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

[David Sterba <dsterba@suse.cz>]

On Fri, May 21, 2021 at 06:45:51PM +0200, David Sterba wrote:
> 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.

So as the cancelling would happen I hope it's ok to return the ECANCELED
error in all cases. Other operations like balance or scrub aren't
interruptible and wait until the condition is satisified, but there's a
different pattern regarding the cancel request so it has to be that way
there. This could be unified but right now I don't see the need for
that.

> 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.

I've asked on IRC what's the expected behaviour of cancel command
regarding waiting/not waiting. Seems that 'wait until it's finished' is
preferred and it's consistent with what scrub and balance (cancel) do.