Messages in this thread | | | Date | Tue, 5 Apr 2022 15:36:03 +1000 | Subject | Re: [RESEND PATCH v7 1/8] kernfs: Introduce interface to access global kernfs_open_file_mutex. | From | Imran Khan <> |
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Hello Al,
On 18/3/22 8:34 am, Al Viro wrote: > On Thu, Mar 17, 2022 at 06:26:05PM +1100, Imran Khan wrote: > >> @@ -570,9 +571,10 @@ static void kernfs_put_open_node(struct kernfs_node *kn, >> struct kernfs_open_file *of) [...]
> As the matter of fact, we can do even better - make freeing > that thing rcu-delayed, use rcu_assign_pointer() for stores, > rcu_dereference() for loads and have kernfs_notify() do > rcu_read_lock(); > on = rcu_dereference(kn->attr.open); > if (on) { > atomic_inc(&on->event); > wake_up_interruptible(&on->poll); > } > rcu_read_unlock(); > and kernfs_open_node_lock becomes useless - all places that > grab it are under kernfs_open_file_mutex.
There are some issues in freeing ->attr.open under RCU callback. There are some users of ->attr.open that can block and hence can't operate under rcu_read_lock. For example in kernfs_drain_open_files we are traversing list of open files corresponding to ->attr.open and unmapping those as well. The unmapping operation can block in i_mmap_lock_write. So even after removing refcnt we will still need kernfs_open_node_lock.
Thanks, -- Imran
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