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

[Peter Lieven]

Am 18.06.2015 um 11:36 schrieb 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().



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