linux-kernel - Re: [Drbd-dev] [PATCH 2/2] drbd: remove check of list iterator against head past the loop body

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Message-ID: <4dcedb78-355f-ed1a-9af1-27e9e63b5643@linbit.com>
Date:   Fri, 1 Apr 2022 00:28:19 +0200
From:   Christoph Böhmwalder 
        <christoph.boehmwalder@...bit.com>
To:     Jakob Koschel <jakobkoschel@...il.com>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
        "Bos, H.J." <h.j.bos@...nl>,
        Brian Johannesmeyer <bjohannesmeyer@...il.com>,
        linux-block@...r.kernel.org,
        Cristiano Giuffrida <c.giuffrida@...nl>,
        Lars Ellenberg <lars.ellenberg@...bit.com>,
        Mike Rapoport <rppt@...nel.org>, drbd-dev@...ts.linbit.com,
        Philipp Reisner <philipp.reisner@...bit.com>
Subject: Re: [Drbd-dev] [PATCH 2/2] drbd: remove check of list iterator
 against head past the loop body

Am 01.04.22 um 00:03 schrieb Jakob Koschel:
> When list_for_each_entry() completes the iteration over the whole list
> without breaking the loop, the iterator value will be a bogus pointer
> computed based on the head element.
> 
> While it is safe to use the pointer to determine if it was computed
> based on the head element, either with list_entry_is_head() or
> &pos->member == head, using the iterator variable after the loop should
> be avoided.
> 
> In preparation to limit the scope of a list iterator to the list
> traversal loop, use a dedicated pointer to point to the found element [1].
> 
> Link: https://lore.kernel.org/all/CAHk-=wgRr_D8CB-D9Kg-c=EHreAsk5SqXPwr9Y7k9sA6cWXJ6w@mail.gmail.com/ [1]
> Signed-off-by: Jakob Koschel <jakobkoschel@...il.com>
> ---
>   drivers/block/drbd/drbd_req.c | 42 ++++++++++++++++++++++-------------
>   1 file changed, 27 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/block/drbd/drbd_req.c b/drivers/block/drbd/drbd_req.c
> index c04394518b07..b2571dc77fe6 100644
> --- a/drivers/block/drbd/drbd_req.c
> +++ b/drivers/block/drbd/drbd_req.c
> @@ -332,17 +332,21 @@ static void set_if_null_req_next(struct drbd_peer_device *peer_device, struct dr
>   static void advance_conn_req_next(struct drbd_peer_device *peer_device, struct drbd_request *req)
>   {
>   	struct drbd_connection *connection = peer_device ? peer_device->connection : NULL;
> +	struct drbd_request *iter = req;
>   	if (!connection)
>   		return;
>   	if (connection->req_next != req)
>   		return;
> -	list_for_each_entry_continue(req, &connection->transfer_log, tl_requests) {
> -		const unsigned s = req->rq_state;
> -		if (s & RQ_NET_QUEUED)
> +
> +	req = NULL;
> +	list_for_each_entry_continue(iter, &connection->transfer_log, tl_requests) {
> +		const unsigned int s = iter->rq_state;
> +
> +		if (s & RQ_NET_QUEUED) {
> +			req = iter;
>   			break;
> +		}
>   	}
> -	if (&req->tl_requests == &connection->transfer_log)
> -		req = NULL;
>   	connection->req_next = req;
>   }
>   
> @@ -358,17 +362,21 @@ static void set_if_null_req_ack_pending(struct drbd_peer_device *peer_device, st
>   static void advance_conn_req_ack_pending(struct drbd_peer_device *peer_device, struct drbd_request *req)
>   {
>   	struct drbd_connection *connection = peer_device ? peer_device->connection : NULL;
> +	struct drbd_request *iter = req;
>   	if (!connection)
>   		return;
>   	if (connection->req_ack_pending != req)
>   		return;
> -	list_for_each_entry_continue(req, &connection->transfer_log, tl_requests) {
> -		const unsigned s = req->rq_state;
> -		if ((s & RQ_NET_SENT) && (s & RQ_NET_PENDING))
> +
> +	req = NULL;
> +	list_for_each_entry_continue(iter, &connection->transfer_log, tl_requests) {
> +		const unsigned int s = iter->rq_state;
> +
> +		if ((s & RQ_NET_SENT) && (s & RQ_NET_PENDING)) {
> +			req = iter;
>   			break;
> +		}
>   	}
> -	if (&req->tl_requests == &connection->transfer_log)
> -		req = NULL;
>   	connection->req_ack_pending = req;
>   }
>   
> @@ -384,17 +392,21 @@ static void set_if_null_req_not_net_done(struct drbd_peer_device *peer_device, s
>   static void advance_conn_req_not_net_done(struct drbd_peer_device *peer_device, struct drbd_request *req)
>   {
>   	struct drbd_connection *connection = peer_device ? peer_device->connection : NULL;
> +	struct drbd_request *iter = req;
>   	if (!connection)
>   		return;
>   	if (connection->req_not_net_done != req)
>   		return;
> -	list_for_each_entry_continue(req, &connection->transfer_log, tl_requests) {
> -		const unsigned s = req->rq_state;
> -		if ((s & RQ_NET_SENT) && !(s & RQ_NET_DONE))
> +
> +	req = NULL;
> +	list_for_each_entry_continue(iter, &connection->transfer_log, tl_requests) {
> +		const unsigned int s = iter->rq_state;
> +
> +		if ((s & RQ_NET_SENT) && !(s & RQ_NET_DONE)) {
> +			req = iter;
>   			break;
> +		}
>   	}
> -	if (&req->tl_requests == &connection->transfer_log)
> -		req = NULL;
>   	connection->req_not_net_done = req;
>   }
>   

Hi Jakob,

Both of these look good to me, thanks.

Reviewed-by: Christoph Böhmwalder <christoph.boehmwalder@...bit.com>

Regards, Christoph