[linux-pm] Freezer: Don't count threads waiting for frozen filesystems.

[Miklos Szeredi]

On Tue, 28 Oct 2008, Nigel Cunningham wrote:
> The answer is to freeze the fuse filesystems first, stopping new
> requests (freezing the processes making them) before they are passed on
> to userspace and allowing existing requests to complete or freeze. Once
> that is done, the userspace fuse processes will be idle, at least as far
> as fuse activity is concerned. After fuse activity is stopped, userspace
> can be frozen without worrying about what processes are fuse and what
> aren't. This is what my patch implements so far.

OK.

> To deal with requests that are already in progress, I'd suggest three
> possibilities, in the order I think they should be preferred.
> 
> 1) Use wait_event_freezeable[_timeout] in fuse code. Probably preferable
> to #2, but I thought of it later :)
> 
> 2) Use freezer_do_not_count as part of FUSE_MIGHT_FREEZE (resetting
> before exiting the callers, of course). If the request doesn't complete
> during the freezing period, it must be because the userspace thread was
> already frozen. If the request does complete, we're counted again during
> the normal freezing of userspace that follows the freezing of
> filesystems.
> 
> 3) Adding a means to check whether processes being frozen are fuse
> requests. The code could then wait for fc->num_waiting - (say)
> fc->num_frozen == 0.

Yup, these fix the freezing of tasks which have outstanding fuse
requests.

However it does not fix the freezing of tasks which are waiting for
VFS locks (i.e. inode->i_mutex) held by the outstanding fuse requests.
This is the tricky part...

Miklos