Re: [Qemu-trivial] [Qemu-devel] [PATCH v4] block: replace fprintf(stderr, ...) with error_report()

[Markus Armbruster]

Kevin Wolf <address@hidden> writes:

> Am 26.05.2014 um 17:02 hat Markus Armbruster geschrieben:
>> Stefan Hajnoczi <address@hidden> writes:
>> 
>> > On Mon, May 26, 2014 at 09:44:03AM +0800, Le Tan wrote:
>> >> Replace fprintf(stderr,...) with error_report() in files block/*, block.c,
>> >> block-migration.c and blockdev.c. The trailing "\n"s of the @fmt argument
>> >> have been removed because @fmt of error_report() should not
>> >> contain newline.
>> >> 
>> >> Signed-off-by: Le Tan <address@hidden>
>> >> ---
>> >>  block-migration.c      |    6 +--
>> >>  block.c                |    4 +-
>> >>  block/qcow2-refcount.c |  115 +++++++++++++++++++++---------------------
>> >>  block/qcow2.c          |   16 +++---
>> >>  block/raw-posix.c      |    8 ++-
>> >>  block/raw-win32.c      |    6 +--
>> >>  block/ssh.c            |    2 +-
>> >>  block/vdi.c            |   14 +++---
>> >>  block/vmdk.c           |   15 +++---
>> >>  block/vpc.c            |    4 +-
>> >>  block/vvfat.c | 129
>> >> ++++++++++++++++++++++++------------------------
>> >>  blockdev.c             |    6 +--
>> >>  12 files changed, 159 insertions(+), 166 deletions(-)
>> >
>> > There is one thing that worries me about this:
>> >
>> > error_report() assumes that the QEMU global mutex is held in order to
>> > access the "current monitor".
>> 
>> Global variable cur_mon, non-null while we're executing a monitor
>> command.
>
> The important part here is that it's indeed global and not thread-local.
>
>> > This is problematic for code in the read/write/flush path (like qcow2
>> > refcount allocation) since it can be invoked from a virtio-blk
>> > data-plane thread (where the QEMU global mutex is not held).
>> 
>> error_report() reports to the current monitor when "in monitor context",
>> i.e. while executing a monitor command, i.e. while cur_mon isn't null.
>> 
>> Can we ever be in monitor context (cur_mon not null) without holding the
>> global mutex?
>
> The right question is: Can a thread (= the main loop thread) ever be in
> monitor context while another thread (= dataplane thread) is executing
> block driver code and doesn't hold the global mutex?
>
> If I understand dataplane correctly, the whole point of it is that the
> answer to this is yes.

Well, the answer used to be "no".  Once upon a time because there was
just a single thread, later on because only one thread ever executed
"interesting" code.

I'm *not* saying this should remain the case.  I'm trying to find out
what needs to be done around cur_mon when we break the assumption behind
it.

Would making cur_mon thread-local suffice?