Hi, Chris Mason

-----Original Message-----
From: Chris Mason [mailto:clm@fb.com]
Sent: Thursday, January 21, 2016 10:15 PM
To: Zhao Lei <redacted>
Cc: linux-btrfs@vger.kernel.org
Subject: Re: [PATCH 1/2] btrfs: reada: limit max works count

On Thu, Jan 21, 2016 at 06:06:21PM +0800, Zhao Lei wrote:

quoted

Hi, Chris Mason

quoted

-----Original Message-----
From: Chris Mason [mailto:clm@fb.com]
Sent: Thursday, January 21, 2016 1:48 AM
To: Zhao Lei <redacted>; 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:

quoted

On Tue, Jan 12, 2016 at 03:46:26PM +0800, Zhao Lei wrote:

quoted

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.

Just running 066 alone doesn't trigger it for me.  I have to run everything from
00->066.

My setup is 5 drives.  I use a script to carve them up into logical volumes, 5 for
the test device and 5 for the scratch pool.  I think it should reproduce with a
single drive, if you still can't trigger I'll confirm that.

quoted

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

100% odds of failing, one time it made it up to btrfs/072.  I think more
important than the drive setup is that I have all the debugging on.
CONFIG_DEBUG_PAGEALLOC, spinlock debugging, mutex debugging and lock
dep enabled.

Thanks for your answer.

But unfortunately I hadn't reproduce the dead_lock in above way today...
Now I queued loop of above reproduce script in more nodes, and hopes
it can happen in this weekend.

And by reviewing code, I found a problem which can introduce similar bad result
in logic, and made a patch for it.
[PATCH] [RFC] btrfs: reada: avoid undone reada extents in btrfs_reada_wait

Because it is only a problem in logic, but rarely happened, I only confirmed
no-problem after patch applied.

Sorry for increased your works, could you apply this patch and test is it
works?

Thanks
Zhaolei

-chris