Re: [PATCH v2] migration/colo: Fix bdrv_graph_rdlock_main_loop: Assertion `!qemu_in_coroutine()' failed.

[Peter Xu <peterx@redhat.com>]

On Sun, Apr 28, 2024 at 03:39:09PM +0300, Michael Tokarev wrote:
> 17.04.2024 05:56, Li Zhijian via wrote:
> > bdrv_activate_all() should not be called from the coroutine context, move
> > it to the QEMU thread colo_process_incoming_thread() with the bql_lock
> > protected.
> > 
> > The backtrace is as follows:
> >   #4  0x0000561af7948362 in bdrv_graph_rdlock_main_loop () at ../block/graph-lock.c:260
> >   #5  0x0000561af7907a68 in graph_lockable_auto_lock_mainloop (x=0x7fd29810be7b) at /patch/to/qemu/include/block/graph-lock.h:259
> >   #6  0x0000561af79167d1 in bdrv_activate_all (errp=0x7fd29810bed0) at ../block.c:6906
> >   #7  0x0000561af762b4af in colo_incoming_co () at ../migration/colo.c:935
> >   #8  0x0000561af7607e57 in process_incoming_migration_co (opaque=0x0) at ../migration/migration.c:793
> >   #9  0x0000561af7adbeeb in coroutine_trampoline (i0=-106876144, i1=22042) at ../util/coroutine-ucontext.c:175
> >   #10 0x00007fd2a5cf21c0 in  () at /lib64/libc.so.6
> > 
> > CC: Fabiano Rosas <farosas@suse.de>
> > Closes: https://gitlab.com/qemu-project/qemu/-/issues/2277
> > Fixes: 2b3912f135 ("block: Mark bdrv_first_blk() and bdrv_is_root_node() GRAPH_RDLOCK")
> > Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
> 
> Commit 2b3912f135 is in 8.2 (v8.1.0-1575-g2b3912f135).  Is this fix supposed to go
> to stable-8.2 series?

Yes we probably should.  I've copied qemu-stable when queuing this patch
for that:

https://lore.kernel.org/r/20240423223813.3237060-27-peterx@redhat.com

> The prob here is that in 8.2, there's no bql_lock/unlock.
> I guess it should use qemu_mutex_lock_iothread() instead, for before
> 195801d700c008 "system/cpus: rename qemu_mutex_lock_iothread() to bql_lock()".

Right, some further care needs to be taken on this one indeed.

> 17.04.2024 05:56, Li Zhijian via wrote:
> > --- a/migration/colo.c
> > +++ b/migration/colo.c
> > @@ -835,6 +835,16 @@ static void *colo_process_incoming_thread(void *opaque)
> >           return NULL;
> >       }
> > +    /* Make sure all file formats throw away their mutable metadata */
> > +    bql_lock();
> > +    bdrv_activate_all(&local_err);
> > +    if (local_err) {
> > +        bql_unlock();
> > +        error_report_err(local_err);
> > +        return NULL;
> > +    }
> > +    bql_unlock();
> 
> FWIW, this can be simplified as follows:
> 
>    bql_lock();
>    bdrv_activate_all(&local_err);
>    bql_unlock();
>    if (local_err) {
>         error_report_err(local_err);
>         return NULL;
>    }
> 
> (I know it is already too late)

Agree. :)

Thanks,

-- 
Peter Xu