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Date:   Mon, 21 Nov 2022 14:35:59 +0100
From:   Jan Kara <jack@suse.cz>
To:     Jeremi Piotrowski <jpiotrowski@linux.microsoft.com>
Cc:     Jan Kara <jack@suse.cz>, Thilo Fromm <t-lo@linux.microsoft.com>,
        Ye Bin <yebin10@huawei.com>, jack@suse.com, tytso@mit.edu,
        linux-ext4@vger.kernel.org, regressions@lists.linux.dev
Subject: Re: [syzbot] possible deadlock in jbd2_journal_lock_updates
Message-ID: <20221121133559.srie6oy47udavj52@quack3>
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On Fri 11-11-22 07:52:38, Jeremi Piotrowski wrote:
> On Fri, Nov 11, 2022 at 07:10:29AM -0800, Jeremi Piotrowski wrote:
> > On Fri, Nov 11, 2022 at 03:24:24PM +0100, Jan Kara wrote:
> > > On Thu 10-11-22 11:27:01, Jeremi Piotrowski wrote:
> > > > On Thu, Nov 10, 2022 at 04:26:37PM +0100, Jan Kara wrote:
> > > > > On Thu 10-11-22 04:57:58, Jeremi Piotrowski wrote:
> > > > > > On Wed, Oct 26, 2022 at 12:18:54PM +0200, Jan Kara wrote:
> > > > > > > On Mon 24-10-22 18:32:51, Thilo Fromm wrote:
> > > > > > > > Hello Honza,
> > > > > > > > 
> > > > > > > > > Yeah, I was pondering about this for some time but still I have no clue who
> > > > > > > > > could be holding the buffer lock (which blocks the task holding the
> > > > > > > > > transaction open) or how this could related to the commit you have
> > > > > > > > > identified. I have two things to try:
> > > > > > > > > 
> > > > > > > > > 1) Can you please check whether the deadlock reproduces also with 6.0
> > > > > > > > > kernel? The thing is that xattr handling code in ext4 has there some
> > > > > > > > > additional changes, commit 307af6c8793 ("mbcache: automatically delete
> > > > > > > > > entries from cache on freeing") in particular.
> > > > > > > > 
> > > > > > > > This would be complex; we currently do not integrate 6.0 with Flatcar and
> > > > > > > > would need to spend quite some effort ingesting it first (mostly, make sure
> > > > > > > > the new kernel does not break something unrelated). Flatcar is an
> > > > > > > > image-based distro, so kernel updates imply full distro updates.
> > > > > > > 
> > > > > > > OK, understood.
> > > > > > > 
> > > > > > > > > 2) I have created a debug patch (against 5.15.x stable kernel). Can you
> > > > > > > > > please reproduce the failure with it and post the output of "echo w
> > > > > > > > > > /proc/sysrq-trigger" and also the output the debug patch will put into the
> > > > > > > > > kernel log? It will dump the information about buffer lock owner if we > cannot get the lock for more than 32 seconds.
> > > > > > > > 
> > > > > > > > This would be more straightforward - I can reach out to one of our users
> > > > > > > > suffering from the issue; they can reliably reproduce it and don't shy away
> > > > > > > > from patching their kernel. Where can I find the patch?
> > > > > > > 
> > > > > > > Ha, my bad. I forgot to attach it. Here it is.
> > > > > > > 
> > > > > > 
> > > > > > Unfortunately this patch produced no output, but I have been able to repro so I
> > > > > > understand why: except for the hung tasks, we have 1+ tasks busy-looping through
> > > > > > the following code in ext4_xattr_block_set():
> > > > > > 
> > > > > > inserted:
> > > > > >         if (!IS_LAST_ENTRY(s->first)) {
> > > > > >                 new_bh = ext4_xattr_block_cache_find(inode, header(s->base),
> > > > > >                                                      &ce);
> > > > > >                 if (new_bh) {
> > > > > >                         /* We found an identical block in the cache. */
> > > > > >                         if (new_bh == bs->bh)
> > > > > >                                 ea_bdebug(new_bh, "keeping");
> > > > > >                         else {
> > > > > >                                 u32 ref;
> > > > > > 
> > > > > >                                 WARN_ON_ONCE(dquot_initialize_needed(inode));
> > > > > > 
> > > > > >                                 /* The old block is released after updating
> > > > > >                                    the inode. */
> > > > > >                                 error = dquot_alloc_block(inode,
> > > > > >                                                 EXT4_C2B(EXT4_SB(sb), 1));
> > > > > >                                 if (error)
> > > > > >                                         goto cleanup;
> > > > > >                                 BUFFER_TRACE(new_bh, "get_write_access");
> > > > > >                                 error = ext4_journal_get_write_access(
> > > > > >                                                 handle, sb, new_bh,
> > > > > >                                                 EXT4_JTR_NONE);
> > > > > >                                 if (error)
> > > > > >                                         goto cleanup_dquot;
> > > > > >                                 lock_buffer(new_bh);
> > > > > >                                 /*
> > > > > >                                  * We have to be careful about races with
> > > > > >                                  * adding references to xattr block. Once we
> > > > > >                                  * hold buffer lock xattr block's state is
> > > > > >                                  * stable so we can check the additional
> > > > > >                                  * reference fits.
> > > > > >                                  */
> > > > > >                                 ref = le32_to_cpu(BHDR(new_bh)->h_refcount) + 1;
> > > > > >                                 if (ref > EXT4_XATTR_REFCOUNT_MAX) {
> > > > > >                                         /*
> > > > > >                                          * Undo everything and check mbcache
> > > > > >                                          * again.
> > > > > >                                          */
> > > > > >                                         unlock_buffer(new_bh);
> > > > > >                                         dquot_free_block(inode,
> > > > > >                                                          EXT4_C2B(EXT4_SB(sb),
> > > > > >                                                                   1));
> > > > > >                                         brelse(new_bh);
> > > > > >                                         mb_cache_entry_put(ea_block_cache, ce);
> > > > > >                                         ce = NULL;
> > > > > >                                         new_bh = NULL;
> > > > > >                                         goto inserted;
> > > > > >                                 }
> > > > > > 
> > > > > > The tasks keep taking the 'goto inserted' branch, and never finish. I've been
> > > > > > able to repro with kernel v6.0.7 as well.
> > > > > 
> > > > > Interesting! That makes is much clearer (and also makes my debug patch
> > > > > unnecessary). So clearly the e_reusable variable in the mb_cache_entry got
> > > > > out of sync with the number of references really in the xattr block - in
> > > > > particular the block likely has h_refcount >= EXT4_XATTR_REFCOUNT_MAX but
> > > > > e_reusable is set to true. Now I can see how e_reusable can stay at false due
> > > > > to a race when refcount is actually smaller but I don't see how it could
> > > > > stay at true when refcount is big enough - that part seems to be locked
> > > > > properly. If you can reproduce reasonably easily, can you try reproducing
> > > > > with attached patch? Thanks!
> > > > > 
> > > > 
> > > > Sure, with that patch I'm getting the following output, reusable is false on
> > > > most items until we hit something with reusable true and then that loops
> > > > indefinitely:
> > > 
> > > Thanks. So that is what I've suspected. I'm still not 100% clear on how
> > > this inconsistency can happen although I have a suspicion - does attached
> > > patch fix the problem for you?
> > > 
> > > Also is it possible to share the reproducer or it needs some special
> > > infrastructure?
> > > 
> > > 								Honza
> > 
> > I'll test the patch and report back.
> > 
> > Attached you'll find the reproducer, for me it reproduces within a few minutes.
> > It brings up a k8s node and then runs 3 instances of the application which
> > creates a lot of small files in a loop. The OS we run it on has selinux enabled
> > in permissive mode, that might play a role.
> > 
> 
> I can still reproduce it with the patch.

Thanks for the answer! So I was trying to make your reproducer work on my
system but it was not so easy on openSUSE ;). Anyway, when working on this
I've realized there may be a simpler way to tickle the bug and indeed, I
can now trigger it with a simple C program. So thanks for your help, I'm
now debugging the issue on my system.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR