Re: [PATCH 4/6] btrfs: add wrapper for conditional start of exclusive operation

[David Sterba <dsterba@suse.cz>]

On Fri, May 21, 2021 at 09:29:16AM -0400, Josef Bacik wrote:
> On 5/21/21 8:06 AM, David Sterba wrote:
> > +		/*
> > +		 * This blocks any exclop finish from setting it to NONE, so we
> > +		 * request cancelation. Either it runs and we will wait for it,
> > +		 * or it has finished and no waiting will happen.
> > +		 */
> > +		atomic_inc(&fs_info->reloc_cancel_req);
> > +		btrfs_exclop_start_unlock(fs_info);
> > +
> > +		if (test_bit(BTRFS_FS_RELOC_RUNNING, &fs_info->flags))
> > +			wait_on_bit(&fs_info->flags, BTRFS_FS_RELOC_RUNNING,
> > +				    TASK_INTERRUPTIBLE);
> 
> Do we want to capture the return value here, in case the user hit's ctrl+c we 
> can return -EINTR instead so we don't think it succeeded?  Thanks,

The cancel request will stay, so only the waiting part won't happen. I'm
not sure if this is worth to distinguish the two states, eg. allow progs
to print a different message.

Maybe a waiting and non-waiting cancel modes would be also useful. As
the interface is string-based we can also add 'status' that would say if
it's running or not. This should cover the usecases, but would be
a bit more complicated in the state transitions.