[PATCH 6.6 074/638] workqueue: RCU protect wq->dfl_pwq and implement accessors for it

Sasha Levin
2024-03-24T23:02:29Z

From: Tejun Heo <tj@kernel.org>

[ Upstream commit 9f66cff212bb3c1cd25996aaa0dfd0c9e9d8baab ]

wq->cpu_pwq is RCU protected but wq->dfl_pwq isn't. This is okay because
currently wq->dfl_pwq is used only accessed to install it into wq->cpu_pwq
which doesn't require RCU access. However, we want to be able to access
wq->dfl_pwq under RCU in the future to access its __pod_cpumask and the code
can be made easier to read by making the two pwq fields behave in the same
way.

- Make wq->dfl_pwq RCU protected.

- Add unbound_pwq_slot() and unbound_pwq() which can access both ->dfl_pwq
and ->cpu_pwq. The former returns the double pointer that can be used
access and update the pwqs. The latter performs locking check and
dereferences the double pointer.

- pwq accesses and updates are converted to use unbound_pwq[_slot]().

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <jiangshanlai@gmail.com>
Stable-dep-of: 5797b1c18919 ("workqueue: Implement system-wide nr_active enforcement for unbound workqueues")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/workqueue.c | 64 +++++++++++++++++++++++++++++-----------------
1 file changed, 40 insertions(+), 24 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 76559b63ee1fc..80c0a84aa147c 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -304,7 +304,7 @@ struct workqueue_struct {
int saved_max_active; /* WQ: saved max_active */

struct workqueue_attrs *unbound_attrs; /* PW: only for unbound wqs */
- struct pool_workqueue *dfl_pwq; /* PW: only for unbound wqs */
+ struct pool_workqueue __rcu *dfl_pwq; /* PW: only for unbound wqs */

#ifdef CONFIG_SYSFS
struct wq_device *wq_dev; /* I: for sysfs interface */
@@ -629,6 +629,23 @@ static int worker_pool_assign_id(struct worker_pool *pool)
return ret;
}

+static struct pool_workqueue __rcu **
+unbound_pwq_slot(struct workqueue_struct *wq, int cpu)
+{
+ if (cpu >= 0)
+ return per_cpu_ptr(wq->cpu_pwq, cpu);
+ else
+ return &wq->dfl_pwq;
+}
+
+/* @cpu < 0 for dfl_pwq */
+static struct pool_workqueue *unbound_pwq(struct workqueue_struct *wq, int cpu)
+{
+ return rcu_dereference_check(*unbound_pwq_slot(wq, cpu),
+ lockdep_is_held(&wq_pool_mutex) ||
+ lockdep_is_held(&wq->mutex));
+}
+
static unsigned int work_color_to_flags(int color)
{
return color << WORK_STRUCT_COLOR_SHIFT;
@@ -4318,10 +4335,11 @@ static void wq_calc_pod_cpumask(struct workqueue_attrs *attrs, int cpu,
"possible intersect\n");
}

-/* install @pwq into @wq's cpu_pwq and return the old pwq */
+/* install @pwq into @wq and return the old pwq, @cpu < 0 for dfl_pwq */
static struct pool_workqueue *install_unbound_pwq(struct workqueue_struct *wq,
int cpu, struct pool_workqueue *pwq)
{
+ struct pool_workqueue __rcu **slot = unbound_pwq_slot(wq, cpu);
struct pool_workqueue *old_pwq;

lockdep_assert_held(&wq_pool_mutex);
@@ -4330,8 +4348,8 @@ static struct pool_workqueue *install_unbound_pwq(struct workqueue_struct *wq,
/* link_pwq() can handle duplicate calls */
link_pwq(pwq);

- old_pwq = rcu_access_pointer(*per_cpu_ptr(wq->cpu_pwq, cpu));
- rcu_assign_pointer(*per_cpu_ptr(wq->cpu_pwq, cpu), pwq);
+ old_pwq = rcu_access_pointer(*slot);
+ rcu_assign_pointer(*slot, pwq);
return old_pwq;
}

@@ -4431,14 +4449,11 @@ static void apply_wqattrs_commit(struct apply_wqattrs_ctx *ctx)

copy_workqueue_attrs(ctx->wq->unbound_attrs, ctx->attrs);

- /* save the previous pwq and install the new one */
+ /* save the previous pwqs and install the new ones */
for_each_possible_cpu(cpu)
ctx->pwq_tbl[cpu] = install_unbound_pwq(ctx->wq, cpu,
ctx->pwq_tbl[cpu]);
-
- /* @dfl_pwq might not have been used, ensure it's linked */
- link_pwq(ctx->dfl_pwq);
- swap(ctx->wq->dfl_pwq, ctx->dfl_pwq);
+ ctx->dfl_pwq = install_unbound_pwq(ctx->wq, -1, ctx->dfl_pwq);

mutex_unlock(&ctx->wq->mutex);
}
@@ -4561,9 +4576,7 @@ static void wq_update_pod(struct workqueue_struct *wq, int cpu,

/* nothing to do if the target cpumask matches the current pwq */
wq_calc_pod_cpumask(target_attrs, cpu, off_cpu);
- pwq = rcu_dereference_protected(*per_cpu_ptr(wq->cpu_pwq, cpu),
- lockdep_is_held(&wq_pool_mutex));
- if (wqattrs_equal(target_attrs, pwq->pool->attrs))
+ if (wqattrs_equal(target_attrs, unbound_pwq(wq, cpu)->pool->attrs))
return;

/* create a new pwq */
@@ -4581,10 +4594,11 @@ static void wq_update_pod(struct workqueue_struct *wq, int cpu,

use_dfl_pwq:
mutex_lock(&wq->mutex);
- raw_spin_lock_irq(&wq->dfl_pwq->pool->lock);
- get_pwq(wq->dfl_pwq);
- raw_spin_unlock_irq(&wq->dfl_pwq->pool->lock);
- old_pwq = install_unbound_pwq(wq, cpu, wq->dfl_pwq);
+ pwq = unbound_pwq(wq, -1);
+ raw_spin_lock_irq(&pwq->pool->lock);
+ get_pwq(pwq);
+ raw_spin_unlock_irq(&pwq->pool->lock);
+ old_pwq = install_unbound_pwq(wq, cpu, pwq);
out_unlock:
mutex_unlock(&wq->mutex);
put_pwq_unlocked(old_pwq);
@@ -4622,10 +4636,13 @@ static int alloc_and_link_pwqs(struct workqueue_struct *wq)

cpus_read_lock();
if (wq->flags & __WQ_ORDERED) {
+ struct pool_workqueue *dfl_pwq;
+
ret = apply_workqueue_attrs(wq, ordered_wq_attrs[highpri]);
/* there should only be single pwq for ordering guarantee */
- WARN(!ret && (wq->pwqs.next != &wq->dfl_pwq->pwqs_node ||
- wq->pwqs.prev != &wq->dfl_pwq->pwqs_node),
+ dfl_pwq = rcu_access_pointer(wq->dfl_pwq);
+ WARN(!ret && (wq->pwqs.next != &dfl_pwq->pwqs_node ||
+ wq->pwqs.prev != &dfl_pwq->pwqs_node),
"ordering guarantee broken for workqueue %s\n", wq->name);
} else {
ret = apply_workqueue_attrs(wq, unbound_std_wq_attrs[highpri]);
@@ -4856,7 +4873,7 @@ static bool pwq_busy(struct pool_workqueue *pwq)
if (pwq->nr_in_flight[i])
return true;

- if ((pwq != pwq->wq->dfl_pwq) && (pwq->refcnt > 1))
+ if ((pwq != rcu_access_pointer(pwq->wq->dfl_pwq)) && (pwq->refcnt > 1))
return true;
if (!pwq_is_empty(pwq))
return true;
@@ -4940,13 +4957,12 @@ void destroy_workqueue(struct workqueue_struct *wq)
rcu_read_lock();

for_each_possible_cpu(cpu) {
- pwq = rcu_access_pointer(*per_cpu_ptr(wq->cpu_pwq, cpu));
- RCU_INIT_POINTER(*per_cpu_ptr(wq->cpu_pwq, cpu), NULL);
- put_pwq_unlocked(pwq);
+ put_pwq_unlocked(unbound_pwq(wq, cpu));
+ RCU_INIT_POINTER(*unbound_pwq_slot(wq, cpu), NULL);
}

- put_pwq_unlocked(wq->dfl_pwq);
- wq->dfl_pwq = NULL;
+ put_pwq_unlocked(unbound_pwq(wq, -1));
+ RCU_INIT_POINTER(*unbound_pwq_slot(wq, -1), NULL);

rcu_read_unlock();
}
--
2.43.0