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Date: Sat, 2 Dec 2017 00:03:36 +0100
From: Christoph Hellwig <hch@....de>
To: Jason Baron <jbaron@...mai.com>
Cc: Christoph Hellwig <hch@....de>,
Andrew Morton <akpm@...ux-foundation.org>,
Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Al Viro <viro@...iv.linux.org.uk>,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: waitqueue lockdep annotation
On Fri, Dec 01, 2017 at 05:34:50PM -0500, Jason Baron wrote:
> hmmm...I'm not sure how this suggestion would change the locking rules
> from what we currently have. Right now, we use ep->lock, if we remove
> that and use ep->wq->lock instead, there is just a 1-to-1 mapping there
> that has not changed, since ep->wq->lock currently is completely not
> being used.
True. The patch below survives the amazing complex booting and starting
systemd with lockdep enabled test. Do we have something resembling a
epoll test suite?
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index afd548ebc328..2b2c5ac80e26 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -182,11 +182,10 @@ struct epitem {
* This structure is stored inside the "private_data" member of the file
* structure and represents the main data structure for the eventpoll
* interface.
+ *
+ * Access to it is protected by the lock inside wq.
*/
struct eventpoll {
- /* Protect the access to this structure */
- spinlock_t lock;
-
/*
* This mutex is used to ensure that files are not removed
* while epoll is using them. This is held during the event
@@ -210,7 +209,7 @@ struct eventpoll {
/*
* This is a single linked list that chains all the "struct epitem" that
* happened while transferring ready events to userspace w/out
- * holding ->lock.
+ * holding ->wq.lock.
*/
struct epitem *ovflist;
@@ -686,17 +685,17 @@ static int ep_scan_ready_list(struct eventpoll *ep,
* because we want the "sproc" callback to be able to do it
* in a lockless way.
*/
- spin_lock_irqsave(&ep->lock, flags);
+ spin_lock_irqsave(&ep->wq.lock, flags);
list_splice_init(&ep->rdllist, &txlist);
ep->ovflist = NULL;
- spin_unlock_irqrestore(&ep->lock, flags);
+ spin_unlock_irqrestore(&ep->wq.lock, flags);
/*
* Now call the callback function.
*/
error = (*sproc)(ep, &txlist, priv);
- spin_lock_irqsave(&ep->lock, flags);
+ spin_lock_irqsave(&ep->wq.lock, flags);
/*
* During the time we spent inside the "sproc" callback, some
* other events might have been queued by the poll callback.
@@ -738,7 +737,7 @@ static int ep_scan_ready_list(struct eventpoll *ep,
if (waitqueue_active(&ep->poll_wait))
pwake++;
}
- spin_unlock_irqrestore(&ep->lock, flags);
+ spin_unlock_irqrestore(&ep->wq.lock, flags);
if (!ep_locked)
mutex_unlock(&ep->mtx);
@@ -782,10 +781,10 @@ static int ep_remove(struct eventpoll *ep, struct epitem *epi)
rb_erase_cached(&epi->rbn, &ep->rbr);
- spin_lock_irqsave(&ep->lock, flags);
+ spin_lock_irqsave(&ep->wq.lock, flags);
if (ep_is_linked(&epi->rdllink))
list_del_init(&epi->rdllink);
- spin_unlock_irqrestore(&ep->lock, flags);
+ spin_unlock_irqrestore(&ep->wq.lock, flags);
wakeup_source_unregister(ep_wakeup_source(epi));
/*
@@ -1015,7 +1014,6 @@ static int ep_alloc(struct eventpoll **pep)
if (unlikely(!ep))
goto free_uid;
- spin_lock_init(&ep->lock);
mutex_init(&ep->mtx);
init_waitqueue_head(&ep->wq);
init_waitqueue_head(&ep->poll_wait);
@@ -1119,7 +1117,7 @@ static int ep_poll_callback(wait_queue_entry_t *wait, unsigned mode, int sync, v
struct eventpoll *ep = epi->ep;
int ewake = 0;
- spin_lock_irqsave(&ep->lock, flags);
+ spin_lock_irqsave(&ep->wq.lock, flags);
ep_set_busy_poll_napi_id(epi);
@@ -1196,7 +1194,7 @@ static int ep_poll_callback(wait_queue_entry_t *wait, unsigned mode, int sync, v
pwake++;
out_unlock:
- spin_unlock_irqrestore(&ep->lock, flags);
+ spin_unlock_irqrestore(&ep->wq.lock, flags);
/* We have to call this outside the lock */
if (pwake)
@@ -1480,7 +1478,7 @@ static int ep_insert(struct eventpoll *ep, struct epoll_event *event,
goto error_remove_epi;
/* We have to drop the new item inside our item list to keep track of it */
- spin_lock_irqsave(&ep->lock, flags);
+ spin_lock_irqsave(&ep->wq.lock, flags);
/* record NAPI ID of new item if present */
ep_set_busy_poll_napi_id(epi);
@@ -1497,7 +1495,7 @@ static int ep_insert(struct eventpoll *ep, struct epoll_event *event,
pwake++;
}
- spin_unlock_irqrestore(&ep->lock, flags);
+ spin_unlock_irqrestore(&ep->wq.lock, flags);
atomic_long_inc(&ep->user->epoll_watches);
@@ -1523,10 +1521,10 @@ static int ep_insert(struct eventpoll *ep, struct epoll_event *event,
* list, since that is used/cleaned only inside a section bound by "mtx".
* And ep_insert() is called with "mtx" held.
*/
- spin_lock_irqsave(&ep->lock, flags);
+ spin_lock_irqsave(&ep->wq.lock, flags);
if (ep_is_linked(&epi->rdllink))
list_del_init(&epi->rdllink);
- spin_unlock_irqrestore(&ep->lock, flags);
+ spin_unlock_irqrestore(&ep->wq.lock, flags);
wakeup_source_unregister(ep_wakeup_source(epi));
@@ -1593,7 +1591,7 @@ static int ep_modify(struct eventpoll *ep, struct epitem *epi, struct epoll_even
* list, push it inside.
*/
if (revents & event->events) {
- spin_lock_irq(&ep->lock);
+ spin_lock_irq(&ep->wq.lock);
if (!ep_is_linked(&epi->rdllink)) {
list_add_tail(&epi->rdllink, &ep->rdllist);
ep_pm_stay_awake(epi);
@@ -1604,7 +1602,7 @@ static int ep_modify(struct eventpoll *ep, struct epitem *epi, struct epoll_even
if (waitqueue_active(&ep->poll_wait))
pwake++;
}
- spin_unlock_irq(&ep->lock);
+ spin_unlock_irq(&ep->wq.lock);
}
/* We have to call this outside the lock */
@@ -1754,7 +1752,7 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
* caller specified a non blocking operation.
*/
timed_out = 1;
- spin_lock_irqsave(&ep->lock, flags);
+ spin_lock_irqsave(&ep->wq.lock, flags);
goto check_events;
}
@@ -1763,7 +1761,7 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
if (!ep_events_available(ep))
ep_busy_loop(ep, timed_out);
- spin_lock_irqsave(&ep->lock, flags);
+ spin_lock_irqsave(&ep->wq.lock, flags);
if (!ep_events_available(ep)) {
/*
@@ -1805,11 +1803,11 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
break;
}
- spin_unlock_irqrestore(&ep->lock, flags);
+ spin_unlock_irqrestore(&ep->wq.lock, flags);
if (!schedule_hrtimeout_range(to, slack, HRTIMER_MODE_ABS))
timed_out = 1;
- spin_lock_irqsave(&ep->lock, flags);
+ spin_lock_irqsave(&ep->wq.lock, flags);
}
__remove_wait_queue(&ep->wq, &wait);
@@ -1819,7 +1817,7 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
/* Is it worth to try to dig for events ? */
eavail = ep_events_available(ep);
- spin_unlock_irqrestore(&ep->lock, flags);
+ spin_unlock_irqrestore(&ep->wq.lock, flags);
/*
* Try to transfer events to user space. In case we get 0 events and
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