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@xxxxxxxxxx> 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@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); + 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
