Re: [Qemu-devel] [PATCH 1/3] Combine bdrv_read and bdrv_write to bdrv_rw

[Paolo Bonzini]

Il 29/02/2012 17:00, Michael Tokarev ha scritto:
> And how it will be a cleanup?
> 
> The whole cow code (and a few others) is not reenterant.  Merely
> moving this lock/unlock stuff inth actual methods eliminates two
> current wrappers in cow_co_write() and cow_co_read(), which are
> exactly the same now, and moves this exactly the same code into
> actual methods, which has nothing to do with locking - they're
> not reenterant, and they deal with internal to the format stuff.
> Having this common locking layer on top and _outside_ of the
> actual work helps removing irrelevant code from important paths.
> Also, it will be too easy to forgot to unlock it there by doing
> just "return" somewhere.

It's not very different from leaking memory.  It's just the way C works.

In the future, you may add unlock around image access like in qcow2, and
then an unlock/lock pair would be confusing without the lock/unlock outside.

If you are worried about forgetting to unlock, add owner tracking to
qemu-coroutine-lock.c, similar to PTHREAD_MUTEX_ERRORCHECK.  That would
be quite useful.

Paolo