srcu: Add srcu_down_read_fast() and srcu_up_read_fast()

A pair of matching srcu_read_lock_fast() and srcu_read_unlock_fast()
invocations must take place within the same context, for example, within
the same task.  Otherwise, lockdep complains, as is the right thing to
do for most use cases.

However, there are use cases involving tracing (for example, uretprobes)
in which an SRCU reader needs to begin in one task and end in a
timer handler, which might interrupt some other task.  This commit
therefore supplies the semaphore-like srcu_down_read_fast() and
srcu_up_read_fast() functions, which act like srcu_read_lock_fast() and
srcu_read_unlock_fast(), but permitting srcu_up_read_fast() to be invoked
in a different context than was the matching srcu_down_read_fast().

Neither srcu_down_read_fast() nor srcu_up_read_fast() may be invoked
from an NMI handler.

Reported-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: <bpf@vger.kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>

Author: paulmckrcu

include/linux/srcu.h

@@ -281,6 +281,24 @@ static inline struct srcu_ctr __percpu *srcu_read_lock_fast(struct srcu_struct *
 	return retval;
 }
 
+/**
+ * srcu_down_read_fast - register a new reader for an SRCU-protected structure.
+ * @ssp: srcu_struct in which to register the new reader.
+ *
+ * Enter a semaphore-like SRCU read-side critical section, but for
+ * a light-weight smp_mb()-free reader.  See srcu_read_lock_fast() and
+ * srcu_down_read() for more information.
+ *
+ * The same srcu_struct may be used concurrently by srcu_down_read_fast()
+ * and srcu_read_lock_fast().
+ */
+static inline struct srcu_ctr __percpu *srcu_down_read_fast(struct srcu_struct *ssp) __acquires(ssp)
+{
+	WARN_ON_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && in_nmi());
+	srcu_check_read_flavor_force(ssp, SRCU_READ_FLAVOR_FAST);
+	return __srcu_read_lock_fast(ssp);
+}
+
 /**
  * srcu_read_lock_lite - register a new reader for an SRCU-protected structure.
  * @ssp: srcu_struct in which to register the new reader.
@@ -400,6 +418,22 @@ static inline void srcu_read_unlock_fast(struct srcu_struct *ssp, struct srcu_ct
 	__srcu_read_unlock_fast(ssp, scp);
 }
 
+/**
+ * srcu_up_read_fast - unregister a old reader from an SRCU-protected structure.
+ * @ssp: srcu_struct in which to unregister the old reader.
+ * @scp: return value from corresponding srcu_read_lock_fast().
+ *
+ * Exit an SRCU read-side critical section, but not necessarily from
+ * the same context as the maching srcu_down_read_fast().
+ */
+static inline void srcu_up_read_fast(struct srcu_struct *ssp, struct srcu_ctr __percpu *scp)
+	__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);
+}
+
 /**
  * srcu_read_unlock_lite - unregister a old reader from an SRCU-protected structure.
  * @ssp: srcu_struct in which to unregister the old reader.