RE: [PATCH v4] hv: hv_fcopy: drop the obsolete message on transfer failure

[KY Srinivasan]

> -----Original Message-----
> From: Dexuan Cui [mailto:decui@xxxxxxxxxxxxx]
> Sent: Wednesday, January 14, 2015 1:55 AM
> To: gregkh@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; driverdev-
> devel@xxxxxxxxxxxxxxxxxxxxxx; olaf@xxxxxxxxx; apw@xxxxxxxxxxxxx;
> jasowang@xxxxxxxxxx; vkuznets@xxxxxxxxxx
> Cc: KY Srinivasan
> Subject: [PATCH v4] hv: hv_fcopy: drop the obsolete message on transfer
> failure
> 
> In the case the user-space daemon crashes, hangs or is killed, we need to
> down the semaphore, otherwise, after the daemon starts next time, the
> obsolete data in fcopy_transaction.message or fcopy_transaction.fcopy_msg
> will be used immediately.
> 
> Cc: Jason Wang <jasowang@xxxxxxxxxx>
> Cc: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> Cc: K. Y. Srinivasan <kys@xxxxxxxxxxxxx>
> Signed-off-by: Dexuan Cui <decui@xxxxxxxxxxxxx>
Signed-off-by: K. Y. Srinivasan <kys@xxxxxxxxxxxxx>
> 
> ---
> 
> v2: I removed the "FCP" prefix as Greg asked.
> 
>     I also updated the output message a little:
>     "FCP: failed to acquire the semaphore" -->
>     "can not acquire the semaphore: it is benign"
> 
> v3: I added the code in fcopy_release() as Jason Wang suggested.
>     I removed the pr_debug (it isn't so meaningful)and added a comment
> instead.
> 
> v4: this is a resend of v3, plus adding a comment before
>     fcopy_release().
> ---
>  drivers/hv/hv_fcopy.c | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/drivers/hv/hv_fcopy.c b/drivers/hv/hv_fcopy.c index
> 23b2ce2..cd453e4 100644
> --- a/drivers/hv/hv_fcopy.c
> +++ b/drivers/hv/hv_fcopy.c
> @@ -86,6 +86,18 @@ static void fcopy_work_func(struct work_struct
> *dummy)
>  	 * process the pending transaction.
>  	 */
>  	fcopy_respond_to_host(HV_E_FAIL);
> +
> +	/* In the case the user-space daemon crashes, hangs or is killed, we
> +	 * need to down the semaphore, otherwise, after the daemon starts
> next
> +	 * time, the obsolete data in fcopy_transaction.message or
> +	 * fcopy_transaction.fcopy_msg will be used immediately.
> +	 *
> +	 * NOTE: fcopy_read() happens to get the semaphore (very rare)?
> We're
> +	 * still OK, because we've reported the failure to the host.
> +	 */
> +	if (down_trylock(&fcopy_transaction.read_sema))
> +		;
> +
>  }
> 
>  static int fcopy_handle_handshake(u32 version) @@ -344,6 +356,14 @@
> static int fcopy_open(struct inode *inode, struct file *f)
>  	return 0;
>  }
> 
> +/* XXX: there are still some tricky corner cases, e.g.,
> + * 1) In a SMP guest, when fcopy_release() runs between
> + * schedule_delayed_work() and fcopy_send_data(), there is
> + * still a chance an obsolete message will be queued.
> + *
> + * 2) When the fcopy daemon is running, if we unload the driver,
> + * we'll notice a kernel oops when we kill the daemon later.
> + */
>  static int fcopy_release(struct inode *inode, struct file *f)  {
>  	/*
> @@ -351,6 +371,13 @@ static int fcopy_release(struct inode *inode, struct
> file *f)
>  	 */
>  	in_hand_shake = true;
>  	opened = false;
> +
> +	if (cancel_delayed_work_sync(&fcopy_work)) {
> +		/* We haven't up()-ed the semaphore(very rare)? */
> +		if (down_trylock(&fcopy_transaction.read_sema))
> +			;
> +		fcopy_respond_to_host(HV_E_FAIL);
> +	}
>  	return 0;
>  }
> 
> --
> 1.9.1

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