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virtio: remove overzealous BUG_ON.

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The 'disable_cb' callback is designed as an optimization to tell the host
we don't need callbacks now.  As it is not reliable, the debug check is
overzealous: it can happen on two CPUs at the same time.  Document this.

Even if it were reliable, the virtio_net driver doesn't disable
callbacks on transmit so the START_USE/END_USE debugging reentrance
protection can be easily tripped even on UP.

Thanks to Balaji Rao for the bug report and testing.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
CC: Balaji Rao <balajirrao@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Rusty Russell 2008-04-07 14:30:28 +10:00 committed by Linus Torvalds
parent 4cac04dd63
commit 2557a933b7
2 changed files with 4 additions and 4 deletions
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3
drivers/virtio/virtio_ring.c
View file

@ -214,10 +214,7 @@ static void vring_disable_cb(struct virtqueue *_vq)
{
struct vring_virtqueue *vq = to_vvq(_vq);
START_USE(vq);
BUG_ON(vq->vring.avail->flags & VRING_AVAIL_F_NO_INTERRUPT);
vq->vring.avail->flags |= VRING_AVAIL_F_NO_INTERRUPT;
END_USE(vq);
}
static bool vring_enable_cb(struct virtqueue *_vq)

5
include/linux/virtio.h
View file

@ -41,6 +41,8 @@ struct virtqueue
* Returns NULL or the "data" token handed to add_buf.
* @disable_cb: disable callbacks
* vq: the struct virtqueue we're talking about.
* Note that this is not necessarily synchronous, hence unreliable and only
* useful as an optimization.
* @enable_cb: restart callbacks after disable_cb.
* vq: the struct virtqueue we're talking about.
* This re-enables callbacks; it returns "false" if there are pending
@ -48,7 +50,8 @@ struct virtqueue
* checking for more work, and enabling callbacks.
*
* Locking rules are straightforward: the driver is responsible for
* locking. No two operations may be invoked simultaneously.
* locking. No two operations may be invoked simultaneously, with the exception
* of @disable_cb.
*
* All operations can be called in any context.
*/