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Subject[PATCH 1/5] workqueue: wq_pool_mutex protects the attrs-installation
Date
Current wq_pool_mutex doesn't proctect the attrs-installation, it results
that ->unbound_attrs, ->numa_pwq_tbl[] and ->dfl_pwq can only be accessed
under wq->mutex and causes some inconveniences. Example, wq_update_unbound_numa()
has to acquire wq->mutex before fetching the wq->unbound_attrs->no_numa
and the old_pwq. After this patch, wq_update_unbound_numa() is simplified.

attrs-installation (including apply_wqattrs_cleanup()) is a short operation,
so this change will no cause any latency for other operations which also
acquire the wq_pool_mutex.

The only unprotected attrs-installation code is in apply_workqueue_attrs(),
so this patch touches code less than comments.

It is also a preparation patch of merging similar code in apply_workqueue_attrs()
and wq_update_unbound_numa() together.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---
kernel/workqueue.c | 44 ++++++++++++++++++++++++++------------------
1 file changed, 26 insertions(+), 18 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index a3915ab..fa8b949 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -127,6 +127,12 @@ enum {
*
* PR: wq_pool_mutex protected for writes. Sched-RCU protected for reads.
*
+ * PW: wq_pool_mutex and wq->mutex protected for writes. Any one of them
+ * protected for reads.
+ *
+ * PWR: wq_pool_mutex and wq->mutex protected for writes. Any one of them
+ * or sched-RCU for reads.
+ *
* WQ: wq->mutex protected.
*
* WR: wq->mutex protected for writes. Sched-RCU protected for reads.
@@ -247,8 +253,8 @@ struct workqueue_struct {
int nr_drainers; /* WQ: drain in progress */
int saved_max_active; /* WQ: saved pwq max_active */

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

#ifdef CONFIG_SYSFS
struct wq_device *wq_dev; /* I: for sysfs interface */
@@ -268,7 +274,7 @@ struct workqueue_struct {
/* hot fields used during command issue, aligned to cacheline */
unsigned int flags ____cacheline_aligned; /* WQ: WQ_* flags */
struct pool_workqueue __percpu *cpu_pwqs; /* I: per-cpu pwqs */
- struct pool_workqueue __rcu *numa_pwq_tbl[]; /* FR: unbound pwqs indexed by node */
+ struct pool_workqueue __rcu *numa_pwq_tbl[]; /* PWR: unbound pwqs indexed by node */
};

static struct kmem_cache *pwq_cache;
@@ -349,6 +355,12 @@ static void workqueue_sysfs_unregister(struct workqueue_struct *wq);
lockdep_is_held(&wq->mutex), \
"sched RCU or wq->mutex should be held")

+#define assert_rcu_or_wq_mutex_or_pool_mutex(wq) \
+ rcu_lockdep_assert(rcu_read_lock_sched_held() || \
+ lockdep_is_held(&wq->mutex) || \
+ lockdep_is_held(&wq_pool_mutex), \
+ "sched RCU, wq->mutex or wq_pool_mutex should be held")
+
#define for_each_cpu_worker_pool(pool, cpu) \
for ((pool) = &per_cpu(cpu_worker_pools, cpu)[0]; \
(pool) < &per_cpu(cpu_worker_pools, cpu)[NR_STD_WORKER_POOLS]; \
@@ -553,7 +565,7 @@ static int worker_pool_assign_id(struct worker_pool *pool)
* @wq: the target workqueue
* @node: the node ID
*
- * This must be called either with pwq_lock held or sched RCU read locked.
+ * This must be called either with wq_pool_mutex held or sched RCU read locked.
* If the pwq needs to be used beyond the locking in effect, the caller is
* responsible for guaranteeing that the pwq stays online.
*
@@ -562,7 +574,7 @@ static int worker_pool_assign_id(struct worker_pool *pool)
static struct pool_workqueue *unbound_pwq_by_node(struct workqueue_struct *wq,
int node)
{
- assert_rcu_or_wq_mutex(wq);
+ assert_rcu_or_wq_mutex_or_pool_mutex(wq);
return rcu_dereference_raw(wq->numa_pwq_tbl[node]);
}

@@ -3480,6 +3492,7 @@ static struct pool_workqueue *numa_pwq_tbl_install(struct workqueue_struct *wq,
struct pool_workqueue *old_pwq;

lockdep_assert_held(&wq->mutex);
+ lockdep_assert_held(&wq_pool_mutex);

/* link_pwq() can handle duplicate calls */
link_pwq(pwq);
@@ -3644,10 +3657,9 @@ int apply_workqueue_attrs(struct workqueue_struct *wq,
* pwqs accordingly.
*/
get_online_cpus();
-
mutex_lock(&wq_pool_mutex);
+
ctx = apply_wqattrs_prepare(wq, attrs);
- mutex_unlock(&wq_pool_mutex);

/* the ctx has been prepared successfully, let's commit it */
if (ctx) {
@@ -3655,10 +3667,11 @@ int apply_workqueue_attrs(struct workqueue_struct *wq,
ret = 0;
}

- put_online_cpus();
-
apply_wqattrs_cleanup(ctx);

+ mutex_unlock(&wq_pool_mutex);
+ put_online_cpus();
+
return ret;
}

@@ -3695,7 +3708,8 @@ static void wq_update_unbound_numa(struct workqueue_struct *wq, int cpu,

lockdep_assert_held(&wq_pool_mutex);

- if (!wq_numa_enabled || !(wq->flags & WQ_UNBOUND))
+ if (!wq_numa_enabled || !(wq->flags & WQ_UNBOUND) ||
+ wq->unbound_attrs->no_numa)
return;

/*
@@ -3706,10 +3720,6 @@ static void wq_update_unbound_numa(struct workqueue_struct *wq, int cpu,
target_attrs = wq_update_unbound_numa_attrs_buf;
cpumask = target_attrs->cpumask;

- mutex_lock(&wq->mutex);
- if (wq->unbound_attrs->no_numa)
- goto out_unlock;
-
copy_workqueue_attrs(target_attrs, wq->unbound_attrs);
pwq = unbound_pwq_by_node(wq, node);

@@ -3721,19 +3731,16 @@ static void wq_update_unbound_numa(struct workqueue_struct *wq, int cpu,
*/
if (wq_calc_node_cpumask(wq->dfl_pwq->pool->attrs, node, cpu_off, cpumask)) {
if (cpumask_equal(cpumask, pwq->pool->attrs->cpumask))
- goto out_unlock;
+ return;
} else {
goto use_dfl_pwq;
}

- mutex_unlock(&wq->mutex);
-
/* create a new pwq */
pwq = alloc_unbound_pwq(wq, target_attrs);
if (!pwq) {
pr_warn("workqueue: allocation failed while updating NUMA affinity of \"%s\"\n",
wq->name);
- mutex_lock(&wq->mutex);
goto use_dfl_pwq;
}

@@ -3748,6 +3755,7 @@ static void wq_update_unbound_numa(struct workqueue_struct *wq, int cpu,
goto out_unlock;

use_dfl_pwq:
+ mutex_lock(&wq->mutex);
spin_lock_irq(&wq->dfl_pwq->pool->lock);
get_pwq(wq->dfl_pwq);
spin_unlock_irq(&wq->dfl_pwq->pool->lock);
--
2.1.0


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