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xen/9pfs: yield when there isn't enough room on the ring

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Instead of truncating replies, which is problematic, wait until the
client reads more data and frees bytes on the reply ring.

Do that by calling qemu_coroutine_yield(). The corresponding
qemu_coroutine_enter_if_inactive() is called from xen_9pfs_bh upon
receiving the next notification from the client.

We need to be careful to avoid races in case xen_9pfs_bh and the
coroutine are both active at the same time. In xen_9pfs_bh, wait until
either the critical section is over (ring->co == NULL) or until the
coroutine becomes inactive (qemu_coroutine_yield() was called) before
continuing. Then, simply wake up the coroutine if it is inactive.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Reviewed-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Message-Id: <20200521192627.15259-2-sstabellini@kernel.org>
Signed-off-by: Greg Kurz <groug@kaod.org>
This commit is contained in:
Stefano Stabellini 2020-05-21 12:26:26 -07:00 committed by Greg Kurz
parent cf45183b71
commit a4c4d46272
1 changed files with 25 additions and 6 deletions
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31
hw/9pfs/xen-9p-backend.c
View file

@ -37,6 +37,7 @@ typedef struct Xen9pfsRing {
struct iovec *sg;
QEMUBH *bh;
Coroutine *co;
/* local copies, so that we can read/write PDU data directly from
* the ring */
@ -198,16 +199,20 @@ static void xen_9pfs_init_in_iov_from_pdu(V9fsPDU *pdu,
g_free(ring->sg);
ring->sg = g_new0(struct iovec, 2);
xen_9pfs_in_sg(ring, ring->sg, &num, pdu->idx, size);
ring->co = qemu_coroutine_self();
/* make sure other threads see ring->co changes before continuing */
smp_wmb();
again:
xen_9pfs_in_sg(ring, ring->sg, &num, pdu->idx, size);
buf_size = iov_size(ring->sg, num);
if (buf_size < size) {
xen_pv_printf(&xen_9pfs->xendev, 0, "Xen 9pfs request type %d"
"needs %zu bytes, buffer has %zu\n", pdu->id, size,
buf_size);
xen_be_set_state(&xen_9pfs->xendev, XenbusStateClosing);
xen_9pfs_disconnect(&xen_9pfs->xendev);
qemu_coroutine_yield();
goto again;
}
ring->co = NULL;
/* make sure other threads see ring->co changes before continuing */
smp_wmb();
*piov = ring->sg;
*pniov = num;
@ -292,6 +297,20 @@ static int xen_9pfs_receive(Xen9pfsRing *ring)
static void xen_9pfs_bh(void *opaque)
{
Xen9pfsRing *ring = opaque;
bool wait;
again:
wait = ring->co != NULL && qemu_coroutine_entered(ring->co);
/* paired with the smb_wmb barriers in xen_9pfs_init_in_iov_from_pdu */
smp_rmb();
if (wait) {
cpu_relax();
goto again;
}
if (ring->co != NULL) {
qemu_coroutine_enter_if_inactive(ring->co);
}
xen_9pfs_receive(ring);
}