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Subject[PATCH 4.19 1/5] wait: add wake_up_pollfree()
Date
From: Eric Biggers <ebiggers@google.com>

commit 42288cb44c4b5fff7653bc392b583a2b8bd6a8c0 upstream.

Several ->poll() implementations are special in that they use a
waitqueue whose lifetime is the current task, rather than the struct
file as is normally the case. This is okay for blocking polls, since a
blocking poll occurs within one task; however, non-blocking polls
require another solution. This solution is for the queue to be cleared
before it is freed, using 'wake_up_poll(wq, EPOLLHUP | POLLFREE);'.

However, that has a bug: wake_up_poll() calls __wake_up() with
nr_exclusive=1. Therefore, if there are multiple "exclusive" waiters,
and the wakeup function for the first one returns a positive value, only
that one will be called. That's *not* what's needed for POLLFREE;
POLLFREE is special in that it really needs to wake up everyone.

Considering the three non-blocking poll systems:

- io_uring poll doesn't handle POLLFREE at all, so it is broken anyway.

- aio poll is unaffected, since it doesn't support exclusive waits.
However, that's fragile, as someone could add this feature later.

- epoll doesn't appear to be broken by this, since its wakeup function
returns 0 when it sees POLLFREE. But this is fragile.

Although there is a workaround (see epoll), it's better to define a
function which always sends POLLFREE to all waiters. Add such a
function. Also make it verify that the queue really becomes empty after
all waiters have been woken up.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20211209010455.42744-2-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
include/linux/wait.h | 26 ++++++++++++++++++++++++++
kernel/sched/wait.c | 7 +++++++
2 files changed, 33 insertions(+)

diff --git a/include/linux/wait.h b/include/linux/wait.h
index ed7c122cb31f4..1d726d79063c5 100644
--- a/include/linux/wait.h
+++ b/include/linux/wait.h
@@ -191,6 +191,7 @@ void __wake_up_locked_key_bookmark(struct wait_queue_head *wq_head,
void __wake_up_sync_key(struct wait_queue_head *wq_head, unsigned int mode, int nr, void *key);
void __wake_up_locked(struct wait_queue_head *wq_head, unsigned int mode, int nr);
void __wake_up_sync(struct wait_queue_head *wq_head, unsigned int mode, int nr);
+void __wake_up_pollfree(struct wait_queue_head *wq_head);

#define wake_up(x) __wake_up(x, TASK_NORMAL, 1, NULL)
#define wake_up_nr(x, nr) __wake_up(x, TASK_NORMAL, nr, NULL)
@@ -217,6 +218,31 @@ void __wake_up_sync(struct wait_queue_head *wq_head, unsigned int mode, int nr);
#define wake_up_interruptible_sync_poll(x, m) \
__wake_up_sync_key((x), TASK_INTERRUPTIBLE, 1, poll_to_key(m))

+/**
+ * wake_up_pollfree - signal that a polled waitqueue is going away
+ * @wq_head: the wait queue head
+ *
+ * In the very rare cases where a ->poll() implementation uses a waitqueue whose
+ * lifetime is tied to a task rather than to the 'struct file' being polled,
+ * this function must be called before the waitqueue is freed so that
+ * non-blocking polls (e.g. epoll) are notified that the queue is going away.
+ *
+ * The caller must also RCU-delay the freeing of the wait_queue_head, e.g. via
+ * an explicit synchronize_rcu() or call_rcu(), or via SLAB_TYPESAFE_BY_RCU.
+ */
+static inline void wake_up_pollfree(struct wait_queue_head *wq_head)
+{
+ /*
+ * For performance reasons, we don't always take the queue lock here.
+ * Therefore, we might race with someone removing the last entry from
+ * the queue, and proceed while they still hold the queue lock.
+ * However, rcu_read_lock() is required to be held in such cases, so we
+ * can safely proceed with an RCU-delayed free.
+ */
+ if (waitqueue_active(wq_head))
+ __wake_up_pollfree(wq_head);
+}
+
#define ___wait_cond_timeout(condition) \
({ \
bool __cond = (condition); \
diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c
index 5dd47f1103d18..a44a57fb95402 100644
--- a/kernel/sched/wait.c
+++ b/kernel/sched/wait.c
@@ -209,6 +209,13 @@ void __wake_up_sync(struct wait_queue_head *wq_head, unsigned int mode, int nr_e
}
EXPORT_SYMBOL_GPL(__wake_up_sync); /* For internal use only */

+void __wake_up_pollfree(struct wait_queue_head *wq_head)
+{
+ __wake_up(wq_head, TASK_NORMAL, 0, poll_to_key(EPOLLHUP | POLLFREE));
+ /* POLLFREE must have cleared the queue. */
+ WARN_ON_ONCE(waitqueue_active(wq_head));
+}
+
/*
* Note: we use "set_current_state()" _after_ the wait-queue add,
* because we need a memory barrier there on SMP, so that any
--
2.34.1
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