On Wed, 01 Sep 2021 18:07:39 +0200
Christian Schoenebeck <qemu_...@crudebyte.com> wrote:
> On Mittwoch, 1. September 2021 17:41:02 CEST Greg Kurz wrote:
> > On Wed, 01 Sep 2021 16:21:06 +0200
> >
> > Christian Schoenebeck <qemu_...@crudebyte.com> wrote:
> > > On Mittwoch, 1. September 2021 14:49:37 CEST Christian Schoenebeck wrote:
> > > > > > And it triggered, however I am not sure if some of those functions I
> > > > > > asserted above are indeed allowed to be executed on a different
> > > > > > thread
> > > > > > than main thread:
> > > > > >
> > > > > > Program terminated with signal SIGABRT, Aborted.
> > > > > > #0 __GI_raise (sig=sig@entry=6) at
> > > > > > ../sysdeps/unix/sysv/linux/raise.c:50
> > > > > > 50 ../sysdeps/unix/sysv/linux/raise.c: No such file or
> > > > > > directory.
> > > > > > [Current thread is 1 (Thread 0x7fd0bcef1700 (LWP 6470))]
> > > > >
> > > > > Based in the thread number, it seems that the signal was raised by
> > > > > the main event thread...
> > > >
> > > > No, it was not main thread actually, gdb's "current thread is 1" output
> > > > is
> > > > misleading.
> > > >
> > > > Following the thread id trace, I extended the thread assertion checks
> > > > over
> > > > to v9fs_walk() as well, like this:
> > > >
> > > > static void coroutine_fn v9fs_walk(void *opaque)
> > > > {
> > > >
> > > > ...
> > > > assert_thread();
> > > > v9fs_co_run_in_worker({
> > > >
> > > > ...
> > > >
> > > > });
> > > > assert_thread();
> > > > ...
> > > >
> > > > }
> > > >
> > > > and made sure the reference thread id to be compared is really the main
> > > > thread.
> > > >
> > > > And what happens here is before v9fs_co_run_in_worker() is entered,
> > > > v9fs_walk() runs on main thread, but after returning from
> > > > v9fs_co_run_in_worker() it runs on a different thread for some reason,
> > > > not
> > > > on main thread as it would be expected at that point.
> > >
> > > Ok, I think I found the root cause: the block is break;-ing out too far.
> > > The
> > That could explain the breakage indeed since the block you've added
> > to v9fs_walk() embeds a bunch of break statements. AFAICT this block
> > breaks on errors... do you know which one ?
>
> Yes, I've verified that. In my case an interrupt of Twalk triggered this bug.
> so it was this path exactly:
>
> v9fs_co_run_in_worker({
> if (v9fs_request_cancelled(pdu)) {
> ...
> break;
> }
> ...
> });
>
> so it was really this break;-ing too far being the root cause of the crash.
>
> > > following patch should fix it:
> > >
> > > diff --git a/hw/9pfs/coth.h b/hw/9pfs/coth.h
> > > index c51289903d..f83c7dda7b 100644
> > > --- a/hw/9pfs/coth.h
> > > +++ b/hw/9pfs/coth.h
> > > @@ -51,7 +51,9 @@
> > >
> > > */ \
> > >
> > > qemu_coroutine_yield(); \
> > > qemu_bh_delete(co_bh); \
> > >
> > > - code_block; \
> > > + do { \
> > > + code_block; \
> > > + } while (0); \
> >
> > Good.
> >
> > > /* re-enter back to qemu thread */ \
> > > qemu_coroutine_yield(); \
> > >
> > > } while (0)
> > >
> > > I haven't triggered a crash with that patch, but due to the occasional
> > > nature of this issue I'll give it some more spins before officially
> > > proclaiming it my bug. :)
> >
> > Well, this is a pre-existing limitation with v9fs_co_run_in_worker().
> > This wasn't documented as such and not really obvious to detect when
> > you optimized TWALK. We've never hit it before because the other
> > v9fs_co_run_in_worker() users don't have break statements.
>
> Yes, I know, this was my bad.
>
No, I mean the opposite actually. You shouldn't feel sorry to have
detected that this macro we're using everywhere is badly broken from
the beginning... even at the cost of a regression we'll fix shortly :)
> > But, indeed, this caused a regression in 6.1 so this will need a Fixes:
> > tag and Cc: qemu-stable.
>
> Yep, I'm preparing a patch now.
>
> Best regards,
> Christian Schoenebeck
>
>
