Re: [Qemu-devel] [PATCH 1/2] iscsi: add support for bdrv_co_is_allocated()

[Peter Lieven <pl@kamp.de>]

Am 21.06.2013 15:14, schrieb Kevin Wolf:
> Am 21.06.2013 um 13:42 hat Peter Lieven geschrieben:
>> Am 21.06.2013 13:07, schrieb Kevin Wolf:
>>> Am 21.06.2013 um 11:45 hat Peter Lieven geschrieben:
>>>> Am 21.06.2013 11:18, schrieb Kevin Wolf:
>>>>> Am 20.06.2013 um 20:20 hat Peter Lieven geschrieben:
>>>>>> Signed-off-by: Peter Lieven <pl@kamp.de>
>>>>>> ---
>>>>>>  block/iscsi.c |   57 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>>>>>>  1 file changed, 57 insertions(+)
>>>>>>
>>>>>> diff --git a/block/iscsi.c b/block/iscsi.c
>>>>>> index 0bbf0b1..e6b966d 100644
>>>>>> --- a/block/iscsi.c
>>>>>> +++ b/block/iscsi.c
>>>>>> @@ -49,6 +49,7 @@ typedef struct IscsiLun {
>>>>>>      uint64_t num_blocks;
>>>>>>      int events;
>>>>>>      QEMUTimer *nop_timer;
>>>>>> +    uint8_t lbpme;
>>>>>>  } IscsiLun;
>>>>>>  
>>>>>>  typedef struct IscsiAIOCB {
>>>>>> @@ -800,6 +801,60 @@ iscsi_getlength(BlockDriverState *bs)
>>>>>>      return len;
>>>>>>  }
>>>>>>  
>>>>>> +static int coroutine_fn iscsi_co_is_allocated(BlockDriverState *bs,
>>>>>> +                                              int64_t sector_num,
>>>>>> +                                              int nb_sectors, int *pnum)
>>>>>> +{
>>>>>> +    IscsiLun *iscsilun = bs->opaque;
>>>>>> +    struct scsi_task *task = NULL;
>>>>>> +    struct scsi_get_lba_status *lbas = NULL;
>>>>>> +    struct scsi_lba_status_descriptor *lbasd = NULL;
>>>>>> +    int ret;
>>>>>> +
>>>>>> +    *pnum = nb_sectors;
>>>>>> +
>>>>>> +    if (iscsilun->lbpme == 0) {
>>>>>> +        return 1;
>>>>>> +    }
>>>>>> +
>>>>>> +    /* in-flight requests could invalidate the lba status result */
>>>>>> +    while (iscsi_process_flush(iscsilun)) {
>>>>>> +        qemu_aio_wait();
>>>>>> +    }
>>>>> Note that you're blocking here. The preferred way would be something
>>>>> involving a yield from the coroutine and a reenter as soon as all
>>>>> requests are done. Maybe a CoRwLock does what you need?
>>>> Is there a document how to use it? Or can you help here?
>>> The idea would be to take a read lock while any request is in flight
>>> (i.e. qemu_co_rwlock_rdlock() before it's started and
>>> qemu_co_rwlock_unlock() when it completes), and to take a write lock
>>> (qemu_co_rwlock_wrlock) for the part of iscsi_co_is_allocated() that
>>> requires that no other request runs in parallel.
>> Wouldn't this require that all the other operations in iscsi.c would
>> also take these lock? Currently there are only aio requests
>> implemented as it seems.
> Hm, okay, that makes it a bit harder because AIO callbacks aren't called
> in coroutine context. So taking the lock would be easy, but releasing
> them could turn out somewhat tricky.
>
>> Would it help here to add sth like this?
>>
>> while (iscsi_process_flush(iscsilun)) {
>>     if (qemu_in_coroutine()) {
>>         qemu_coroutine_yield();
>>     } else {
>>         qemu_aio_wait();
>>     }       
>> }
> You're always in a coroutine here.
>
> The problem is that if you yield, you need to reenter the coroutine at
> some point or the is_allocated request would never complete. That's
> basically what the coroutine locks do for you: They yield and only
> reenter when the lock can be taken.
would it ok for you to stay with qemu_aio_wait() and add a remark
that it blocks and its a TODO. i need the is_allocated support in
iscsi only to improve bdrv_has_zero_init().

Peter