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Date: Thu, 10 Nov 2022 16:26:37 +0100
From: Jan Kara <jack@...e.cz>
To: Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>
Cc: Jan Kara <jack@...e.cz>, Thilo Fromm <t-lo@...ux.microsoft.com>,
Ye Bin <yebin10@...wei.com>, jack@...e.com, tytso@....edu,
linux-ext4@...r.kernel.org, regressions@...ts.linux.dev
Subject: Re: [syzbot] possible deadlock in jbd2_journal_lock_updates
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!
Honza
--
Jan Kara <jack@...e.com>
SUSE Labs, CR
View attachment "0001-ext4-Debug-xattr-refcount.patch" of type "text/x-patch" (718 bytes)
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