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Subject[PATCH 14/31] workqueue: replace POOL_MANAGING_WORKERS flag with worker_pool->manager_mutex
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POOL_MANAGING_WORKERS is used to synchronize the manager role.
Synchronizing among workers doesn't need blocking and that's why it's
implemented as a flag.

It got converted to a mutex a while back to add blocking wait from CPU
hotplug path - 6037315269 ("workqueue: use mutex for global_cwq
manager exclusion"). Later it turned out that synchronization among
workers and cpu hotplug need to be done separately. Eventually,
POOL_MANAGING_WORKERS is restored and workqueue->manager_mutex got
morphed into workqueue->assoc_mutex - 552a37e936 ("workqueue: restore
POOL_MANAGING_WORKERS") and b2eb83d123 ("workqueue: rename
manager_mutex to assoc_mutex").

Now, we're gonna need to be able to lock out managers from
destroy_workqueue() to support multiple unbound pools with custom
attributes making it again necessary to be able to block on the
manager role. This patch replaces POOL_MANAGING_WORKERS with
worker_pool->manager_mutex.

This patch doesn't introduce any behavior changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
kernel/workqueue.c | 13 ++++++-------
1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 2645218..68b3443 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -64,7 +64,6 @@ enum {
* create_worker() is in progress.
*/
POOL_MANAGE_WORKERS = 1 << 0, /* need to manage workers */
- POOL_MANAGING_WORKERS = 1 << 1, /* managing workers */
POOL_DISASSOCIATED = 1 << 2, /* cpu can't serve workers */
POOL_FREEZING = 1 << 3, /* freeze in progress */

@@ -145,6 +144,7 @@ struct worker_pool {
DECLARE_HASHTABLE(busy_hash, BUSY_WORKER_HASH_ORDER);
/* L: hash of busy workers */

+ struct mutex manager_mutex; /* the holder is the manager */
struct mutex assoc_mutex; /* protect POOL_DISASSOCIATED */
struct ida worker_ida; /* L: for worker IDs */

@@ -702,7 +702,7 @@ static bool need_to_manage_workers(struct worker_pool *pool)
/* Do we have too many workers and should some go away? */
static bool too_many_workers(struct worker_pool *pool)
{
- bool managing = pool->flags & POOL_MANAGING_WORKERS;
+ bool managing = mutex_is_locked(&pool->manager_mutex);
int nr_idle = pool->nr_idle + managing; /* manager is considered idle */
int nr_busy = pool->nr_workers - nr_idle;

@@ -2027,15 +2027,13 @@ static bool manage_workers(struct worker *worker)
struct worker_pool *pool = worker->pool;
bool ret = false;

- if (pool->flags & POOL_MANAGING_WORKERS)
+ if (!mutex_trylock(&pool->manager_mutex))
return ret;

- pool->flags |= POOL_MANAGING_WORKERS;
-
/*
* To simplify both worker management and CPU hotplug, hold off
* management while hotplug is in progress. CPU hotplug path can't
- * grab %POOL_MANAGING_WORKERS to achieve this because that can
+ * grab @pool->manager_mutex to achieve this because that can
* lead to idle worker depletion (all become busy thinking someone
* else is managing) which in turn can result in deadlock under
* extreme circumstances. Use @pool->assoc_mutex to synchronize
@@ -2075,8 +2073,8 @@ static bool manage_workers(struct worker *worker)
ret |= maybe_destroy_workers(pool);
ret |= maybe_create_worker(pool);

- pool->flags &= ~POOL_MANAGING_WORKERS;
mutex_unlock(&pool->assoc_mutex);
+ mutex_unlock(&pool->manager_mutex);
return ret;
}

@@ -3805,6 +3803,7 @@ static int __init init_workqueues(void)
setup_timer(&pool->mayday_timer, pool_mayday_timeout,
(unsigned long)pool);

+ mutex_init(&pool->manager_mutex);
mutex_init(&pool->assoc_mutex);
ida_init(&pool->worker_ida);

--
1.8.1.2


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