Re: [Xen-devel] [PATCH v4] pvcalls-front: Avoid get_free_pages(GFP_KERNEL) under spinlock

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 12/1/18 1:33 AM, Wen Yang wrote:
> The problem is that we call this with a spin lock held.
> The call tree is:
> pvcalls_front_accept() holds bedata->socket_lock.
>     -> create_active()
>         -> __get_free_pages() uses GFP_KERNEL
>
> The create_active() function is only called from pvcalls_front_accept()
> with a spin_lock held, The allocation is not allowed to sleep and
> GFP_KERNEL is not sufficient.
>
> This issue was detected by using the Coccinelle software.
>
> v2: Add a function doing the allocations which is called
>     outside the lock and passing the allocated data to
>     create_active().
>
> v3: Use the matching deallocators i.e., free_page()
>     and free_pages(), respectively.
>
> v4: It would be better to pre-populate map (struct sock_mapping),
>     rather than introducing one more new struct.
>
> Suggested-by: Juergen Gross <jgross@xxxxxxxx>
> Suggested-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> Suggested-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> Signed-off-by: Wen Yang <wen.yang99@xxxxxxxxxx>
> CC: Julia Lawall <julia.lawall@xxxxxxx>
> CC: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> CC: Juergen Gross <jgross@xxxxxxxx>
> CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> CC: xen-devel@xxxxxxxxxxxxxxxxxxxx
> CC: linux-kernel@xxxxxxxxxxxxxxx
> ---
>  drivers/xen/pvcalls-front.c | 57 ++++++++++++++++++++++++++++++-------
>  1 file changed, 46 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/xen/pvcalls-front.c b/drivers/xen/pvcalls-front.c
> index 77224d8f3e6f..555c9abdf58f 100644
> --- a/drivers/xen/pvcalls-front.c
> +++ b/drivers/xen/pvcalls-front.c
> @@ -335,13 +335,18 @@ int pvcalls_front_socket(struct socket *sock)
>       return ret;
>  }
>  
> -static int create_active(struct sock_mapping *map, int *evtchn)
> +static void free_active_ring(struct sock_mapping *map)
>  {
> -     void *bytes;
> -     int ret = -ENOMEM, irq = -1, i;
> +     if (!map)
> +             return;
> +     free_pages((unsigned long)map->active.data.in,
> +                     map->active.ring->ring_order);
> +     free_page((unsigned long)map->active.ring);
> +}
>  
> -     *evtchn = -1;
> -     init_waitqueue_head(&map->active.inflight_conn_req);
> +static int alloc_active_ring(struct sock_mapping *map)
> +{
> +     void *bytes;
>  
>       map->active.ring = (struct pvcalls_data_intf *)
>               __get_free_page(GFP_KERNEL | __GFP_ZERO);
> @@ -352,6 +357,26 @@ static int create_active(struct sock_mapping *map, int 
> *evtchn)
>                                       PVCALLS_RING_ORDER);
>       if (bytes == NULL)
>               goto out_error;
> +     map->active.data.in = bytes;
> +     map->active.data.out = bytes +
> +             XEN_FLEX_RING_SIZE(PVCALLS_RING_ORDER);
> +
> +     return 0;
> +
> +out_error:
> +     free_active_ring(map);
> +     return -ENOMEM;
> +}
> +
> +static int create_active(struct sock_mapping *map, int *evtchn)
> +{
> +     void *bytes;
> +     int ret = -ENOMEM, irq = -1, i;
> +
> +     *evtchn = -1;
> +     init_waitqueue_head(&map->active.inflight_conn_req);
> +
> +     bytes = map->active.data.in;

Why is this needed?

I may not be be reading the diff correctly, but your patch appears to be
whitespace-damaged and I can't apply it.

>       for (i = 0; i < (1 << PVCALLS_RING_ORDER); i++)
>               map->active.ring->ref[i] = gnttab_grant_foreign_access(
>                       pvcalls_front_dev->otherend_id,
> @@ -361,10 +386,6 @@ static int create_active(struct sock_mapping *map, int 
> *evtchn)
>               pvcalls_front_dev->otherend_id,
>               pfn_to_gfn(virt_to_pfn((void *)map->active.ring)), 0);
>  
> -     map->active.data.in = bytes;
> -     map->active.data.out = bytes +
> -             XEN_FLEX_RING_SIZE(PVCALLS_RING_ORDER);
> -
>       ret = xenbus_alloc_evtchn(pvcalls_front_dev, evtchn);
>       if (ret)
>               goto out_error;
> @@ -385,8 +406,7 @@ static int create_active(struct sock_mapping *map, int 
> *evtchn)
>  out_error:
>       if (*evtchn >= 0)
>               xenbus_free_evtchn(pvcalls_front_dev, *evtchn);
> -     free_pages((unsigned long)map->active.data.in, PVCALLS_RING_ORDER);
> -     free_page((unsigned long)map->active.ring);
> +     free_active_ring(map);


I think that since you are allocating the data outside of this call it
should also be freed outside, when create_active() fails.

-boris


>       return ret;
>  }
>  
> @@ -406,11 +426,17 @@ int pvcalls_front_connect(struct socket *sock, struct 
> sockaddr *addr,
>               return PTR_ERR(map);
>  
>       bedata = dev_get_drvdata(&pvcalls_front_dev->dev);
> +     ret = alloc_active_ring(map);
> +     if (ret < 0) {
> +             pvcalls_exit_sock(sock);
> +             return ret;
> +     }
>  
>       spin_lock(&bedata->socket_lock);
>       ret = get_request(bedata, &req_id);
>       if (ret < 0) {
>               spin_unlock(&bedata->socket_lock);
> +             free_active_ring(map);
>               pvcalls_exit_sock(sock);
>               return ret;
>       }
> @@ -780,12 +806,20 @@ int pvcalls_front_accept(struct socket *sock, struct 
> socket *newsock, int flags)
>               }
>       }
>  
> +     ret = alloc_active_ring(map);
> +     if (ret < 0) {
> +             clear_bit(PVCALLS_FLAG_ACCEPT_INFLIGHT,
> +                             (void *)&map->passive.flags);
> +             pvcalls_exit_sock(sock);
> +             return ret;
> +     }
>       spin_lock(&bedata->socket_lock);
>       ret = get_request(bedata, &req_id);
>       if (ret < 0) {
>               clear_bit(PVCALLS_FLAG_ACCEPT_INFLIGHT,
>                         (void *)&map->passive.flags);
>               spin_unlock(&bedata->socket_lock);
> +             free_active_ring(map);
>               pvcalls_exit_sock(sock);
>               return ret;
>       }
> @@ -794,6 +828,7 @@ int pvcalls_front_accept(struct socket *sock, struct 
> socket *newsock, int flags)
>               clear_bit(PVCALLS_FLAG_ACCEPT_INFLIGHT,
>                         (void *)&map->passive.flags);
>               spin_unlock(&bedata->socket_lock);
> +             free_active_ring(map);
>               pvcalls_exit_sock(sock);
>               return -ENOMEM;
>       }


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