Re: [PATCH v2] fuse: permit freezing while waiting for request answer

[Sergey Senozhatsky]

On (26/08/20 11:10), Peter Zijlstra wrote:
> On Thu, Aug 20, 2026 at 06:07:18PM +0900, Sergey Senozhatsky wrote:
> > On (26/08/20 10:49), Peter Zijlstra wrote:
> > > Message-ID: <20260820084933.GA4036497@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
> > > 
> > > On Wed, Aug 19, 2026 at 12:03:32PM +0200, Miklos Szeredi wrote:
> > > > Why is this unsafe exactly?  Does that unsafeness apply to
> > > > filesystems?  If so why do we allow freezing while blocked on
> > > > sb_start_write()?
> > > 
> > > Getting frozen with lock A held, while another task is blocked on A in
> > > an unfreezable state results in the system not being freezable.
> > 
> > Right, but then the system says "suspend failed" (tasks refuse to freeze
> > after 20sec) and just thaws everything back in?
> 
> People don't like suspend failing. People like to close their lid, throw
> laptop in bag, and expect laptop to not cook itself to death.

Sure, that's exactly the problem I'm looking at.  Throwing TASK_FREEZABLE
addresses some of the cases.  Failing laptop suspend because of uncontended
VFS lock is not an uncommon scenario for us.  Ideally, however, we need
some sort of server/client aware suspend, maybe moving clients to cgroup C
and server to cgroup S, and freezing those in strict order.  Or teaching PM
that some tasks cannot be frozen in random order during suspend (e.g. a
special flag PM_FREEZE_ME_LAST).

fuse is not the only subsystem that doesn't fit current random order suspend.
Another troublemaker for us is notify, which basically has the same server/client
architecture (where both sides are user-space processes).