RE: [PATCH 1/2] btrfs: reada: limit max works count

[Zhao Lei <zhaolei@cn.fujitsu.com>]

Hi, Chris Mason

> -----Original Message-----
> From: Chris Mason [mailto:clm@fb.com]
> Sent: Thursday, January 21, 2016 1:48 AM
> To: Zhao Lei <zhaolei@cn.fujitsu.com>; linux-btrfs@vger.kernel.org
> Subject: Re: [PATCH 1/2] btrfs: reada: limit max works count
> 
> On Wed, Jan 20, 2016 at 10:16:27AM -0500, Chris Mason wrote:
> > On Tue, Jan 12, 2016 at 03:46:26PM +0800, Zhao Lei wrote:
> > > reada create 2 works for each level of tree in recursion.
> > >
> > > In case of a tree having many levels, the number of created works is
> > > 2^level_of_tree.
> > > Actually we don't need so many works in parallel, this patch limit
> > > max works to BTRFS_MAX_MIRRORS * 2.
> >
> > Hi,
> >
> > I don't think you end up calling atomic_dec() for every time that
> > reada_start_machine() is called.  Also, I'd rather not have a global
> > static variable to limit the parallel workers, when we have more than
> > one FS mounted it'll end up limiting things too much.
> >
> > With this patch applied, I'm seeing deadlocks during btrfs/066.    You
> > have to run the scrub tests as well, basically we're just getting
> > fsstress run alongside scrub.
> >
> > I'll run a few more times with it reverted to make sure, but I think
> > it's the root cause.
> 
> I spoke too soon, it ended up deadlocking a few tests later.
>
In logic, even if the calculation of atomic_dec() in this patch having bug,
in worst condition, reada will works in single-thread mode, and will not
introduce deadlock.

And by looking the backtrace in this mail, maybe it is caused by
reada_control->elems in someplace of this patchset.

I recheck xfstests/066 in both vm and physical machine, on top of my pull-request
git today, with btrfs-progs 4.4 for many times, but had not triggered the bug.

Could you tell me your test environment(TEST_DEV size, mount option),
and odds of fails in btrfs/066?

Thanks
Zhaolei

> Sorry for now I'm pulling all the reada patches.  We'll sort out bug fixes vs cleanups in later rcs.
> 
> With all of the reada patches removed, the deadlocks are gone.
> 
> -chris