Re: [Qemu-devel] [PATCH v6 2/2] block: Support GlusterFS as a QEMU block backend

[Kevin Wolf]

Am 05.09.2012 09:41, schrieb Bharata B Rao:
> On Thu, Aug 09, 2012 at 06:32:16PM +0530, Bharata B Rao wrote:
>> +static void qemu_gluster_complete_aio(GlusterAIOCB *acb)
>> +{
>> +    int ret;
>> +
>> +    if (acb->canceled) {
>> +        qemu_aio_release(acb);
>> +        return;
>> +    }
>> +
>> +    if (acb->ret == acb->size) {
>> +        ret = 0; /* Success */
>> +    } else if (acb->ret < 0) {
>> +        ret = acb->ret; /* Read/Write failed */
>> +    } else {
>> +        ret = -EIO; /* Partial read/write - fail it */
>> +    }
>> +    acb->common.cb(acb->common.opaque, ret);
> 
> The .cb() here is bdrv_co_io_em_complete(). It does qemu_coroutine_enter(),
> handles the return value and comes back here.

Right.

.cb is set by qemu_gluster_aio_rw/flush(), and the only way these can be
called is through bdrv_co_io_em() and bdrv_co_flush(), which both set
bdrv_co_io_em_complete as the callback.

> But if the bdrv_read or bdrv_write or bdrv_flush was called from a
> coroutine context (as against they themselves creating a new coroutine),
> the above .cb() call above doesn't return to this point.

Why?

A coroutine that yields before it's completed must be reentered, no
matter whether it's been created for a single request or if it already
existed.

Conversely, a coroutine that you enter, always yields at some point and
then you return from the qemu_coroutine_enter() and get back to this
line of code.

If you never come back to this point, there's a bug somewhere.

> Hence I won't
> be able to release the acb and decrement the qemu_aio_count.
> 
> What could be the issue here ? In general, how do I ensure that my
> aio calls get completed correctly in such scenarios where bdrv_read etc
> are called from coroutine context rather than from main thread context ?
> 
> Creating qcow2 image would lead to this scenario where
> ->bdrv_create (=qcow2_create) will create a coroutine and subsequently
> read and write are called within qcow2_create in coroutine context itself.

Can you describe in more detail what code paths it's taking and at which
point you're thinking it's wrong?

Kevin