rcutorture: Fix rcutorture shutdown races

Not all of the rcutorture kthreads waited for kthread_should_stop()
before returning from their top-level functions, and none of them
used torture_shutdown_absorb() properly.  These problems can result in
segfaults and hangs at shutdown time, and some recent changes perturbed
timing sufficiently to make them much more probable.  This commit
therefore creates a torture_kthread_stopping() function that does the
proper kthread shutdown dance in one centralized location.

Accommodate this grouping by making VERBOSE_TOROUT_STRING() capable of
taking a non-const string as its argument, which allows the new
torture_kthread_stopping() to pass its "title" argument directly to
the updated version of VERBOSE_TOROUT_STRING().
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

include/linux/torture.h

@@ -45,7 +45,7 @@
 #define TOROUT_STRING(s) \
 	pr_alert("%s" TORTURE_FLAG s "\n", torture_type)
 #define VERBOSE_TOROUT_STRING(s) \
-	do { if (verbose) pr_alert("%s" TORTURE_FLAG s "\n", torture_type); } while (0)
+	do { if (verbose) pr_alert("%s" TORTURE_FLAG " %s\n", torture_type, s); } while (0)
 #define VERBOSE_TOROUT_ERRSTRING(s) \
 	do { if (verbose) pr_alert("%s" TORTURE_FLAG "!!! " s "\n", torture_type); } while (0)
 
@@ -88,5 +88,6 @@ void torture_init_end(void);
 bool torture_cleanup(void);
 bool torture_must_stop(void);
 bool torture_must_stop_irq(void);
+void torture_kthread_stopping(char *title);
 
 #endif /* __LINUX_TORTURE_H */

kernel/rcu/rcutorture.c

@@ -602,12 +602,13 @@ checkwait:	stutter_wait("rcu_torture_boost");
 	} while (!torture_must_stop());
 
 	/* Clean up and exit. */
-	VERBOSE_TOROUT_STRING("rcu_torture_boost task stopping");
-	torture_shutdown_absorb("rcu_torture_boost");
-	while (!kthread_should_stop() || rbi.inflight)
+	while (!kthread_should_stop() || rbi.inflight) {
+		torture_shutdown_absorb("rcu_torture_boost");
 		schedule_timeout_uninterruptible(1);
+	}
 	smp_mb(); /* order accesses to ->inflight before stack-frame death. */
 	destroy_rcu_head_on_stack(&rbi.rcu);
+	torture_kthread_stopping("rcu_torture_boost");
 	return 0;
 }
 
@@ -638,10 +639,7 @@ rcu_torture_fqs(void *arg)
 		}
 		stutter_wait("rcu_torture_fqs");
 	} while (!torture_must_stop());
-	VERBOSE_TOROUT_STRING("rcu_torture_fqs task stopping");
-	torture_shutdown_absorb("rcu_torture_fqs");
-	while (!kthread_should_stop())
-		schedule_timeout_uninterruptible(1);
+	torture_kthread_stopping("rcu_torture_fqs");
 	return 0;
 }
 
@@ -710,10 +708,7 @@ rcu_torture_writer(void *arg)
 		rcutorture_record_progress(++rcu_torture_current_version);
 		stutter_wait("rcu_torture_writer");
 	} while (!torture_must_stop());
-	VERBOSE_TOROUT_STRING("rcu_torture_writer task stopping");
-	torture_shutdown_absorb("rcu_torture_writer");
-	while (!kthread_should_stop())
-		schedule_timeout_uninterruptible(1);
+	torture_kthread_stopping("rcu_torture_writer");
 	return 0;
 }
 
@@ -748,10 +743,7 @@ rcu_torture_fakewriter(void *arg)
 		stutter_wait("rcu_torture_fakewriter");
 	} while (!torture_must_stop());
 
-	VERBOSE_TOROUT_STRING("rcu_torture_fakewriter task stopping");
-	torture_shutdown_absorb("rcu_torture_fakewriter");
-	while (!kthread_should_stop())
-		schedule_timeout_uninterruptible(1);
+	torture_kthread_stopping("rcu_torture_fakewriter");
 	return 0;
 }
 
@@ -892,12 +884,9 @@ rcu_torture_reader(void *arg)
 		schedule();
 		stutter_wait("rcu_torture_reader");
 	} while (!torture_must_stop());
-	VERBOSE_TOROUT_STRING("rcu_torture_reader task stopping");
-	torture_shutdown_absorb("rcu_torture_reader");
 	if (irqreader && cur_ops->irq_capable)
 		del_timer_sync(&t);
-	while (!kthread_should_stop())
-		schedule_timeout_uninterruptible(1);
+	torture_kthread_stopping("rcu_torture_reader");
 	return 0;
 }
 
@@ -1010,7 +999,7 @@ rcu_torture_stats(void *arg)
 		rcu_torture_stats_print();
 		torture_shutdown_absorb("rcu_torture_stats");
 	} while (!torture_must_stop());
-	VERBOSE_TOROUT_STRING("rcu_torture_stats task stopping");
+	torture_kthread_stopping("rcu_torture_stats");
 	return 0;
 }
 
@@ -1171,12 +1160,9 @@ static int rcu_torture_barrier_cbs(void *arg)
 		if (atomic_dec_and_test(&barrier_cbs_count))
 			wake_up(&barrier_wq);
 	} while (!torture_must_stop());
-	VERBOSE_TOROUT_STRING("rcu_torture_barrier_cbs task stopping");
-	torture_shutdown_absorb("rcu_torture_barrier_cbs");
-	while (!kthread_should_stop())
-		schedule_timeout_interruptible(1);
 	cur_ops->cb_barrier();
 	destroy_rcu_head_on_stack(&rcu);
+	torture_kthread_stopping("rcu_torture_barrier_cbs");
 	return 0;
 }
 
@@ -1207,10 +1193,7 @@ static int rcu_torture_barrier(void *arg)
 		n_barrier_successes++;
 		schedule_timeout_interruptible(HZ / 10);
 	} while (!torture_must_stop());
-	VERBOSE_TOROUT_STRING("rcu_torture_barrier task stopping");
-	torture_shutdown_absorb("rcu_torture_barrier");
-	while (!kthread_should_stop())
-		schedule_timeout_interruptible(1);
+	torture_kthread_stopping("rcu_torture_barrier");
 	return 0;
 }
 

kernel/torture.c

@@ -169,7 +169,7 @@ torture_onoff(void *arg)
 		}
 		schedule_timeout_interruptible(onoff_interval);
 	}
-	VERBOSE_TOROUT_STRING("torture_onoff task stopping");
+	torture_kthread_stopping("torture_onoff");
 	return 0;
 }
 
@@ -370,7 +370,7 @@ static int torture_shuffle(void *arg)
 		torture_shuffle_tasks();
 		torture_shutdown_absorb("torture_shuffle");
 	} while (!torture_must_stop());
-	VERBOSE_TOROUT_STRING("torture_shuffle task stopping");
+	torture_kthread_stopping("torture_shuffle");
 	return 0;
 }
 
@@ -465,7 +465,7 @@ static int torture_shutdown(void *arg)
 		jiffies_snap = jiffies;
 	}
 	if (torture_must_stop()) {
-		VERBOSE_TOROUT_STRING("torture_shutdown task stopping");
+		torture_kthread_stopping("torture_shutdown");
 		return 0;
 	}
 
@@ -583,7 +583,7 @@ static int torture_stutter(void *arg)
 		ACCESS_ONCE(stutter_pause_test) = 0;
 		torture_shutdown_absorb("torture_stutter");
 	} while (!torture_must_stop());
-	VERBOSE_TOROUT_STRING("torture_stutter task stopping");
+	torture_kthread_stopping("torture_stutter");
 	return 0;
 }
 
@@ -696,3 +696,21 @@ bool torture_must_stop_irq(void)
 	return ACCESS_ONCE(fullstop) != FULLSTOP_DONTSTOP;
 }
 EXPORT_SYMBOL_GPL(torture_must_stop_irq);
+
+/*
+ * Each kthread must wait for kthread_should_stop() before returning from
+ * its top-level function, otherwise segfaults ensue.  This function
+ * prints a "stopping" message and waits for kthread_should_stop(), and
+ * should be called from all torture kthreads immediately prior to
+ * returning.
+ */
+void torture_kthread_stopping(char *title)
+{
+	if (verbose)
+		VERBOSE_TOROUT_STRING(title);
+	while (!kthread_should_stop()) {
+		torture_shutdown_absorb(title);
+		schedule_timeout_uninterruptible(1);
+	}
+}
+EXPORT_SYMBOL_GPL(torture_kthread_stopping);