Re: [LSF/MM/BPF TOPIC] Replacing TASK_(UN)INTERRUPTIBLE with regions of uninterruptibility

[David Howells <dhowells@redhat.com>]

Miklos Szeredi <miklos@szeredi.hu> wrote:

> > We have various locks, mutexes, etc., that are taken on entry to
> > filesystem code, for example, and a bunch of them are taken interruptibly
> > or killably (or ought to be) - but filesystem code might be called into
> > from uninterruptible code, such as the memory allocator, fscache, etc..
> 
> Are you suggesting to make lots more filesystem/vfs/mm sleeps
> killable?  That would present problems with being called from certain
> contexts.

No, it wouldn't.  What I'm suggesting is something like:

	overlayfs_mkdir(inode)
	{
		inode_lock(inode);  <---  This could be interruptible
		...
		begin_task_uninterruptible();
		...
		do_stuff();
		...
		inode->lower->inode->mkdir(inode->lower->inode);
					^--- say ext4_mkdir
		...
		do_more_stuff();
		end_task_uninterruptible();
		inode_unlock(inode);
	}

	ext4_mkdir(inode)
	{
		inode_lock(inode);  <--- This would be interruptible, but
					 called from overlayfs above is now
					 uninterruptible
		...
	}

You bracket the context where interruptibility is bad and then everyone you
call is forced to be uninterruptible.  This would need to be nestable also:

	begin_task_uninterruptible();
	  begin_task_uninterruptible();
	    begin_task_uninterruptible();
	    ...
	    end_task_uninterruptible();  // must not become interruptible
	  end_task_uninterruptible();  // nor this
	end_task_uninterruptible();  // from here *could* be interruptible again

Obviously, we would need to gradate this to accommodate killability also.

David