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McKenney" , Joel Fernandes , Frederic Weisbecker , Boqun Feng Cc: RCU , LKML , Uladzislau Rezki , Saravana Kannan Subject: [PATCH -next v1 01/12] rcutorture: Fully test lazy RCU Date: Mon, 11 May 2026 19:54:37 +0200 Message-ID: <20260511175448.153326-2-urezki@gmail.com> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260511175448.153326-1-urezki@gmail.com> References: <20260511175448.153326-1-urezki@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: "Paul E. McKenney" Currently, rcutorture bypasses lazy RCU by using call_rcu_hurry(). This works, avoiding the dreaded rtort_pipe_count WARN(), but fails to fully test lazy RCU. The rtort_pipe_count WARN() splats because lazy RCU could delay the start of an RCU grace period for a full stutter period, which defaults to only three seconds. This commit therefore reverts the call_rcu_hurry() instances back to call_rcu(), but, in kernels built with CONFIG_RCU_LAZY=y, queues a workqueue handler just before the call to stutter_wait() in rcu_torture_writer(). This workqueue handler invokes rcu_barrier(), which motivates any lingering lazy callbacks, thus avoiding the splat. Questions for review: 1. Should we avoid queueing work for RCU implementations not supporting lazy callbacks? 2. Should we avoid queueing work in kernels built with CONFIG_RCU_LAZY=y, but that were not booted with the rcutree.enable_rcu_lazy kernel boot parameter set? (Note that this requires some ugliness to access this parameter, and must also handle Tiny RCU.) 3. Does the rcu_torture_ops structure need a ->call_hurry() field, and if so, why? If not, why not? 4. Your additional questions here! Reported-by: Saravana Kannan Signed-off-by: Paul E. McKenney Signed-off-by: Uladzislau Rezki (Sony) --- kernel/rcu/rcutorture.c | 21 ++++++++++++++++++--- 1 file changed, 18 insertions(+), 3 deletions(-) diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c index 5f2848b828dc..91ba3160ba6a 100644 --- a/kernel/rcu/rcutorture.c +++ b/kernel/rcu/rcutorture.c @@ -572,7 +572,7 @@ static unsigned long rcu_no_completed(void) static void rcu_torture_deferred_free(struct rcu_torture *p) { - call_rcu_hurry(&p->rtort_rcu, rcu_torture_cb); + call_rcu(&p->rtort_rcu, rcu_torture_cb); } static void rcu_sync_torture_init(void) @@ -619,7 +619,7 @@ static struct rcu_torture_ops rcu_ops = { .poll_gp_state_exp = poll_state_synchronize_rcu, .cond_sync_exp = cond_synchronize_rcu_expedited, .cond_sync_exp_full = cond_synchronize_rcu_expedited_full, - .call = call_rcu_hurry, + .call = call_rcu, .cb_barrier = rcu_barrier, .fqs = rcu_force_quiescent_state, .gp_kthread_dbg = show_rcu_gp_kthreads, @@ -1145,7 +1145,7 @@ static void rcu_tasks_torture_deferred_free(struct rcu_torture *p) static void synchronize_rcu_mult_test(void) { - synchronize_rcu_mult(call_rcu_tasks, call_rcu_hurry); + synchronize_rcu_mult(call_rcu_tasks, call_rcu); } static struct rcu_torture_ops tasks_ops = { @@ -1631,6 +1631,17 @@ static void do_rtws_sync(struct torture_random_state *trsp, void (*sync)(void)) cpus_read_unlock(); } +/* + * Do an rcu_barrier() to motivate lazy callbacks during a stutter + * pause. Without this, we can get false-positives rtort_pipe_count + * splats. + */ +static void rcu_torture_writer_work(struct work_struct *work) +{ + if (cur_ops->cb_barrier) + cur_ops->cb_barrier(); +} + /* * RCU torture writer kthread. Repeatedly substitutes a new structure * for that pointed to by rcu_torture_current, freeing the old structure @@ -1651,6 +1662,7 @@ rcu_torture_writer(void *arg) int i; int idx; unsigned long j; + struct work_struct lazy_work; int oldnice = task_nice(current); struct rcu_gp_oldstate *rgo = NULL; int rgo_size = 0; @@ -1667,6 +1679,7 @@ rcu_torture_writer(void *arg) stallsdone += (stall_cpu_holdoff + stall_gp_kthread + stall_cpu + 60) * HZ * (stall_cpu_repeat + 1); VERBOSE_TOROUT_STRING("rcu_torture_writer task started"); + INIT_WORK_ONSTACK(&lazy_work, rcu_torture_writer_work); if (!can_expedite) pr_alert("%s" TORTURE_FLAG " GP expediting controlled from boot/sysfs for %s.\n", @@ -1895,6 +1908,8 @@ rcu_torture_writer(void *arg) !rcu_gp_is_normal(); } rcu_torture_writer_state = RTWS_STUTTER; + if (IS_ENABLED(CONFIG_RCU_LAZY)) + queue_work(system_percpu_wq, &lazy_work); stutter_waited = stutter_wait("rcu_torture_writer"); if (stutter_waited && !atomic_read(&rcu_fwd_cb_nodelay) && -- 2.47.3