Re: [PATCH 07/36] aio: add delayed cancel support

Darrick J. Wong
2018-03-20T03:20:06Z

On Mon, Mar 05, 2018 at 01:27:14PM -0800, Christoph Hellwig wrote:
> The upcoming aio poll support would like to be able to complete the
> iocb inline from the cancellation context, but that would cause
> a lock order reversal. Add support for optionally moving the cancelation
> outside the context lock to avoid this reversal.

I started to wonder which lock order reversal the commit message refers
to?

I think the reason for adding delayed cancellations is that we want to
be able to call io_cancel -> kiocb_cancel -> aio_poll_cancel ->
aio_complete without double locking ctx_lock?

--D

> Signed-off-by: Christoph Hellwig <hch@lst.de>
> Acked-by: Jeff Moyer <jmoyer@redhat.com>
> ---
> fs/aio.c | 49 ++++++++++++++++++++++++++++++++++++++-----------
> 1 file changed, 38 insertions(+), 11 deletions(-)
>
> diff --git a/fs/aio.c b/fs/aio.c
> index 0b6394b4e528..9d7d6e4cde87 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -170,6 +170,10 @@ struct aio_kiocb {
> struct list_head ki_list; /* the aio core uses this
> * for cancellation */
>
> + unsigned int flags; /* protected by ctx->ctx_lock */
> +#define AIO_IOCB_DELAYED_CANCEL (1 << 0)
> +#define AIO_IOCB_CANCELLED (1 << 1)
> +
> /*
> * If the aio_resfd field of the userspace iocb is not zero,
> * this is the underlying eventfd context to deliver events to.
> @@ -536,9 +540,9 @@ static int aio_setup_ring(struct kioctx *ctx, unsigned int nr_events)
> #define AIO_EVENTS_FIRST_PAGE ((PAGE_SIZE - sizeof(struct aio_ring)) / sizeof(struct io_event))
> #define AIO_EVENTS_OFFSET (AIO_EVENTS_PER_PAGE - AIO_EVENTS_FIRST_PAGE)
>
> -void kiocb_set_cancel_fn(struct kiocb *iocb, kiocb_cancel_fn *cancel)
> +static void __kiocb_set_cancel_fn(struct aio_kiocb *req,
> + kiocb_cancel_fn *cancel, unsigned int iocb_flags)
> {
> - struct aio_kiocb *req = container_of(iocb, struct aio_kiocb, rw);
> struct kioctx *ctx = req->ki_ctx;
> unsigned long flags;
>
> @@ -548,8 +552,15 @@ void kiocb_set_cancel_fn(struct kiocb *iocb, kiocb_cancel_fn *cancel)
> spin_lock_irqsave(&ctx->ctx_lock, flags);
> list_add_tail(&req->ki_list, &ctx->active_reqs);
> req->ki_cancel = cancel;
> + req->flags |= iocb_flags;
> spin_unlock_irqrestore(&ctx->ctx_lock, flags);
> }
> +
> +void kiocb_set_cancel_fn(struct kiocb *iocb, kiocb_cancel_fn *cancel)
> +{
> + return __kiocb_set_cancel_fn(container_of(iocb, struct aio_kiocb, rw),
> + cancel, 0);
> +}
> EXPORT_SYMBOL(kiocb_set_cancel_fn);
>
> /*
> @@ -603,17 +614,27 @@ static void free_ioctx_users(struct percpu_ref *ref)
> {
> struct kioctx *ctx = container_of(ref, struct kioctx, users);
> struct aio_kiocb *req;
> + LIST_HEAD(list);
>
> spin_lock_irq(&ctx->ctx_lock);
> -
> while (!list_empty(&ctx->active_reqs)) {
> req = list_first_entry(&ctx->active_reqs,
> struct aio_kiocb, ki_list);
> - kiocb_cancel(req);
> - }
>
> + if (req->flags & AIO_IOCB_DELAYED_CANCEL) {
> + req->flags |= AIO_IOCB_CANCELLED;
> + list_move_tail(&req->ki_list, &list);
> + } else {
> + kiocb_cancel(req);
> + }
> + }
> spin_unlock_irq(&ctx->ctx_lock);
>
> + while (!list_empty(&list)) {
> + req = list_first_entry(&list, struct aio_kiocb, ki_list);
> + kiocb_cancel(req);
> + }
> +
> percpu_ref_kill(&ctx->reqs);
> percpu_ref_put(&ctx->reqs);
> }
> @@ -1785,15 +1806,22 @@ SYSCALL_DEFINE3(io_cancel, aio_context_t, ctx_id, struct iocb __user *, iocb,
> if (unlikely(!ctx))
> return -EINVAL;
>
> - spin_lock_irq(&ctx->ctx_lock);
> + ret = -EINVAL;
>
> + spin_lock_irq(&ctx->ctx_lock);
> kiocb = lookup_kiocb(ctx, iocb, key);
> + if (kiocb) {
> + if (kiocb->flags & AIO_IOCB_DELAYED_CANCEL) {
> + kiocb->flags |= AIO_IOCB_CANCELLED;
> + } else {
> + ret = kiocb_cancel(kiocb);
> + kiocb = NULL;
> + }
> + }
> + spin_unlock_irq(&ctx->ctx_lock);
> +
> if (kiocb)
> ret = kiocb_cancel(kiocb);
> - else
> - ret = -EINVAL;
> -
> - spin_unlock_irq(&ctx->ctx_lock);
>
> if (!ret) {
> /*
> @@ -1805,7 +1833,6 @@ SYSCALL_DEFINE3(io_cancel, aio_context_t, ctx_id, struct iocb __user *, iocb,
> }
>
> percpu_ref_put(&ctx->users);
> -
> return ret;
> }
>
> --
> 2.14.2
>