Re: [Qemu-devel] [PATCH 19/19] block: make accounting thread-safe

[Paolo Bonzini]

On 24/05/2018 15:35, Alberto Garcia wrote:
> On Mon 05 Jun 2017 02:39:08 PM CEST, Paolo Bonzini wrote:
>> @@ -102,6 +107,8 @@ static void block_account_one_io(BlockAcctStats *stats, 
>> BlockAcctCookie *cookie,
>>  
>>      assert(cookie->type < BLOCK_MAX_IOTYPE);
>>  
>> +    qemu_mutex_lock(&stats->lock);
> 
> QEMU crashes because this lock is not initialized if you do aio_read
> directly from a BDS not attached to a BlockBackend (should that
> operation be legal at all?)
> 
> $ qemu-system-x86_64 -monitor stdio \
> -blockdev driver=qcow2,file.driver=file,file.filename=hd0.img,node-name=hd0
> 
> (qemu) qemu-io hd0 "aio_read 0 1M"
> qemu-system-x86_64: util/qemu-thread-posix.c:64: qemu_mutex_lock_impl: 
> Assertion `mutex->initialized' failed.
> Aborted

The lock is initialized but it has been destroyed via qemu_mutex_destroy
before you reach this point.  So the bug is actually much more than just
the uninitialized lock!

The destruction happens when blk_delete calls block_acct_cleanup.  It
should be fixed by Greg Kurz's patch that adds a bdrv_drained_begin/end
pair to bdrv_root_unref_child.

Thanks,

Paolo