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

From: "Darrick J. Wong" <[email-masked]>
Date: Mon, 19 Mar 2018 20:19:57 -0700

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 <[email-masked]>
> Acked-by: Jeff Moyer <[email-masked]>
> ---
>  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
> 

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