Re: [Qemu-devel] [PULL v2 6/7] exec: allow to get a pointer for some mmio memory region

[Paolo Bonzini <pbonzini@redhat.com>]

On 10/08/2017 14:56, Peter Maydell wrote:
>> +    qemu_mutex_lock_iothread();
>> +
>> +    /* Reset dirty so this doesn't happen later. */
>> +    cpu_physical_memory_test_and_clear_dirty(offset, size, 1);
>> +
>> +    if (section.mr != mr) {
>> +        /* memory_region_find add a ref on section.mr */
>> +        memory_region_unref(section.mr);
>> +        if (MMIO_INTERFACE(section.mr->owner)) {
> 
> Could somebody explain why it's OK to unref section.mr here before
> we go on to do things with it, rather than only unrefing it after
> we've finished using it?

The memory region won't disappear until you release the BQL and/or
RCU-read-lock, but yes it's cleaner to move it later, and yes there is a
leak.

Paolo

> Also, by my reading memory_region_find() will always ref
> ret.mr (if it's not NULL), whereas this code only unrefs it
> if section.mr == mr. Does this leak a reference in the case
> where section.mr != mr, or am I missing something ?
>