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, 35 insertions, 4 deletions

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 2f8aa280911e..ea3f128de06f 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -309,13 +309,24 @@ int __init_srcu_struct_fast(struct srcu_struct *ssp, const char *name, struct lo
 }
 EXPORT_SYMBOL_GPL(__init_srcu_struct_fast);
 
+int __init_srcu_struct_fast_updown(struct srcu_struct *ssp, const char *name,
+				   struct lock_class_key *key)
+{
+	ssp->srcu_reader_flavor = SRCU_READ_FLAVOR_FAST_UPDOWN;
+	return __init_srcu_struct_common(ssp, name, key);
+}
+EXPORT_SYMBOL_GPL(__init_srcu_struct_fast_updown);
+
 #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 
 /**
  * init_srcu_struct - initialize a sleep-RCU structure
  * @ssp: structure to initialize.
  *
- * Must invoke this on a given srcu_struct before passing that srcu_struct
+ * Use this in place of DEFINE_SRCU() and DEFINE_STATIC_SRCU()
+ * for non-static srcu_struct structures that are to be passed to
+ * srcu_read_lock(), srcu_read_lock_nmisafe(), and friends.  It is necessary
+ * to invoke this on a given srcu_struct before passing that srcu_struct
  * to any other function.  Each srcu_struct represents a separate domain
  * of SRCU protection.
  */
@@ -330,9 +341,11 @@ EXPORT_SYMBOL_GPL(init_srcu_struct);
  * init_srcu_struct_fast - initialize a fast-reader sleep-RCU structure
  * @ssp: structure to initialize.
  *
- * Must invoke this on a given srcu_struct before passing that srcu_struct
- * to any other function.  Each srcu_struct represents a separate domain
- * of SRCU protection.
+ * Use this in place of DEFINE_SRCU_FAST() and DEFINE_STATIC_SRCU_FAST()
+ * for non-static srcu_struct structures that are to be passed to
+ * srcu_read_lock_fast() and friends.  It is necessary to invoke this on a
+ * given srcu_struct before passing that srcu_struct to any other function.
+ * Each srcu_struct represents a separate domain of SRCU protection.
  */
 int init_srcu_struct_fast(struct srcu_struct *ssp)
 {
@@ -341,6 +354,24 @@ int init_srcu_struct_fast(struct srcu_struct *ssp)
 }
 EXPORT_SYMBOL_GPL(init_srcu_struct_fast);
 
+/**
+ * init_srcu_struct_fast_updown - initialize a fast-reader up/down sleep-RCU structure
+ * @ssp: structure to initialize.
+ *
+ * Use this function in place of DEFINE_SRCU_FAST_UPDOWN() and
+ * DEFINE_STATIC_SRCU_FAST_UPDOWN() for non-static srcu_struct
+ * structures that are to be passed to srcu_read_lock_fast_updown(),
+ * srcu_down_read_fast(), and friends.  It is necessary to invoke this on a
+ * given srcu_struct before passing that srcu_struct to any other function.
+ * Each srcu_struct represents a separate domain of SRCU protection.
+ */
+int init_srcu_struct_fast_updown(struct srcu_struct *ssp)
+{
+	ssp->srcu_reader_flavor = SRCU_READ_FLAVOR_FAST_UPDOWN;
+	return init_srcu_struct_fields(ssp, false);
+}
+EXPORT_SYMBOL_GPL(init_srcu_struct_fast_updown);
+
 #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 
 /*