Kevin Wolf <kw...@redhat.com> writes: > Am 26.05.2014 um 17:02 hat Markus Armbruster geschrieben: >> Stefan Hajnoczi <stefa...@gmail.com> 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 <tamlokv...@gmail.com> >> >> --- >> >> 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?
