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

From: Frederic Weisbecker
Date: Wed Aug 25 2021 - 08:24:54 EST

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@xxxxxxxxxx>
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);
+ read_lock_irqsave(&ep->lock, flags);
+ else
+ write_lock_irqsave(&ep->lock, flags);


@@ -1197,7 +1200,10 @@ static int ep_poll_callback(wait_queue_entry_t *wait, unsigned mode, int sync, v

- read_unlock_irqrestore(&ep->lock, flags);
+ read_unlock_irqrestore(&ep->lock, flags);
+ else
+ write_unlock_irqrestore(&ep->lock, flags);

/* We have to call this outside the lock */
if (pwake)