torture: Add get_torture_init_jiffies() for test-start time

This commit adds a get_torture_init_jiffies() function that returns the
value of the jiffies counter at the start of the test, that is, at the
point where torture_init_begin() was invoked.

This will be used to enable torture-test holdoffs for tests implemented
using per-CPU kthreads, which are created and deleted by CPU-hotplug
operations, and thus (unlike normal kthreads) don't automatically know
when the test started.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>

Author: paulmckrcu

include/linux/torture.h

@@ -104,6 +104,7 @@ int torture_stutter_init(int s, int sgap);
 /* Initialization and cleanup. */
 bool torture_init_begin(char *ttype, int v);
 void torture_init_end(void);
+unsigned long get_torture_init_jiffies(void);
 bool torture_cleanup_begin(void);
 void torture_cleanup_end(void);
 bool torture_must_stop(void);

kernel/torture.c

@@ -792,6 +792,8 @@ static void torture_stutter_cleanup(void)
 	stutter_task = NULL;
 }
 
+static unsigned long torture_init_jiffies;
+
 static void
 torture_print_module_parms(void)
 {
@@ -821,6 +823,7 @@ bool torture_init_begin(char *ttype, int v)
 	torture_type = ttype;
 	verbose = v;
 	fullstop = FULLSTOP_DONTSTOP;
+	WRITE_ONCE(torture_init_jiffies, jiffies); // Lockless reads.
 	torture_print_module_parms();
 	return true;
 }
@@ -836,6 +839,15 @@ void torture_init_end(void)
 }
 EXPORT_SYMBOL_GPL(torture_init_end);
 
+/*
+ * Get the torture_init_begin()-time value of the jiffies counter.
+ */
+unsigned long get_torture_init_jiffies(void)
+{
+	return READ_ONCE(torture_init_jiffies);
+}
+EXPORT_SYMBOL_GPL(get_torture_init_jiffies);
+
 /*
  * Clean up torture module.  Please note that this is -not- invoked via
  * the usual module_exit() mechanism, but rather by an explicit call from