Re: [Qemu-devel] [Qemu-block] RFC cdrom in own thread?

[Peter Lieven <pl@kamp.de>]

Am 18.06.2015 um 11:36 schrieb Stefan Hajnoczi:
> On Thu, Jun 18, 2015 at 10:29 AM, Peter Lieven <pl@kamp.de> wrote:
>> Am 18.06.2015 um 10:42 schrieb Kevin Wolf:
>>> Am 18.06.2015 um 10:30 hat Peter Lieven geschrieben:
>>>> Am 18.06.2015 um 09:45 schrieb Kevin Wolf:
>>>>> Am 18.06.2015 um 09:12 hat Peter Lieven geschrieben:
>>>>>> Thread 2 (Thread 0x7ffff5550700 (LWP 2636)):
>>>>>> #0  0x00007ffff5d87aa3 in ppoll () from /lib/x86_64-linux-gnu/libc.so.6
>>>>>> No symbol table info available.
>>>>>> #1  0x0000555555955d91 in qemu_poll_ns (fds=0x5555563889c0, nfds=3,
>>>>>>       timeout=4999424576) at qemu-timer.c:326
>>>>>>           ts = {tv_sec = 4, tv_nsec = 999424576}
>>>>>>           tvsec = 4
>>>>>> #2  0x0000555555956feb in aio_poll (ctx=0x5555563528e0, blocking=true)
>>>>>>       at aio-posix.c:231
>>>>>>           node = 0x0
>>>>>>           was_dispatching = false
>>>>>>           ret = 1
>>>>>>           progress = false
>>>>>> #3  0x000055555594aeed in bdrv_prwv_co (bs=0x55555637eae0,
>>>>>> offset=4292007936,
>>>>>>       qiov=0x7ffff554f760, is_write=false, flags=0) at block.c:2699
>>>>>>           aio_context = 0x5555563528e0
>>>>>>           co = 0x5555563888a0
>>>>>>           rwco = {bs = 0x55555637eae0, offset = 4292007936,
>>>>>>             qiov = 0x7ffff554f760, is_write = false, ret = 2147483647,
>>>>>> flags = 0}
>>>>>> #4  0x000055555594afa9 in bdrv_rw_co (bs=0x55555637eae0,
>>>>>> sector_num=8382828,
>>>>>>       buf=0x7ffff44cc800 "(", nb_sectors=4, is_write=false, flags=0)
>>>>>>       at block.c:2722
>>>>>>           qiov = {iov = 0x7ffff554f780, niov = 1, nalloc = -1, size =
>>>>>> 2048}
>>>>>>           iov = {iov_base = 0x7ffff44cc800, iov_len = 2048}
>>>>>> #5  0x000055555594b008 in bdrv_read (bs=0x55555637eae0,
>>>>>> sector_num=8382828,
>>>>>>       buf=0x7ffff44cc800 "(", nb_sectors=4) at block.c:2730
>>>>>> No locals.
>>>>>> #6  0x000055555599acef in blk_read (blk=0x555556376820,
>>>>>> sector_num=8382828,
>>>>>>       buf=0x7ffff44cc800 "(", nb_sectors=4) at block/block-backend.c:404
>>>>>> No locals.
>>>>>> #7  0x0000555555833ed2 in cd_read_sector (s=0x555556408f88,
>>>>>> lba=2095707,
>>>>>>       buf=0x7ffff44cc800 "(", sector_size=2048) at hw/ide/atapi.c:116
>>>>>>           ret = 32767
>>>>> Here is the problem: The ATAPI emulation uses synchronous blk_read()
>>>>> instead of the AIO or coroutine interfaces. This means that it keeps
>>>>> polling for request completion while it holds the BQL until the request
>>>>> is completed.
>>>> I will look at this.
>>
>> I need some further help. My way to "emulate" a hung NFS Server is to
>> block it in the Firewall. Currently I face the problem that I cannot mount
>> a CD Iso via libnfs (nfs://) without hanging Qemu (i previously tried with
>> a kernel NFS mount). It reads a few sectors and then stalls (maybe another
>> bug):
>>
>> (gdb) thread apply all bt full
>>
>> Thread 3 (Thread 0x7ffff0c21700 (LWP 29710)):
>> #0  qemu_cond_broadcast (cond=cond@entry=0x555556259940) at
>> util/qemu-thread-posix.c:120
>>          err = <optimized out>
>>          __func__ = "qemu_cond_broadcast"
>> #1  0x0000555555911164 in rfifolock_unlock (r=r@entry=0x555556259910) at
>> util/rfifolock.c:75
>>          __PRETTY_FUNCTION__ = "rfifolock_unlock"
>> #2  0x0000555555875921 in aio_context_release (ctx=ctx@entry=0x5555562598b0)
>> at async.c:329
>> No locals.
>> #3  0x000055555588434c in aio_poll (ctx=ctx@entry=0x5555562598b0,
>> blocking=blocking@entry=true) at aio-posix.c:272
>>          node = <optimized out>
>>          was_dispatching = false
>>          i = <optimized out>
>>          ret = <optimized out>
>>          progress = false
>>          timeout = 611734526
>>          __PRETTY_FUNCTION__ = "aio_poll"
>> #4  0x00005555558bc43d in bdrv_prwv_co (bs=bs@entry=0x55555627c0f0,
>> offset=offset@entry=7038976, qiov=qiov@entry=0x7ffff0c208f0,
>> is_write=is_write@entry=false, flags=flags@entry=(unknown: 0)) at
>> block/io.c:552
>>          aio_context = 0x5555562598b0
>>          co = <optimized out>
>>          rwco = {bs = 0x55555627c0f0, offset = 7038976, qiov =
>> 0x7ffff0c208f0, is_write = false, ret = 2147483647, flags = (unknown: 0)}
>> #5  0x00005555558bc533 in bdrv_rw_co (bs=0x55555627c0f0,
>> sector_num=sector_num@entry=13748, buf=buf@entry=0x555557874800 "(",
>> nb_sectors=nb_sectors@entry=4, is_write=is_write@entry=false,
>>      flags=flags@entry=(unknown: 0)) at block/io.c:575
>>          qiov = {iov = 0x7ffff0c208e0, niov = 1, nalloc = -1, size = 2048}
>>          iov = {iov_base = 0x555557874800, iov_len = 2048}
>> #6  0x00005555558bc593 in bdrv_read (bs=<optimized out>,
>> sector_num=sector_num@entry=13748, buf=buf@entry=0x555557874800 "(",
>> nb_sectors=nb_sectors@entry=4) at block/io.c:583
>> No locals.
>> #7  0x00005555558af75d in blk_read (blk=<optimized out>,
>> sector_num=sector_num@entry=13748, buf=buf@entry=0x555557874800 "(",
>> nb_sectors=nb_sectors@entry=4) at block/block-backend.c:493
>>          ret = <optimized out>
>> #8  0x00005555557abb88 in cd_read_sector (sector_size=<optimized out>,
>> buf=0x555557874800 "(", lba=3437, s=0x55555760db70) at hw/ide/atapi.c:116
>>          ret = <optimized out>
>> #9  ide_atapi_cmd_reply_end (s=0x55555760db70) at hw/ide/atapi.c:190
>>          byte_count_limit = <optimized out>
>>          size = <optimized out>
>>          ret = 2
> This is still the same scenario Kevin explained.
>
> The ATAPI CD-ROM emulation code is using synchronous blk_read().  This
> function holds the QEMU global mutex while waiting for the I/O request
> to complete.  This blocks other vcpu threads and the main loop thread.
>
> The solution is to convert the CD-ROM emulation code to use
> blk_aio_readv() instead of blk_read().



Yes, my problem was that I was using a libnfs version that was broken.
I can now continue debugging. Sorry for the noise.

A problem I faced is that 'info block' queries the allocated file size of the
CDROM Image and thus hangs. I think nfs_get_allocated_file_size additionally
needs a timeout?

Peter