Commit 03e02acd authored by Jens Axboe's avatar Jens Axboe
Browse files

eventfd: provide a eventfd_signal_mask() helper 



This is identical to eventfd_signal(), but it allows the caller to pass
in a mask to be used for the poll wakeup key. The use case is avoiding
repeated multishot triggers if we have a dependency between eventfd and
io_uring.

If we setup an eventfd context and register that as the io_uring eventfd,
and at the same time queue a multishot poll request for the eventfd
context, then any CQE posted will repeatedly trigger the multishot request
until it terminates when the CQ ring overflows.

In preparation for io_uring detecting this circular dependency, add the
mentioned helper so that io_uring can pass in EPOLL_URING as part of the
poll wakeup key.

Cc: stable@vger.kernel.org # 6.0
[axboe: fold in !CONFIG_EVENTFD fix from Zhang Qilong]
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent caf1aeaf

fs/eventfd.c

+21 −16
Original line number Diff line number Diff line
@@ -43,21 +43,7 @@ struct eventfd_ctx { 
	int id;

};



/**

 * eventfd_signal - Adds @n to the eventfd counter.

 * @ctx: [in] Pointer to the eventfd context.

 * @n: [in] Value of the counter to be added to the eventfd internal counter.

 *          The value cannot be negative.

 *

 * This function is supposed to be called by the kernel in paths that do not

 * allow sleeping. In this function we allow the counter to reach the ULLONG_MAX

 * value, and we signal this as overflow condition by returning a EPOLLERR

 * to poll(2).

 *

 * Returns the amount by which the counter was incremented.  This will be less

 * than @n if the counter has overflowed.

 */

__u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n)

__u64 eventfd_signal_mask(struct eventfd_ctx *ctx, __u64 n, unsigned mask)

{

	unsigned long flags;
@@ -78,12 +64,31 @@ __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n) 
		n = ULLONG_MAX - ctx->count;

	ctx->count += n;

	if (waitqueue_active(&ctx->wqh))

		wake_up_locked_poll(&ctx->wqh, EPOLLIN);

		wake_up_locked_poll(&ctx->wqh, EPOLLIN | mask);

	current->in_eventfd = 0;

	spin_unlock_irqrestore(&ctx->wqh.lock, flags);



	return n;

}



/**

 * eventfd_signal - Adds @n to the eventfd counter.

 * @ctx: [in] Pointer to the eventfd context.

 * @n: [in] Value of the counter to be added to the eventfd internal counter.

 *          The value cannot be negative.

 *

 * This function is supposed to be called by the kernel in paths that do not

 * allow sleeping. In this function we allow the counter to reach the ULLONG_MAX

 * value, and we signal this as overflow condition by returning a EPOLLERR

 * to poll(2).

 *

 * Returns the amount by which the counter was incremented.  This will be less

 * than @n if the counter has overflowed.

 */

__u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n)

{

	return eventfd_signal_mask(ctx, n, 0);

}

EXPORT_SYMBOL_GPL(eventfd_signal);



static void eventfd_free_ctx(struct eventfd_ctx *ctx)

include/linux/eventfd.h

+7 −0
Original line number Diff line number Diff line
@@ -40,6 +40,7 @@ struct file *eventfd_fget(int fd); 
struct eventfd_ctx *eventfd_ctx_fdget(int fd);

struct eventfd_ctx *eventfd_ctx_fileget(struct file *file);

__u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n);

__u64 eventfd_signal_mask(struct eventfd_ctx *ctx, __u64 n, unsigned mask);

int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, wait_queue_entry_t *wait,

				  __u64 *cnt);

void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt);
@@ -66,6 +67,12 @@ static inline int eventfd_signal(struct eventfd_ctx *ctx, int n) 
	return -ENOSYS;

}



static inline int eventfd_signal_mask(struct eventfd_ctx *ctx, __u64 n,

				      unsigned mask)

{

	return -ENOSYS;

}



static inline void eventfd_ctx_put(struct eventfd_ctx *ctx)

{