[patch 138/166] fs/epoll: make nesting accounting safe for -rt kernel

Andrew Morton
2020-04-07T03:11:26Z

From: Jason Baron <jbaron@akamai.com>
Subject: fs/epoll: make nesting accounting safe for -rt kernel

Davidlohr Bueso pointed out that when CONFIG_DEBUG_LOCK_ALLOC is set
ep_poll_safewake() can take several non-raw spinlocks after disabling
interrupts. Since a spinlock can block in the -rt kernel, we can't take a
spinlock after disabling interrupts. So let's re-work how we determine
the nesting level such that it plays nicely with the -rt kernel.

Let's introduce a 'nests' field in struct eventpoll that records the
current nesting level during ep_poll_callback(). Then, if we nest again
we can find the previous struct eventpoll that we were called from and
increase our count by 1. The 'nests' field is protected by
ep->poll_wait.lock.

I've also moved the visited field to reduce the size of struct eventpoll
from 184 bytes to 176 bytes on x86_64 for !CONFIG_DEBUG_LOCK_ALLOC, which
is typical for a production config.

Link: http://lkml.kernel.org/r/1582739816-13167-1-git-send-email-jbaron@akamai.com
Reported-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Jason Baron <jbaron@akamai.com>
Reviewed-by: Davidlohr Bueso <dbueso@suse.de>
Cc: Roman Penyaev <rpenyaev@suse.de>
Cc: Eric Wong <normalperson@yhbt.net>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

fs/eventpoll.c | 64 +++++++++++++++++++++++++++++++----------------
1 file changed, 43 insertions(+), 21 deletions(-)

--- a/fs/eventpoll.c~fs-epoll-make-nesting-accounting-safe-for-rt-kernel
+++ a/fs/eventpoll.c
@@ -218,13 +218,18 @@ struct eventpoll {
struct file *file;

/* used to optimize loop detection check */
- int visited;
struct list_head visited_list_link;
+ int visited;

#ifdef CONFIG_NET_RX_BUSY_POLL
/* used to track busy poll napi_id */
unsigned int napi_id;
#endif
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+ /* tracks wakeup nests for lockdep validation */
+ u8 nests;
+#endif
};

/* Wait structure used by the poll hooks */
@@ -545,30 +550,47 @@ out_unlock:
*/
#ifdef CONFIG_DEBUG_LOCK_ALLOC

-static DEFINE_PER_CPU(int, wakeup_nest);
-
-static void ep_poll_safewake(wait_queue_head_t *wq)
+static void ep_poll_safewake(struct eventpoll *ep, struct epitem *epi)
{
+ struct eventpoll *ep_src;
unsigned long flags;
- int subclass;
+ u8 nests = 0;

- local_irq_save(flags);
- preempt_disable();
- subclass = __this_cpu_read(wakeup_nest);
- spin_lock_nested(&wq->lock, subclass + 1);
- __this_cpu_inc(wakeup_nest);
- wake_up_locked_poll(wq, POLLIN);
- __this_cpu_dec(wakeup_nest);
- spin_unlock(&wq->lock);
- local_irq_restore(flags);
- preempt_enable();
+ /*
+ * To set the subclass or nesting level for spin_lock_irqsave_nested()
+ * it might be natural to create a per-cpu nest count. However, since
+ * we can recurse on ep->poll_wait.lock, and a non-raw spinlock can
+ * schedule() in the -rt kernel, the per-cpu variable are no longer
+ * protected. Thus, we are introducing a per eventpoll nest field.
+ * If we are not being call from ep_poll_callback(), epi is NULL and
+ * we are at the first level of nesting, 0. Otherwise, we are being
+ * called from ep_poll_callback() and if a previous wakeup source is
+ * not an epoll file itself, we are at depth 1 since the wakeup source
+ * is depth 0. If the wakeup source is a previous epoll file in the
+ * wakeup chain then we use its nests value and record ours as
+ * nests + 1. The previous epoll file nests value is stable since its
+ * already holding its own poll_wait.lock.
+ */
+ if (epi) {
+ if ((is_file_epoll(epi->ffd.file))) {
+ ep_src = epi->ffd.file->private_data;
+ nests = ep_src->nests;
+ } else {
+ nests = 1;
+ }
+ }
+ spin_lock_irqsave_nested(&ep->poll_wait.lock, flags, nests);
+ ep->nests = nests + 1;
+ wake_up_locked_poll(&ep->poll_wait, EPOLLIN);
+ ep->nests = 0;
+ spin_unlock_irqrestore(&ep->poll_wait.lock, flags);
}

#else

-static void ep_poll_safewake(wait_queue_head_t *wq)
+static void ep_poll_safewake(struct eventpoll *ep, struct epitem *epi)
{
- wake_up_poll(wq, EPOLLIN);
+ wake_up_poll(&ep->poll_wait, EPOLLIN);
}

#endif
@@ -789,7 +811,7 @@ static void ep_free(struct eventpoll *ep

/* We need to release all tasks waiting for these file */
if (waitqueue_active(&ep->poll_wait))
- ep_poll_safewake(&ep->poll_wait);
+ ep_poll_safewake(ep, NULL);

/*
* We need to lock this because we could be hit by
@@ -1258,7 +1280,7 @@ out_unlock:

/* We have to call this outside the lock */
if (pwake)
- ep_poll_safewake(&ep->poll_wait);
+ ep_poll_safewake(ep, epi);

if (!(epi->event.events & EPOLLEXCLUSIVE))
ewake = 1;
@@ -1562,7 +1584,7 @@ static int ep_insert(struct eventpoll *e

/* We have to call this outside the lock */
if (pwake)
- ep_poll_safewake(&ep->poll_wait);
+ ep_poll_safewake(ep, NULL);

return 0;

@@ -1666,7 +1688,7 @@ static int ep_modify(struct eventpoll *e

/* We have to call this outside the lock */
if (pwake)
- ep_poll_safewake(&ep->poll_wait);
+ ep_poll_safewake(ep, NULL);

return 0;
}
_