[Libguestfs] a question about multithreading with libguestfs

[Maxim Kozover]

Hi Richard!
Maybe the function guestfs_mount_local_run shouldn't
ACQUIRE_LOCK_FOR_CURRENT_SCOPE as it doesn't talk with the daemon and sits
in the loop? What do you think?
If I remove it from guestfs_mount_local_run (in lib/action-1.c, don't know
how to properly remove it from ml generator), fuse_loop_mt works, but I
still don't understand how it worked with fuse_loop (single threaded) when
guestfs_mount_local_run did ACQUIRE_LOCK_FOR_CURRENT_SCOPE.
Unfortunately multiple ls -lR to the same directory tree lead to crash
while it seems multiple ls -lR to different directory trees (if not
repeated for a while) don't lead to crash, so I suspect directory cache in
fuse without guard.
Is there a quick way to disable the directory cache in fuse so I can see if
it is directory cache that leads to crash or we have to look at other parts?

Thanks much,

Maxim.

On Tue, Oct 17, 2017 at 11:25 PM, Richard W.M. Jones <rjones at redhat.com>
wrote:

> [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
> libguestfs lets you edit virtual machines.  Supports shell scripting,
> bindings from many languages.  http://libguestfs.org
>
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