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wait: add wake_up_pollfree()
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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>
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ebiggers committed Dec 9, 2021
1 parent 0fcfb00 commit 42288cb
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26 changes: 26 additions & 0 deletions include/linux/wait.h
Original file line number Diff line number Diff line change
Expand Up @@ -217,6 +217,7 @@ void __wake_up_sync_key(struct wait_queue_head *wq_head, unsigned int mode, void
void __wake_up_locked_sync_key(struct wait_queue_head *wq_head, unsigned int mode, 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);
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)
Expand Down Expand Up @@ -245,6 +246,31 @@ void __wake_up_sync(struct wait_queue_head *wq_head, unsigned int mode);
#define wake_up_interruptible_sync_poll_locked(x, m) \
__wake_up_locked_sync_key((x), TASK_INTERRUPTIBLE, 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); \
Expand Down
7 changes: 7 additions & 0 deletions kernel/sched/wait.c
Original file line number Diff line number Diff line change
Expand Up @@ -238,6 +238,13 @@ void __wake_up_sync(struct wait_queue_head *wq_head, unsigned int mode)
}
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
Expand Down

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