From: Bart Van Assche <bvanassche@acm.org>
To: Johannes Berg <johannes@sipsolutions.net>, Tejun Heo <tj@kernel.org>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Christoph Hellwig <hch@lst.de>, Sagi Grimberg <sagi@grimberg.me>,
"tytso@mit.edu" <tytso@mit.edu>
Subject: Re: [PATCH 3/3] kernel/workqueue: Suppress a false positive lockdep complaint
Date: Thu, 25 Oct 2018 13:39:31 -0700 [thread overview]
Message-ID: <1540499971.66186.51.camel@acm.org> (raw)
In-Reply-To: <11db45f85e2f763c23ae32834dcd016adfce8ad6.camel@sipsolutions.net>
On Thu, 2018-10-25 at 21:51 +0200, Johannes Berg wrote:
> [ ... ]
> diff --git a/fs/direct-io.c b/fs/direct-io.c
> index 093fb54cd316..9ef33d6cba56 100644
> --- a/fs/direct-io.c
> +++ b/fs/direct-io.c
> @@ -629,9 +629,16 @@ int sb_init_dio_done_wq(struct super_block *sb)
> * This has to be atomic as more DIOs can race to create the workqueue
> */
> old = cmpxchg(&sb->s_dio_done_wq, NULL, wq);
> - /* Someone created workqueue before us? Free ours... */
> + /*
> + * Someone created workqueue before us? Free ours...
> + * Note the _nested(), that pushes down to the (in this case actually
> + * pointless) flush_workqueue() happening inside, since this function
> + * might be called in contexts that hold the same locks that an fs may
> + * take while being called from dio_aio_complete_work() from another
> + * instance of the workqueue we allocate here.
> + */
> if (old)
> - destroy_workqueue(wq);
> + destroy_workqueue_nested(wq, SINGLE_DEPTH_NESTING);
> return 0;
> }
> [ ... ]
> /**
> - * flush_workqueue - ensure that any scheduled work has run to completion.
> + * flush_workqueue_nested - ensure that any scheduled work has run to completion.
> * @wq: workqueue to flush
> + * @subclass: subclass for lockdep
> *
> * This function sleeps until all work items which were queued on entry
> * have finished execution, but it is not livelocked by new incoming ones.
> */
> -void flush_workqueue(struct workqueue_struct *wq)
> +void flush_workqueue_nested(struct workqueue_struct *wq, int subclass)
> {
> struct wq_flusher this_flusher = {
> .list = LIST_HEAD_INIT(this_flusher.list),
> @@ -2652,7 +2653,7 @@ void flush_workqueue(struct workqueue_struct *wq)
> if (WARN_ON(!wq_online))
> return;
>
> - lock_map_acquire(&wq->lockdep_map);
> + lock_acquire_exclusive(&wq->lockdep_map, subclass, 0, NULL, _THIS_IP_);
> lock_map_release(&wq->lockdep_map);
>
> mutex_lock(&wq->mutex);
> [ ... ]
I don't like this approach because it doesn't match how other kernel code uses
lockdep annotations. All other kernel code I know of only annotates lock depmaps
as nested if the same kernel thread calls lock_acquire() twice for the same depmap
without intervening lock_release(). My understanding is that with your patch
applied flush_workqueue_nested(wq, 1) calls lock_acquire() only once and with the
subclass argument set to one. I think this will confuse other people who will read
the workqueue implementation and who have not followed this conversation.
I like Tejuns proposal much better than the above proposal.
Bart.
next prev parent reply other threads:[~2018-10-25 20:39 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-25 15:05 [PATCH 0/3] Suppress false positives triggered by workqueue lockdep annotations Bart Van Assche
2018-10-25 15:05 ` [PATCH 1/3] kernel/workqueue: Remove lockdep annotation from __flush_work() Bart Van Assche
2018-10-25 15:31 ` Johannes Berg
2018-10-25 15:57 ` Johannes Berg
2018-10-25 16:01 ` Bart Van Assche
2018-10-25 15:05 ` [PATCH 2/3] kernel/workqueue: Surround work execution with shared lock annotations Bart Van Assche
2018-10-25 16:53 ` Johannes Berg
2018-10-25 17:22 ` Bart Van Assche
2018-10-25 19:17 ` Johannes Berg
2018-10-25 15:05 ` [PATCH 3/3] kernel/workqueue: Suppress a false positive lockdep complaint Bart Van Assche
2018-10-25 15:34 ` Johannes Berg
2018-10-25 15:55 ` Bart Van Assche
2018-10-25 19:59 ` Johannes Berg
2018-10-25 20:21 ` Theodore Y. Ts'o
2018-10-25 20:26 ` Johannes Berg
2018-10-25 15:36 ` Tejun Heo
2018-10-25 15:37 ` Tejun Heo
2018-10-25 20:13 ` Johannes Berg
2018-10-25 15:40 ` Theodore Y. Ts'o
2018-10-25 17:02 ` Johannes Berg
2018-10-25 17:11 ` Bart Van Assche
2018-10-25 19:51 ` Johannes Berg
2018-10-25 20:39 ` Bart Van Assche [this message]
2018-10-25 20:47 ` Johannes Berg
2018-10-25 15:27 ` [PATCH 0/3] Suppress false positives triggered by workqueue lockdep annotations Johannes Berg
2018-10-25 15:47 ` Bart Van Assche
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