linux-kernel - Re: [PATCH] Retry infinitely for hypercall

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Date:   Thu, 5 Jan 2017 00:47:34 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Long Li <longli@...hange.microsoft.com>
Cc:     "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        devel@...uxdriverproject.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Retry infinitely for hypercall

Fix the subsystem prefix in the subject.

On Wed, Jan 04, 2017 at 02:39:31PM -0800, Long Li wrote:
> From: Long Li <longli@...rosoft.com>
> 
> Hyper-v host guarantees that a hypercall will succeed. Retry infinitely to avoid returning transient failures to upper layer.
> 
> Signed-off-by: Long Li <longli@...rosoft.com>
> ---
>  drivers/hv/connection.c | 17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/hv/connection.c b/drivers/hv/connection.c
> index 6ce8b87..4bcb099 100644
> --- a/drivers/hv/connection.c
> +++ b/drivers/hv/connection.c
> @@ -439,7 +439,6 @@ int vmbus_post_msg(void *buffer, size_t buflen)
>  {
>  	union hv_connection_id conn_id;
>  	int ret = 0;

Btw, when you disable GCC's uninitialized variable checking by storing
bogus values in "ret", it's eventually going to bite you in the bum.
Eventually you're going to get a bug that should have been detected
through static analysis if only you hadn't disabled it.

> -	int retries = 0;
>  	u32 usec = 1;
>  
>  	conn_id.asu32 = 0;
> @@ -447,10 +446,10 @@ int vmbus_post_msg(void *buffer, size_t buflen)
>  
>  	/*
>  	 * hv_post_message() can have transient failures because of
> -	 * insufficient resources. Retry the operation a couple of
> -	 * times before giving up.
> +	 * insufficient resources. We retry infinitely on these failures
> +	 * because host guarantees hypercall will eventually succeed.
>  	 */
> -	while (retries < 20) {
> +	while (1) {
>  		ret = hv_post_message(conn_id, 1, buffer, buflen);
>  
>  		switch (ret) {
> @@ -459,11 +458,11 @@ int vmbus_post_msg(void *buffer, size_t buflen)
>  			 * We could get this if we send messages too
>  			 * frequently.
>  			 */

Move the comment above the code it's commenting about.

		/* 
		 * We could get INVALID_CONNECTION_ID if we flood the
		 * host with too many messages.
		 */
		case HV_STATUS_INVALID_CONNECTION_ID:
		case HV_STATUS_INSUFFICIENT_MEMORY:
		case HV_STATUS_INSUFFICIENT_BUFFERS:
			break;



> -			ret = -EAGAIN;
> -			break;
>  		case HV_STATUS_INSUFFICIENT_MEMORY:
>  		case HV_STATUS_INSUFFICIENT_BUFFERS:
> -			ret = -ENOMEM;
> +			/*
> +			 * Temporary failure out of resources
> +			 */
>  			break;
>  		case HV_STATUS_SUCCESS:
>  			return ret;

			return 0;

Better to be more explicit.  When I looked at this I got briefly
confused if this function was supposed to return HV_ statuses or
standard kernel error codes.  It turns out that HV_STATUS_SUCCESS is
zero the success returns map directly to linux kernel code for success
but it's clearer to be explicit.

> @@ -472,12 +471,12 @@ int vmbus_post_msg(void *buffer, size_t buflen)
>  			return -EINVAL;
>  		}
  
> -		retries++;
>  		udelay(usec);
>  		if (usec < 2048)
>  			usec *= 2;
>  	}
> -	return ret;
> +	/* Impossible to get here */
> +	BUG_ON(1);

Remove the comment and the BUG_ON().

regards,
dan carpenter