[PATCH -rcu/dev 1/4] 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 first, then the
writer, then 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.  Readers that have left
their loop hold no hazard pointer, so the writer waits on one of the 25
deferred hazard pointers above (reldefer - relundefer) that nothing
releases.

Stop the readers before do_pending, and do_pending before the writer.
do_pending still leaves its loop at torture_must_stop(), and now
releases the deferred hazard pointers of all CPUs once
torture_kthread_stopping() returns, that is, once
hazptr_torture_cleanup() stops it, after the readers.  The writer can
still wait in ->sync(), but only until then.

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.
The readers do not need the writer to stop: they leave their loop at
torture_must_stop(), after at most one stutter_wait().

do_pending keeps the torture_must_stop() check before its sleep.  A
kthread_stop() that comes after that check and before the sleep starts
still delays do_pending's exit by a full kthread_do_pending_ms sleep,
however long that is.  With the readers stopped first, this happens
only if do_pending passed the check before the test began stopping and
had not yet gone to sleep when kthread_stop() came, after all the
readers had stopped.

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: 983727 relundefer
983727).

The hang was found with the help of an LLM.

Fixes: f23d7b47ced9 ("hazptrtorture: Add kthread to release deferred hazard pointers")
Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
---
 kernel/rcu/hazptrtorture.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/hazptrtorture.c b/kernel/rcu/hazptrtorture.c
index 7c8b5899fb011..c46083a8af093 100644
--- a/kernel/rcu/hazptrtorture.c
+++ b/kernel/rcu/hazptrtorture.c
@@ -560,6 +560,9 @@ static int hazptr_torture_do_pending(void *arg)
 		// Omit stutter_wait() because this function needs to do cleanup.
 	} while (!torture_must_stop());
 	torture_kthread_stopping("hazptr_torture_do_pending");
+	/* The readers have stopped, so nothing new can be deferred. */
+	for_each_possible_cpu(cpu)
+		hazptr_torture_do_one_pending(cpu, &rand);
 	return 0;
 }
 
@@ -773,9 +776,7 @@ 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);
 
 	if (reader_tasks) {
 		for (i = 0; i < nrealreaders; i++)
@@ -785,6 +786,14 @@ hazptr_torture_cleanup(void)
 		reader_tasks = NULL;
 	}
 
+	/*
+	 * Stop the writer only after do_pending, and do_pending only after
+	 * the readers: the writer may be waiting on hazard pointers that
+	 * readers deferred, which do_pending releases once they stopped.
+	 */
+	torture_stop_kthread(hazptr_torture_do_pending, do_pending_task);
+	torture_stop_kthread(hazptr_torture_writer, writer_task);
+
 	torture_stop_kthread(hazptr_torture_stats, stats_task);
 
 	/* Do torture-type-specific cleanup operations.  */
-- 
2.53.0