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 SUSE Labs, CR