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SubjectRe: [PATCH 0/8] simplify ep_poll
On Fri,  6 Nov 2020 18:16:27 -0500 Soheil Hassas Yeganeh <soheil.kdev@gmail.com> wrote:

> From: Soheil Hassas Yeganeh <soheil@google.com>
>
> This patch series is a follow up based on the suggestions and feedback by Linus:
> https://lkml.kernel.org/r/CAHk-=wizk=OxUyQPbO8MS41w2Pag1kniUV5WdD5qWL-gq1kjDA@mail.gmail.com

Al Viro has been playing in here as well - see the below, from
linux-next.

I think I'll leave it to you folks to sort this out, please.


commit 57804b1cc4616780c72a2d0930d1bd0d5bd3ed4c
Author: Al Viro <viro@zeniv.linux.org.uk>
AuthorDate: Mon Aug 31 13:41:30 2020 -0400
Commit: Al Viro <viro@zeniv.linux.org.uk>
CommitDate: Sun Oct 25 20:02:01 2020 -0400

lift locking/unlocking ep->mtx out of ep_{start,done}_scan()

get rid of depth/ep_locked arguments there and document
the kludge in ep_item_poll() that has lead to ep_locked existence in
the first place

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index ac996b959e94..f9c567af1f5f 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -554,20 +554,13 @@ static inline void ep_pm_stay_awake_rcu(struct epitem *epi)
rcu_read_unlock();
}

-static void ep_start_scan(struct eventpoll *ep,
- int depth, bool ep_locked,
- struct list_head *txlist)
-{
- lockdep_assert_irqs_enabled();
-
- /*
- * We need to lock this because we could be hit by
- * eventpoll_release_file() and epoll_ctl().
- */
-
- if (!ep_locked)
- mutex_lock_nested(&ep->mtx, depth);

+/*
+ * ep->mutex needs to be held because we could be hit by
+ * eventpoll_release_file() and epoll_ctl().
+ */
+static void ep_start_scan(struct eventpoll *ep, struct list_head *txlist)
+{
/*
* Steal the ready list, and re-init the original one to the
* empty list. Also, set ep->ovflist to NULL so that events
@@ -576,6 +569,7 @@ static void ep_start_scan(struct eventpoll *ep,
* because we want the "sproc" callback to be able to do it
* in a lockless way.
*/
+ lockdep_assert_irqs_enabled();
write_lock_irq(&ep->lock);
list_splice_init(&ep->rdllist, txlist);
WRITE_ONCE(ep->ovflist, NULL);
@@ -583,7 +577,6 @@ static void ep_start_scan(struct eventpoll *ep,
}

static void ep_done_scan(struct eventpoll *ep,
- int depth, bool ep_locked,
struct list_head *txlist)
{
struct epitem *epi, *nepi;
@@ -624,9 +617,6 @@ static void ep_done_scan(struct eventpoll *ep,
list_splice(txlist, &ep->rdllist);
__pm_relax(ep->ws);
write_unlock_irq(&ep->lock);
-
- if (!ep_locked)
- mutex_unlock(&ep->mtx);
}

static void epi_rcu_free(struct rcu_head *head)
@@ -763,11 +753,16 @@ static __poll_t ep_item_poll(const struct epitem *epi, poll_table *pt,

ep = epi->ffd.file->private_data;
poll_wait(epi->ffd.file, &ep->poll_wait, pt);
- locked = pt && (pt->_qproc == ep_ptable_queue_proc);

- ep_start_scan(ep, depth, locked, &txlist);
+ // kludge: ep_insert() calls us with ep->mtx already locked
+ locked = pt && (pt->_qproc == ep_ptable_queue_proc);
+ if (!locked)
+ mutex_lock_nested(&ep->mtx, depth);
+ ep_start_scan(ep, &txlist);
res = ep_read_events_proc(ep, &txlist, depth + 1);
- ep_done_scan(ep, depth, locked, &txlist);
+ ep_done_scan(ep, &txlist);
+ if (!locked)
+ mutex_unlock(&ep->mtx);
return res & epi->event.events;
}

@@ -809,9 +804,11 @@ static __poll_t ep_eventpoll_poll(struct file *file, poll_table *wait)
* Proceed to find out if wanted events are really available inside
* the ready list.
*/
- ep_start_scan(ep, 0, false, &txlist);
+ mutex_lock(&ep->mtx);
+ ep_start_scan(ep, &txlist);
res = ep_read_events_proc(ep, &txlist, 1);
- ep_done_scan(ep, 0, false, &txlist);
+ ep_done_scan(ep, &txlist);
+ mutex_unlock(&ep->mtx);
return res;
}

@@ -1573,15 +1570,13 @@ static int ep_send_events(struct eventpoll *ep,

init_poll_funcptr(&pt, NULL);

- ep_start_scan(ep, 0, false, &txlist);
+ mutex_lock(&ep->mtx);
+ ep_start_scan(ep, &txlist);

/*
* We can loop without lock because we are passed a task private list.
- * Items cannot vanish during the loop because ep_scan_ready_list() is
- * holding "mtx" during this call.
+ * Items cannot vanish during the loop we are holding ep->mtx.
*/
- lockdep_assert_held(&ep->mtx);
-
list_for_each_entry_safe(epi, tmp, &txlist, rdllink) {
struct wakeup_source *ws;
__poll_t revents;
@@ -1609,9 +1604,8 @@ static int ep_send_events(struct eventpoll *ep,

/*
* If the event mask intersect the caller-requested one,
- * deliver the event to userspace. Again, ep_scan_ready_list()
- * is holding ep->mtx, so no operations coming from userspace
- * can change the item.
+ * deliver the event to userspace. Again, we are holding ep->mtx,
+ * so no operations coming from userspace can change the item.
*/
revents = ep_item_poll(epi, &pt, 1);
if (!revents)
@@ -1645,7 +1639,8 @@ static int ep_send_events(struct eventpoll *ep,
ep_pm_stay_awake(epi);
}
}
- ep_done_scan(ep, 0, false, &txlist);
+ ep_done_scan(ep, &txlist);
+ mutex_unlock(&ep->mtx);

return res;
}
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