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Re: [Qemu-devel] Help with deadlock when using sound

From: Programmingkid
Subject: Re: [Qemu-devel] Help with deadlock when using sound
Date: Sun, 10 May 2015 15:41:46 -0400

On May 10, 2015, at 10:54 AM, Paolo Bonzini wrote:

> On 06/05/2015 18:40, Programmingkid wrote:
>> When I try to use the pcspk sound hardware, QEMU freezes and uses
>> 100% of the cpu time. This is the command I use:
>> qemu-system-i386 -cdrom <anything you wan here> -soundhw pcspk
>> This looks like a deadlock situation because some unknown code called
>> qemu_mutex_lock(). Here is the stack trace at the freeze:
>> (gdb) bt #0  0x00007fff824e2db6 in semaphore_wait_trap () #1
>> 0x00007fff824e8417 in pthread_mutex_lock () #2  0x0000000100267199 in
>> qemu_mutex_lock (mutex=<value temporarily unavailable, due to
>> optimizations>) at util/qemu-thread-posix.c:73 #3  0x003c44016e95153b
>> in ?? ()
>> My host is Mac OS 10.6.8. My guest isn't really anything. I have used
>> Windows XP before but it isn't necessary to reproduce the problem.
>> The ?? is what appears to be the problem. I can't even print
>> instructions at that address. Any ideas as to what is calling the
>> qemu_mutex_lock() function could help.
> Reproduced with a FreeDOS image from QEMU Advent Calendar.  It locks up
> as soon as you type "beep".
> It works with the PulseAudio and ALSA backends, but it doesn't with the
> SDL backend, even on Linux.
> Also, it deadlocks even with KVM enabled.
> Paolo

Thank you very much for finding this out. Any theories as to what is wrong?

This is my list of theories:
- Compiler bug
- Bug with a dependency 
- Host operating system bug/untrue assumption
- Emulated sound cards not implementing some required functionality
- Missing/broken deadlock prevention code

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