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

[Stefan Hajnoczi]

On Thu, Jun 18, 2015 at 10:29 AM, Peter Lieven <address@hidden> 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 (address@hidden) at
> util/qemu-thread-posix.c:120
>         err = <optimized out>
>         __func__ = "qemu_cond_broadcast"
> #1  0x0000555555911164 in rfifolock_unlock (address@hidden) at
> util/rfifolock.c:75
>         __PRETTY_FUNCTION__ = "rfifolock_unlock"
> #2  0x0000555555875921 in aio_context_release (address@hidden)
> at async.c:329
> No locals.
> #3  0x000055555588434c in aio_poll (address@hidden,
> address@hidden) 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 (address@hidden,
> address@hidden, address@hidden,
> address@hidden, address@hidden(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,
> address@hidden, address@hidden "(",
> address@hidden, address@hidden,
>     address@hidden(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>,
> address@hidden, address@hidden "(",
> address@hidden) at block/io.c:583
> No locals.
> #7  0x00005555558af75d in blk_read (blk=<optimized out>,
> address@hidden, address@hidden "(",
> address@hidden) 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().

Stefan