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authorAmit Shah <amit.shah@redhat.com>2010-10-20 13:45:43 +1030
committerLinus Torvalds <torvalds@linux-foundation.org>2010-10-20 13:18:04 -0700
commit531295e63be8c2b8b909d7400739e8b8df60d61f (patch)
tree1ae5e6429e3fb52104220e543534049a6832ba1a
parent30c278192f9ab06125fb042f6e46763e0fd7140a (diff)
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virtio: console: Don't block entire guest if host doesn't read data
If the host is slow in reading data or doesn't read data at all, blocking write calls not only blocked the program that called write() but the entire guest itself. To overcome this, let's not block till the host signals it has given back the virtio ring element we passed it. Instead, send the buffer to the host and return to userspace. This operation then becomes similar to how non-blocking writes work, so let's use the existing code for this path as well. This code change also ensures blocking write calls do get blocked if there's not enough room in the virtio ring as well as they don't return -EAGAIN to userspace. Signed-off-by: Amit Shah <amit.shah@redhat.com> Acked-by: Hans de Goede <hdegoede@redhat.com> CC: stable@kernel.org Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--drivers/char/virtio_console.c17
1 files changed, 14 insertions, 3 deletions
diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
index c810481a5bc..0f69c5ec0ec 100644
--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -459,9 +459,12 @@ static ssize_t send_buf(struct port *port, void *in_buf, size_t in_count,
/*
* Wait till the host acknowledges it pushed out the data we
- * sent. This is done for ports in blocking mode or for data
- * from the hvc_console; the tty operations are performed with
- * spinlocks held so we can't sleep here.
+ * sent. This is done for data from the hvc_console; the tty
+ * operations are performed with spinlocks held so we can't
+ * sleep here. An alternative would be to copy the data to a
+ * buffer and relax the spinning requirement. The downside is
+ * we need to kmalloc a GFP_ATOMIC buffer each time the
+ * console driver writes something out.
*/
while (!virtqueue_get_buf(out_vq, &len))
cpu_relax();
@@ -626,6 +629,14 @@ static ssize_t port_fops_write(struct file *filp, const char __user *ubuf,
goto free_buf;
}
+ /*
+ * We now ask send_buf() to not spin for generic ports -- we
+ * can re-use the same code path that non-blocking file
+ * descriptors take for blocking file descriptors since the
+ * wait is already done and we're certain the write will go
+ * through to the host.
+ */
+ nonblock = true;
ret = send_buf(port, buf, count, nonblock);
if (nonblock && ret > 0)