Re: [COMMAND HANGS] The command 'btrfs subvolume sync -s 2 xyz' can hangs.

[Giuseppe Della Bianca <giusdbg@gmail.com>]

In data venerdì 4 gennaio 2019 21:34:03 CET, Jeff Mahoney ha scritto:
]zac[
> > 
> > root     17133 17127  0 17:17 ?        00:00:00 btrfs subvolume sync -s 2
> > /
> > tmp/tmp.p9SiQ1GnpV
> > 
> > cat /proc/17133/stack
> > [<0>] hrtimer_nanosleep+0xce/0x1e0
> > [<0>] __x64_sys_nanosleep+0x77/0xa0
> > [<0>] do_syscall_64+0x5b/0x160
> > [<0>] entry_SYSCALL_64_after_hwframe+0x44/0xa9
> > [<0>] 0xffffffffffffffff
> 
> Ok, so this is it just sleeping between tree searches.
]zac[
> This part is actually important since we see below that we're searching
> for bytenr 76428623872 which, if present, would be in the cut portion of
> your log.
]zac[

If you want, I can send you the full log (very long).
From what you wrote below it seems to me that you do not need it

]zac]
> 
> This is the more important part.  The file system has gone read-only due
> to a missing extent backref.  This is corruption.

Yes, but this is an autocorruption of btrfs, which occurred (it seems to me), 
during a cleanup after a deleting of a snapshoot of an operating system 
installation (perhaps interrupted by an umount).

> It also means that the subvolume is never going to disappear during this
> mount and 'btrfs subvol sync' will wait forever.
]zac[

In my opinion this is bad.

The infinite wait occurs during the execution of a backup script, so I will 
have to find a bypass even for this problem (the sync was a patch to another 
autocorruption problem).

Gdb