srcu: Create an SRCU-fast-updown API

This commit creates an SRCU-fast-updown API, including
DEFINE_SRCU_FAST_UPDOWN(), DEFINE_STATIC_SRCU_FAST_UPDOWN(),
__init_srcu_struct_fast_updown(), init_srcu_struct_fast_updown(),
srcu_read_lock_fast_updown(), srcu_read_unlock_fast_updown(),
__srcu_read_lock_fast_updown(), and __srcu_read_unlock_fast_updown().

These are initially identical to their SRCU-fast counterparts, but both
SRCU-fast and SRCU-fast-updown will be optimized in different directions
by later commits. SRCU-fast will lack any sort of srcu_down_read() and
srcu_up_read() APIs, which will enable extremely efficient NMI safety.
For its part, SRCU-fast-updown will not be NMI safe, which will enable
reasonably efficient implementations of srcu_down_read_fast() and
srcu_up_read_fast().

This API fork happens to meet two different future use cases.

* SRCU-fast will become the reimplementation basis for RCU-TASK-TRACE
  for consolidation. Since RCU-TASK-TRACE must be NMI safe, SRCU-fast
  must be as well.

* SRCU-fast-updown will be needed for uretprobes code in order to get
  rid of the read-side memory barriers while still allowing entering the
  reader at task level while exiting it in a timer handler.

This commit also adds rcutorture tests for the new APIs.  This
(annoyingly) needs to be in the same commit for bisectability.  With this
commit, the 0x8 value tests SRCU-fast-updown.  However, most SRCU-fast
testing will be via the RCU Tasks Trace wrappers.

[ paulmck: Apply s/0x8/0x4/ missing change per Boqun Feng feedback. ]
[ paulmck: Apply Akira Yokosawa feedback. ]

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: <bpf@vger.kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

1 files changed, 72 insertions, 5 deletions

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index 1dd6812aabe7..344ad51c8f6c 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -28,6 +28,8 @@ struct srcu_struct;
 int __init_srcu_struct(struct srcu_struct *ssp, const char *name, struct lock_class_key *key);
 #ifndef CONFIG_TINY_SRCU
 int __init_srcu_struct_fast(struct srcu_struct *ssp, const char *name, struct lock_class_key *key);
+int __init_srcu_struct_fast_updown(struct srcu_struct *ssp, const char *name,
+				   struct lock_class_key *key);
 #endif // #ifndef CONFIG_TINY_SRCU
 
 #define init_srcu_struct(ssp) \
@@ -44,12 +46,20 @@ int __init_srcu_struct_fast(struct srcu_struct *ssp, const char *name, struct lo
 	__init_srcu_struct_fast((ssp), #ssp, &__srcu_key); \
 })
 
+#define init_srcu_struct_fast_updown(ssp) \
+({ \
+	static struct lock_class_key __srcu_key; \
+	\
+	__init_srcu_struct_fast_updown((ssp), #ssp, &__srcu_key); \
+})
+
 #define __SRCU_DEP_MAP_INIT(srcu_name)	.dep_map = { .name = #srcu_name },
 #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 
 int init_srcu_struct(struct srcu_struct *ssp);
 #ifndef CONFIG_TINY_SRCU
 int init_srcu_struct_fast(struct srcu_struct *ssp);
+int init_srcu_struct_fast_updown(struct srcu_struct *ssp);
 #endif // #ifndef CONFIG_TINY_SRCU
 
 #define __SRCU_DEP_MAP_INIT(srcu_name)
@@ -305,6 +315,46 @@ static inline struct srcu_ctr __percpu *srcu_read_lock_fast(struct srcu_struct *
 	return retval;
 }
 
+/**
+ * srcu_read_lock_fast_updown - register a new reader for an SRCU-fast-updown 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.
+ * This function is compatible with srcu_down_read_fast(), but is not
+ * NMI-safe.
+ *
+ * For srcu_read_lock_fast_updown() to be used on an srcu_struct
+ * structure, that structure must have been defined using either
+ * DEFINE_SRCU_FAST_UPDOWN() or DEFINE_STATIC_SRCU_FAST_UPDOWN() on the one
+ * hand or initialized with init_srcu_struct_fast_updown() on the other.
+ * Such an srcu_struct structure cannot be passed to any non-fast-updown
+ * variant of srcu_read_{,un}lock() or srcu_{down,up}_read().  In kernels
+ * built with CONFIG_PROVE_RCU=y, __srcu_check_read_flavor() will complain
+ * bitterly if you ignore this * restriction.
+ *
+ * Grace-period auto-expediting is disabled for SRCU-fast-updown
+ * srcu_struct structures because SRCU-fast-updown expedited grace periods
+ * invoke synchronize_rcu_expedited(), IPIs and all.  If you need expedited
+ * SRCU-fast-updown grace periods, use synchronize_srcu_expedited().
+ *
+ * The srcu_read_lock_fast_updown() function 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 struct srcu_ctr __percpu *srcu_read_lock_fast_updown(struct srcu_struct *ssp)
+__acquires(ssp)
+{
+	struct srcu_ctr __percpu *retval;
+
+	RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_read_lock_fast_updown().");
+	srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_FAST_UPDOWN);
+	retval = __srcu_read_lock_fast_updown(ssp);
+	rcu_try_lock_acquire(&ssp->dep_map);
+	return retval;
+}
+
 /*
  * Used by tracing, cannot be traced and cannot call lockdep.
  * See srcu_read_lock_fast() for more information.
@@ -335,8 +385,8 @@ static inline struct srcu_ctr __percpu *srcu_down_read_fast(struct srcu_struct *
 {
 	WARN_ON_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && in_nmi());
 	RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_down_read_fast().");
-	srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_FAST);
-	return __srcu_read_lock_fast(ssp);
+	srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_FAST_UPDOWN);
+	return __srcu_read_lock_fast_updown(ssp);
 }
 
 /**
@@ -432,6 +482,23 @@ static inline void srcu_read_unlock_fast(struct srcu_struct *ssp, struct srcu_ct
 	RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_read_unlock_fast().");
 }
 
+/**
+ * srcu_read_unlock_fast_updown - unregister a old reader from an SRCU-fast-updown structure.
+ * @ssp: srcu_struct in which to unregister the old reader.
+ * @scp: return value from corresponding srcu_read_lock_fast_updown().
+ *
+ * Exit an SRCU-fast-updown read-side critical section.
+ */
+static inline void
+srcu_read_unlock_fast_updown(struct srcu_struct *ssp, struct srcu_ctr __percpu *scp) __releases(ssp)
+{
+	srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_FAST_UPDOWN);
+	srcu_lock_release(&ssp->dep_map);
+	__srcu_read_unlock_fast_updown(ssp, scp);
+	RCU_LOCKDEP_WARN(!rcu_is_watching(),
+			 "RCU must be watching srcu_read_unlock_fast_updown().");
+}
+
 /*
  * Used by tracing, cannot be traced and cannot call lockdep.
  * See srcu_read_unlock_fast() for more information.
@@ -455,9 +522,9 @@ static inline void srcu_up_read_fast(struct srcu_struct *ssp, struct srcu_ctr __
 	__releases(ssp)
 {
 	WARN_ON_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && in_nmi());
-	srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_FAST);
-	__srcu_read_unlock_fast(ssp, scp);
-	RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_up_read_fast().");
+	srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_FAST_UPDOWN);
+	__srcu_read_unlock_fast_updown(ssp, scp);
+	RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_up_read_fast_updown().");
 }
 
 /**