srcu_read_lock_nmisafe(ssp);
 	rcu_try_lock_acquire(&ssp->dep_map);
 	return retval;
@@ -274,7 +304,7 @@ srcu_read_lock_notrace(struct srcu_struct *ssp) __acquires(ssp)
 {
 	int retval;
 
-	srcu_check_nmi_safety(ssp, false);
+	srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_NORMAL);
 	retval = __srcu_read_lock(ssp);
 	return retval;
 }
@@ -303,7 +333,7 @@ srcu_read_lock_notrace(struct srcu_struct *ssp) __acquires(ssp)
 static inline int srcu_down_read(struct srcu_struct *ssp) __acquires(ssp)
 {
 	WARN_ON_ONCE(in_nmi());
-	srcu_check_nmi_safety(ssp, false);
+	srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_NORMAL);
 	return __srcu_read_lock(ssp);
 }
 
@@ -318,12 +348,28 @@ static inline void srcu_read_unlock(struct srcu_struct *ssp, int idx)
 	__releases(ssp)
 {
 	WARN_ON_ONCE(idx & ~0x1);
-	srcu_check_nmi_safety(ssp, false);
+	srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_NORMAL);
 	srcu_lock_release(&ssp->dep_map);
 	__srcu_read_unlock(ssp, idx);
 }
 
 /**
+ * srcu_read_unlock_lite - unregister a old reader from an SRCU-protected structure.
+ * @ssp: srcu_struct in which to unregister the old reader.
+ * @idx: return value from corresponding srcu_read_lock().
+ *
+ * Exit a light-weight SRCU read-side critical section.
+ */
+static inline void srcu_read_unlock_lite(struct srcu_struct *ssp, int idx)
+	__releases(ssp)
+{
+	WARN_ON_ONCE(idx & ~0x1);
+	srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_LITE);
+	srcu_lock_release(&ssp->dep_map);
+	__srcu_read_unlock_lite(ssp, idx);
+}
+
+/**
  * srcu_read_unlock_nmisafe - unregister a old reader from an SRCU-protected structure.
  * @ssp: srcu_struct in which to unregister the old reader.
  * @idx: return value from corresponding srcu_read_lock().
@@ -334,7 +380,7 @@ static inline void srcu_read_unlock_nmisafe(struct srcu_struct *ssp, int idx)
 	__releases(ssp)
 {
 	WARN_ON_ONCE(idx & ~0x1);
-	srcu_check_nmi_safety(ssp, true);
+	srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_NMI);
 	rcu_lock_release(&ssp->dep_map);
 	__srcu_read_unlock_nmisafe(ssp, idx);
 }
@@ -343,7 +389,7 @@ static inline void srcu_read_unlock_nmisafe(struct srcu_struct *ssp, int idx)
 static inline notrace void
 srcu_read_unlock_notrace(struct srcu_struct *ssp, int idx) __releases(ssp)
 {
-	srcu_check_nmi_safety(ssp, false);
+	srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_NORMAL);
 	__srcu_read_unlock(ssp, idx);
 }
 
@@ -360,7 +406,7 @@ static inline void srcu_up_read(struct srcu_struct *ssp, int idx)
 {
 	WARN_ON_ONCE(idx & ~0x1);
 	WARN_ON_ONCE(in_nmi());
-	srcu_check_nmi_safety(ssp, false);
+	srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_NORMAL);
 	__srcu_read_unlock(ssp, idx);
 }
 
diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
index 4d96bbdb45f0..1321da803274 100644
--- a/include/linux/srcutiny.h
+++ b/include/linux/srcutiny.h
@@ -81,6 +81,9 @@ static inline void srcu_barrier(struct srcu_struct *ssp)
 	synchronize_srcu(ssp);
 }
 
+#define srcu_check_read_flavor(ssp, read_flavor) do { } while (0)
+#define srcu_check_read_flavor_lite(ssp) do { } while (0)
+
 /* Defined here to avoid size increase for non-torture kernels. */
 static inline void srcu_torture_stats_print(struct srcu_struct *ssp,
 					    char *tt, char *tf)
diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
index ed57598394de..490aeecc6bb4 100644
--- a/include/linux/srcutree.h
+++ b/include/linux/srcutree.h
@@ -25,7 +25,7 @@ struct srcu_data {
 	/* Read-side state. */
 	atomic_long_t srcu_lock_count[2];	/* Locks per CPU. */
 	atomic_long_t srcu_unlock_count[2];	/* Unlocks per CPU. */
-	int srcu_nmi_safety;			/* NMI-safe srcu_struct structure? */
+	int srcu_reader_flavor;			/* Reader flavor for srcu_struct structure? */
 
 	/* Update-side state. */
 	spinlock_t __private lock ____cacheline_internodealigned_in_smp;
@@ -43,6 +43,11 @@ struct srcu_data {
 	struct srcu_struct *ssp;
 };
 
+/* Values for ->srcu_reader_flavor. */
+#define SRCU_READ_FLAVOR_NORMAL	0x1		// srcu_read_lock().
+#define SRCU_READ_FLAVOR_NMI	0x2		// srcu_read_lock_nmisafe().
+#define SRCU_READ_FLAVOR_LITE	0x4		// srcu_read_lock_lite().
+
 /*
  * Node in SRCU combining tree, similar in function to rcu_data.
  */
@@ -204,4 +209,64 @@ void synchronize_srcu_expedited(struct srcu_struct *ssp);
 void srcu_barrier(struct srcu_struct *ssp);
 void srcu_torture_stats_print(struct srcu_struct *ssp, char *tt, char *tf);
 
+/*
+ * Counts the new reader in the appropriate per-CPU element of the
+ * srcu_struct.  Returns an index that must be passed to the matching
+ * srcu_read_unlock_lite().
+ *
+ * Note that this_cpu_inc() is an RCU read-side critical section either
+ * because it disables interrupts, because it is a single instruction,
+ * or because it is a read-modify-write atomic operation, depending on
+ * the whims of the architecture.
+ */
+static inline int __srcu_read_lock_lite(struct srcu_struct *ssp)
+{
+	int idx;
+
+	RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_read_lock_lite().");
+	idx = READ_ONCE(ssp->srcu_idx) & 0x1;
+	this_cpu_inc(ssp->sda->srcu_lock_count[idx].counter); /* Y */
+	barrier(); /* Avoid leaking the critical section. */
+	return idx;
+}
+
+/*
+ * Removes the count for the old reader from the appropriate
+ * per-CPU element of the srcu_struct.  Note that this may well be a
+ * different CPU than that which was incremented by the corresponding
+ * srcu_read_lock_lite(), but it must be within the same task.
+ *
+ * Note that this_cpu_inc() is an RCU read-side critical section either
+ * because it disables interrupts, because it is a single instruction,
+ * or because it is a read-modify-write atomic operation, depending on
+ * the whims of the architecture.
+ */
+static inline void __srcu_read_unlock_lite(struct srcu_struct *ssp, int idx)
+{
+	barrier();  /* Avoid leaking the critical section. */
+	this_cpu_inc(ssp->sda->srcu_unlock_count[idx].counter);  /* Z */
+	RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_read_unlock_lite().");
+}
+
+void __srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor);
+
+// Record _lite() usage even for CONFIG_PROVE_RCU=n kernels.
+static inline void srcu_check_read_flavor_lite(struct srcu_struct *ssp)
+{
+	struct srcu_data *sdp = raw_cpu_ptr(ssp->sda);
+
+	if (likely(READ_ONCE(sdp->srcu_reader_flavor) & SRCU_READ_FLAVOR_LITE))
+		return;
+
+	// Note that the cmpxchg() in srcu_check_read_flavor() is fully ordered.
+	__srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_LITE);
+}
+
+// Record non-_lite() usage only for CONFIG_PROVE_RCU=y kernels.
+static inline void srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor)
+{
+	if (IS_ENABLED(CONFIG_PROVE_RCU))
+		__srcu_check_read_flavor(ssp, read_flavor);
+}
+
 #endif