[Libguestfs] a question about multithreading with libguestfs

[Richard W.M. Jones]

[Please keep replies on the mailing list so that others can benefit]

On Tue, Oct 17, 2017 at 10:07:31PM +0300, Maxim Kozover wrote:
> Hi Richard,
> Unfortunately I achieved a negative result, maybe you could help, please?
.
> I'm using libguestfs-1.36.4 as a base since I changed for myself a bit some
> detection stuff that you recently moved from C to OCaml and I can't rewrite
> it immediately.
>
> WIth vanilla 1.36.4 just changing fuse_loop to fuse_loop_mt gives almost
> immediate crash when the filesystem is accessed.

That's expected because plain 1.36 doesn't support multithreading, but ...

> I've put the most recent gnulib, changed a bit bootstrap etc, put the
> patches from https://www.redhat.com/archives/libguestfs/2017-June/
> msg00287.html and rebuilt.

... OK.

> The system seems to work (with single-threaded fuse).
> Changed fuse_loop to fuse_loop_mt, rebuilt and fuse is stuck on the first
> call to appropriate fuse "system call" like mount_local_getattr in case of
> ls.
> It is stuck at guestfs_lstatns possibly at ACQUIRE_LOCK_FOR_CURRENT_SCOPE,
> while being called from fuse.

Can you get a stack trace from gdb.  Use the command ‘t a a bt’ to
show stacks from all threads at the same time.

> What do you think could be a problem? Some lock already taken?
> Also, maybe less concern for me as I use fuse read-only, is the directory
> cache guarded?

Possibly not, I've not really looked at the fuse code w.r.t
multithreading at all.  Patches welcome as usual.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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