[RFC PATCH -RT] epoll: Fix eventpoll read-lock not writer-fair in PREEMPT_RT

[Frederic Weisbecker <frederic@kernel.org>]

Hi,

Ok the patch is gross but at least this lets me start a discussion
about the issue.

---
From d9d66d650b3dac8947a34464dd2e0b546a8c6b63 Mon Sep 17 00:00:00 2001
From: Frederic Weisbecker <frederic@kernel.org>
Date: Wed, 25 Aug 2021 14:24:54 +0200
Subject: [RFC PATCH -RT] epoll: Fix eventpoll read-lock not writer-fair in PREEMPT_RT

The eventpoll lock has been converted to an rwlock some time ago with:

	a218cc491420 (epoll: use rwlock in order to reduce ep_poll
					callback() contention)

Unfortunately this can result in scenarios where a high priority caller
of epoll_wait() need to wait for the completion of lower priority wakers.

The typical scenario is:

1) epoll_wait() waits and sleeps for new events in the ep_poll() loop.

2) new events arrive in ep_poll_callback(), the waiter is awaken while
   ep->lock is read-acquired.

3) The high priority waiter preempts the waker but it can't acquire the
   write lock in epoll_wait() so it blocks waiting for the low prio waker
   without priority inheritance.

I guess making readlock writer fair is still not the plan so all I can
propose is to make that rwlock build-conditional.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
 fs/eventpoll.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 1e596e1d0bba..c1fb4b01ea4f 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1133,7 +1133,10 @@ static int ep_poll_callback(wait_queue_entry_t *wait, unsigned mode, int sync, v
 	unsigned long flags;
 	int ewake = 0;
 
-	read_lock_irqsave(&ep->lock, flags);
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+		read_lock_irqsave(&ep->lock, flags);
+	else
+		write_lock_irqsave(&ep->lock, flags);
 
 	ep_set_busy_poll_napi_id(epi);
 
@@ -1197,7 +1200,10 @@ static int ep_poll_callback(wait_queue_entry_t *wait, unsigned mode, int sync, v
 		pwake++;
 
 out_unlock:
-	read_unlock_irqrestore(&ep->lock, flags);
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+		read_unlock_irqrestore(&ep->lock, flags);
+	else
+		write_unlock_irqrestore(&ep->lock, flags);
 
 	/* We have to call this outside the lock */
 	if (pwake)
-- 
2.25.1