[PATCH RFC v2 22/24] hazptrtorture: Fix hazptr ownership issue

["Paul E. McKenney" <paulmck@kernel.org>]

From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

hazptr_torture_acquire is invoked from IPI on remote CPUs. It needs to
immediately detach the hazptr from its local task so it can be released
from other tasks.

Without this fix, the hazptrtorture test fails loudly with a OOPS. It
passes with the fix applied.

[ This applies on top of linux-rcu dev branch at
  commit e5ee2ea2d00ae5a0ca53ccf9c9100f2357e1bd08 ]

[ paulmck: s/hazptr_detach_from_task/hazptr_detach/ per Mathieu. ]

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/hazptrtorture.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/kernel/rcu/hazptrtorture.c b/kernel/rcu/hazptrtorture.c
index ac0cb5b9f4f0f6..b2999f577af8b1 100644
--- a/kernel/rcu/hazptrtorture.c
+++ b/kernel/rcu/hazptrtorture.c
@@ -370,6 +370,11 @@ static void hazptr_torture_acquire(void *hppp_in)
 	struct hazptr_pending *hppp = hppp_in;
 
 	hppp->hpp_htp = cur_ops->readlock(&hppp->hpp_hc);
+	/*
+	 * Acquiring a hazard pointer from a remote CPU.
+	 * Detach hazptr from its task so it can be released by another task.
+	 */
+	hazptr_detach(&hppp->hpp_hc);
 	atomic_long_inc(per_cpu_ptr(&hazptr_torture_acquires_irq, raw_smp_processor_id()));
 }
 
@@ -411,6 +416,10 @@ hazptr_torture_reader_tail(struct hazptr_pending *hppp, struct torture_random_st
 	preempt_enable();
 	if (irq_release && !(torture_random(trsp) % irq_release)) {
 		guard(preempt)();
+		/*
+		 * Handling release from a remote CPU.
+		 * Detach hazptr from its task so it can be released by another task.
+		 */
 		hazptr_detach(&hppp->hpp_hc);
 		cpu = cpumask_next_wrap(smp_processor_id(), cpu_online_mask);
 		smp_call_function_single(cpu, hazptr_torture_release, hppp, 1);
@@ -429,6 +438,10 @@ static void hazptr_torture_defer(struct hazptr_pending *hppp, struct torture_ran
 	struct llist_head *llhp;
 
 	guard(preempt)();
+	/*
+	 * Handling release from a remote CPU.
+	 * Detach hazptr from its task so it can be released by another task.
+	 */
 	hazptr_detach(&hppp->hpp_hc);
 	cpu = cpumask_next_wrap(cpu, cpu_online_mask);
 	llhp = per_cpu_ptr(&hazptr_pending, cpu);
-- 
2.40.1