Re: [PATCH] btrfs: let btrfs_delete_unused_bgs() to clean relocated bgs

[Naohiro Aota]

2016-09-02 (é) ã 09:35 -0400 ã Josef Bacik ãããæãããã:
> On 09/02/2016 03:46 AM, Naohiro Aota wrote:
> > 
> > Currently, btrfs_relocate_chunk() is removing relocated BG by
> > itself. But
> > the work can be done by btrfs_delete_unused_bgs() (and it's better
> > since it
> > trim the BG). Let's dedupe the code.
> > 
> > While btrfs_delete_unused_bgs() is already hitting the relocated
> > BG, it
> > skip the BG since the BG has "ro" flag set (to keep balancing BG
> > intact).
> > On the other hand, btrfs cannot drop "ro" flag here to prevent
> > additional
> > writes. So this patch make use of "removed" flag.
> > btrfs_delete_unused_bgs() now detect the flag to distinguish
> > whether a
> > read-only BG is relocating or not.
> > 
> 
> This seems racey to me.ÂÂWe remove the last part of the block group,
> it ends upÂ
> on the unused_bgs_list, we process this list, see that removed isn't
> set and weÂ
> skip it, then later we set removed, but it's too late.ÂÂI think the
> right way isÂ
> to actually do a transaction, set ->removed, manually add it to theÂ
> unused_bgs_list if it's not already, then end the transaction.ÂÂThis
> way we areÂ
> guaranteed to have the bg on the list when it is ready to be
> removed.ÂÂThis isÂ
> my analysis after looking at it for 10 seconds after being awake for
> like 30Â
> minutes so if I'm missing something let me know.ÂÂThanks,

I don't think a race will happen. Since we are holding
delete_unused_bgs_mutex here, btrfs_delte_unused_bgs() checks ->removed
flag after we unlock the mutex i.e. we setup the flag properly. For a
case btrfs_delete_usused_bgs() checks the BG before we hold
delte_unused_bgs_mutex, then that BG is removed by it (if it's empty)
and btrfs_relocate_chunk() should never see it.

Regards,
Naohiro