Re: [Xen-changelog] [linux-2.6.18-xen] scsifront: avoid acquiring same lock twice if ring is full

["Jürgen Groß" <jgross@suse.com>]

On 01/14/2015 11:22 AM, Xen patchbot-linux-2.6.18-xen wrote:
> # HG changeset patch
> # User Juergen Gross <jgross@suse.com>
> # Date 1421228828 -3600
> # Node ID 3015a92b2b53825d00dc81c2dd131fc77ce8ab00
> # Parent  078f1bb69ea5e3772f3df4b4ee21f3c52e381e51
> scsifront: avoid acquiring same lock twice if ring is full
>
> The locking in scsifront_dev_reset_handler() as introduced by c/s
> 1209:4d1471ca031e is obviously wrong. In case of a full ring the host
> lock is acquired twice.
>
> Fixing this issue enables to get rid of the endless fo loop with an
> explicit break statement.
>
> Signed-off-by: Juergen Gross <jgross@suse.com>
>
> Also add a missing version check.
>
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> Committed-by: Jan Beulich <jbeulich@suse.com>
> ---
>
>
> diff -r 078f1bb69ea5 -r 3015a92b2b53 drivers/xen/scsifront/scsifront.c
> --- a/drivers/xen/scsifront/scsifront.c	Wed Dec 10 10:22:39 2014 +0100
> +++ b/drivers/xen/scsifront/scsifront.c	Wed Jan 14 10:47:08 2015 +0100
> @@ -447,12 +447,10 @@ static int scsifront_dev_reset_handler(s
>   	uint16_t rqid;
>   	int err = 0;
>
> -	for (;;) {
>   #if LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,12)
> -		spin_lock_irq(host->host_lock);
> +	spin_lock_irq(host->host_lock);
>   #endif
> -		if (!RING_FULL(&info->ring))
> -			break;
> +	while (RING_FULL(&info->ring)) {
>   		if (err) {
>   #if LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,12)
>   			spin_unlock_irq(host->host_lock);
> @@ -463,7 +461,9 @@ static int scsifront_dev_reset_handler(s
>   		spin_unlock_irq(host->host_lock);
>   		err = wait_event_interruptible(info->wq_sync,
>   					       !info->waiting_sync);
> +#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,12)

Is this correct? I didn't check, but thought up to 2.6.12 the reset
handler would be called with the lock already held.

In any case: either this test must be removed again or two lines above
another test should be added for the unlock.

Juergen

>   		spin_lock_irq(host->host_lock);
> +#endif
>   	}
>
>   	ring_req      = scsifront_pre_request(info);
>
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