[PATCH RFC -rcu/dev] hazptrtorture: Release all deferred hazard pointers before stopping

[Andrea Parri <parri.andrea@gmail.com>]

hazptrtorture can hang at shutdown when deferral is enabled, as in the
PREEMPT scenario: the writer stays in ->sync() and kthread_stop() on it
never returns, until kvm.sh kills QEMU:

    [ 1800.899278] hazptr-torture: Stopping hazptr_torture_do_pending task
    [ 1800.899447] hazptr-torture: Stopping hazptr_torture_writer task
    [ 1812.713870] hazptr-torture: rtc: 00000000225df70a ver: 12436 HTWS_SYNC tfle: 0 rta: 12437 rtaf: 0 rtf: 12427 onoff: 48783/48783:48786/48786 3,254:3,280 611671:731494 (HZ=1000) acq: 5720353149 rel: 5720210186 acqirq: 571927 relirq: 714778 reldefer: 714139 relundefer 714114
    [ 1812.713971] hazptr-torture: hazptr_torture_stats is stopping

    Summary: Potential hang (QEMU killed)
    !!! PID 1393523 hung at 1980 vs. 1800 seconds

hazptr_torture_cleanup() stops the do_pending kthread before the writer
and the readers.  Once torture_must_stop(), do_pending makes one last
pass and leaves its loop, but unless stutter_will_wait(), that pass
releases the deferred hazard pointers of a single CPU.

Readers can also still defer after that pass, from the iteration they
are in when torture_must_stop() becomes true.  Stopped readers hold no
hazard pointer, so the writer waits on one of the 25 deferred hazard
pointers above (reldefer - relundefer) that nothing releases.

Keep do_pending running until hazptr_torture_cleanup() stops it, after
the writer and the readers, releasing the deferred hazard pointers of
all CPUs on each pass once torture_must_stop() and once more on exit.
It now also sleeps between those passes, and kthread_stop() wakes it.

This should be safe because readers defer only from their own loop,
which they have left once torture_stop_kthread() returns for them, and
the hazptr_torture_acquire() IPI runs synchronously within that loop.

With this change, a 30-minute PREEMPT run on x86_64 with 8 CPUs ends
with "End of test: SUCCESS" and with as many hazard pointers released
from the pending lists as were deferred (reldefer: 960658 relundefer
960658).

Fixes: f23d7b47ced9 ("hazptrtorture: Add kthread to release deferred hazard pointers")
Assisted-by: LLM
Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
---
 kernel/rcu/hazptrtorture.c | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/kernel/rcu/hazptrtorture.c b/kernel/rcu/hazptrtorture.c
index 7c8b5899fb011..5153720074644 100644
--- a/kernel/rcu/hazptrtorture.c
+++ b/kernel/rcu/hazptrtorture.c
@@ -547,18 +547,25 @@ static int hazptr_torture_do_pending(void *arg)
 	DEFINE_TORTURE_RANDOM(rand);
 
 	VERBOSE_TOROUT_STRING("hazptr_torture_do_pending task started");
-	do {
-		if (stutter_will_wait()) {
+	/*
+	 * Run until hazptr_torture_cleanup() stops this kthread, not just
+	 * until torture_must_stop(): the writer can be waiting in ->sync()
+	 * for a deferred hazard pointer while it is being stopped.
+	 */
+	while (!kthread_should_stop()) {
+		if (torture_must_stop() || stutter_will_wait()) {
 			for_each_possible_cpu(cpu)
 				hazptr_torture_do_one_pending(cpu, &rand);
 		} else {
 			cpu = cpumask_next_wrap(cpu, cpu_possible_mask);
 			hazptr_torture_do_one_pending(cpu, &rand);
 		}
-		if (!torture_must_stop())
-			torture_hrtimeout_ms(kthread_do_pending_ms, USEC_PER_MSEC, &rand);
+		torture_hrtimeout_ms(kthread_do_pending_ms, USEC_PER_MSEC, &rand);
 		// Omit stutter_wait() because this function needs to do cleanup.
-	} while (!torture_must_stop());
+	}
+	/* The readers have stopped, so nothing new can be deferred. */
+	for_each_possible_cpu(cpu)
+		hazptr_torture_do_one_pending(cpu, &rand);
 	torture_kthread_stopping("hazptr_torture_do_pending");
 	return 0;
 }
@@ -773,7 +780,6 @@ hazptr_torture_cleanup(void)
 		return;
 	}
 
-	torture_stop_kthread(hazptr_torture_do_pending, do_pending_task);
 	torture_stop_kthread(hazptr_torture_preempt, preempt_task);
 	torture_stop_kthread(hazptr_torture_writer, writer_task);
 
@@ -785,6 +791,12 @@ hazptr_torture_cleanup(void)
 		reader_tasks = NULL;
 	}
 
+	/*
+	 * Stop do_pending only after the writer and the readers: the writer
+	 * may be waiting on hazard pointers that readers deferred.
+	 */
+	torture_stop_kthread(hazptr_torture_do_pending, do_pending_task);
+
 	torture_stop_kthread(hazptr_torture_stats, stats_task);
 
 	/* Do torture-type-specific cleanup operations.  */

-- 
2.53.0