workqueue: restore POOL_MANAGING_WORKERS

This patch restores POOL_MANAGING_WORKERS which was replaced by
pool->manager_mutex by 6037315269 "workqueue: use mutex for global_cwq
manager exclusion".

There's a subtle idle worker depletion bug across CPU hotplug events
and we need to distinguish an actual manager and CPU hotplug
preventing management.  POOL_MANAGING_WORKERS will be used for the
former and manager_mutex the later.

This patch just lays POOL_MANAGING_WORKERS on top of the existing
manager_mutex and doesn't introduce any synchronization changes.  The
next patch will update it.

Note that this patch fixes a non-critical anomaly where
too_many_workers() may return %true spuriously while CPU hotplug is in
progress.  While the issue could schedule idle timer spuriously, it
didn't trigger any actual misbehavior.

tj: Rewrote patch description.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index dc7b8458e275f73a38278974676c22d1a5c84d2a..383548ed0b54ad1d12b84e0934e542b7b432622d 100644 (file)
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -66,6 +66,7 @@ enum {
 
        /* pool flags */
        POOL_MANAGE_WORKERS     = 1 << 0,       /* need to manage workers */
+       POOL_MANAGING_WORKERS   = 1 << 1,       /* managing workers */
 
        /* worker flags */
        WORKER_STARTED          = 1 << 0,       /* started */
@@ -652,7 +653,7 @@ 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 = mutex_is_locked(&pool->manager_mutex);
+       bool managing = pool->flags & POOL_MANAGING_WORKERS;
        int nr_idle = pool->nr_idle + managing; /* manager is considered idle */
        int nr_busy = pool->nr_workers - nr_idle;
 
@@ -1827,6 +1828,7 @@ static bool manage_workers(struct worker *worker)
        if (!mutex_trylock(&pool->manager_mutex))
                return ret;
 
+       pool->flags |= POOL_MANAGING_WORKERS;
        pool->flags &= ~POOL_MANAGE_WORKERS;
 
        /*
@@ -1836,6 +1838,7 @@ 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->manager_mutex);
        return ret;
 }