RE: [PATCH] usb: gadget: f_fs: add "zombie" mode

[Krzysztof Opasiak]

Hi,

> -----Original Message-----
> From: Felipe Balbi [mailto:balbi@xxxxxx]
> Sent: Tuesday, October 07, 2014 4:07 PM
> To: Robert Baldyga
> Cc: balbi@xxxxxx; gregkh@xxxxxxxxxxxxxxxxxxx; linux-
> usb@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> mina86@xxxxxxxxxx; andrzej.p@xxxxxxxxxxx; k.opasiak@xxxxxxxxxxx
> Subject: Re: [PATCH] usb: gadget: f_fs: add "zombie" mode
> 
> Hi,
> 
> On Tue, Oct 07, 2014 at 08:33:16AM +0200, Robert Baldyga wrote:
> > On 10/07/2014 04:28 AM, Felipe Balbi wrote:
> > > Hi,
> > >
> > > On Mon, Oct 06, 2014 at 01:25:14PM +0200, Robert Baldyga wrote:
> > >> Since we can compose gadgets from many functions, there is the
> > >> problem related to gadget breakage while FunctionFS daemon
> being
> > >> closed. In some cases it's strongly desired to keep gadget
> alive
> > >> for a while, despite FunctionFS files are closed, to allow
> another
> > >> functions to complete some presumably critical operations.
> > >>
> > >> For this purpose this patch introduces "zombie" mode. It can
> be
> > >> enabled by setting mount option "zombie=1", and results with
> > >> defering function closure to the moment of reopening ep0 file
> or filesystem umount.
> > >>
> > >> When ffs->state == FFS_ZOMBIE:
> > >> - function is still binded and visible to host,
> > >> - setup requests are automatically stalled,
> > >> - all another transfers are refused,
> > >> - opening ep0 causes function close, and then FunctionFS is
> ready for
> > >>   descriptors and string write,
> > >> - umount of functionfs cause function close.
> > >>
> > >> Signed-off-by: Robert Baldyga <r.baldyga@xxxxxxxxxxx>
> > >
> > > Can you further explain how do you trigger this ? Do I
> understand
> > > correctly that you composed a gadget using configfs and that
> gadget
> > > has functionfs + another gadget ?
> > >
> >
> > Yes, I compose configfs gadget from functionfs + another gadget,
> and
> > when functionfs daemon closes ep files, entire gadget get
> disconnected
> > from host. FFS function is userspace code so there is no way to
> know
> > when it will close files (it doesn't matter what is the reason of
> this
> > situation, it can be daemon logic, program breakage, process kill
> or
> > any other). So when we have another function in gadget which, for
> > example, sends some amount of data, does some software update or
> > implements some real-time functionality, we may want to keep the
> > gadget connected despite FFS function is no longer functional. We
> > can't just remove one of functions from gadget since it has been
> > enumerated, so the only way we can do that is to make broken FFS
> > function "zombie". It will be still visible to host but it will
> no longer implement it's functionality.
> 
> now that's an explanation. Can you update commit log with some of
> this info (once we agree on how to go about fixing this) ?
> 
> I'm not sure we should try to fix this. The only case where this
> could trigger is if ffs daemon crashes and dies or somebody sends a
> bogus signal to kill it.
> 
> A function cannot communicate with the host if it isn't functional
> and ffs depends on its userland daemon. If daemon is crashing, why
> not print a big WARN("closed %s while connected to host\n") ? That
> seems like it's as much as we can do from the kernel. Userland
> should know that they can't have a buggy ffs daemon.

It's not a problem of buggy ffs daemon. The problem is that there are
some non deterministic mechanisms in userspace like OOM killer. FFS
daemon can be written very well but if we are out of memory it may
become a victim. In this case reliability of whole gadget hurts a lot.

If it's going about WARN(). I'm not enthusiastic about it. Userspace
process dies all the time, that's quite normal;) I don't think that it
is good idea to generate a warning on kernel level when some process
dies. Kernel should be resistant for such situations and know how to
deal with them (maybe user could select exact behavior, but it should be
done on kernel site)

-- 
Krzysztof Opasiak
Samsung R&D Institute Poland
Samsung Electronics
k.opasiak@xxxxxxxxxxx



--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/