Re: [PATCH 14/31] workqueue: replace POOL_MANAGING_WORKERS flag with worker_pool->manager_mutex

Lai Jiangshan
2013-03-10T10:07:29Z

On 02/03/13 11:24, Tejun Heo wrote:
> 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;

if mutex_trylock(&pool->manager_mutex) fails, it does not mean
the pool is managing workers. (although current code does).
so I recommend to keep POOL_MANAGING_WORKERS.

I suggest that you reuse assoc_mutex for your purpose(later patches).
(and rename assoc_mutex back to manager_mutex)

> -
> /*
> * 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);
>