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

[Bradley Morgan <brads@mainlining.org>]

On 19 October 2026 17:20:03 BST, Andrea Parri <parri.andrea@gmail.com>
wrote:
>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

Why not put this into a series?

>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.
>+	 */

Could this be shorter?

>+	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)

Is the comment needed? I don't know if it's nessesary.

>+		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.
>+	 */

May we shorten this? Feel free to bikeshed that one.

>+	torture_stop_kthread(hazptr_torture_do_pending, do_pending_task);
>+
> 	torture_stop_kthread(hazptr_torture_stats, stats_task);
> 
> 	/* Do torture-type-specific cleanup operations.  */
>
>

--- Thanks!
"I'm not a very positive person" - Linus torvalds