On 02/10/2012 04:43 PM, Eric Sandeen wrote:
> On 02/10/2012 04:31 AM, Martin Wilck wrote:
>> I don't want to bother, but could someone have a look at this
>> patch please?
>
> You have to bother, to get attention ;)
>
>> Martin
>>
>> Processes hang forever on a sync-mounted ext2 file system that
>> is mounted with the ext4 module (default in Fedora 16).
>>
>> I can reproduce this reliably by mounting an ext2 partition with
>> "-o sync" and opening a new file an that partition with vim. vim
>> will hang in "D" state forever.
>>
>> I am attaching a small patch here that solves this issue for me.
>> In the sync mounted case, ext4_handle_dirty_metadata() may call
>> sync_dirty_buffer(), which can't be called with buffer lock held.
>>
>> My knowledge of locking in ext4 is insufficient to judge if this
>> patch is correct, but it may serve as starting point for others.
>
> This looks an awful lot like:
>
> b2f49033d80c952a0ffc2d5647bc1a0b8a09c1b3 [PATCH] fix deadlock in ext2
>
> from 2006! :)
>
> But I'm also eyeballing:
>
> 8a2bfdcbfa441d8b0e5cb9c9a7f45f77f80da465 [PATCH] ext[34]: EA block
> reference count racing fix
>
> to be sure it doesn't regress anything there.
>
> And I'd really like to know why this only happens on ext2-with-ext4
> (ext4 nojournal seems ok... hm, actually, I can't repro it on
> ext2 either...)
>
> Ahah. Ok, makes sense, it has to be mkfs'd with a 128-byte inode, so
> that selinux xattrs will go into an external block. Now I can reproduce
> it on ext2. But still not on ext4, with 128-byte-inodes
> and nojournal. Odd. I'd like to sort that out.
I take it back, ext4, 128-byte inodes, -o sync, and ^has_journal does
the trick too. Ok, good, my world is sane(r) again.
So that means whenever there is no journal, we follow a different path
in __ext4_handle_dirty_metadata(), down to sync_dirty_buffer()
I think moving mb_cache_entry_release() prior to your unlock_buffer
should keep the race closed, like this (not tested):
@@ -487,18 +487,19 @@ ext4_xattr_release_block(handle_t *handle, struct
inode *inode,
ext4_free_blocks(handle, inode, bh, 0, 1,
EXT4_FREE_BLOCKS_METADATA |
EXT4_FREE_BLOCKS_FORGET);
+ unlock_buffer(bh);
} else {
le32_add_cpu(&BHDR(bh)->h_refcount, -1);
+ if (ce)
+ mb_cache_entry_release(ce);
+ unlock_buffer(bh);
error = ext4_handle_dirty_metadata(handle, inode, bh);
if (IS_SYNC(inode))
ext4_handle_sync(handle);
dquot_free_block(inode, 1);
ea_bdebug(bh, "refcount now=%d; releasing",
le32_to_cpu(BHDR(bh)->h_refcount));
- if (ce)
- mb_cache_entry_release(ce);
}
- unlock_buffer(bh);
out:
ext4_std_error(inode->i_sb, error);
return;
That's what ext2 does, and what ext3 did, before a bunch of refactoring
a while ago. Care to send again w/ those 2 lines movedd?
-Eric
> The patch also unlocks prior to mb_cache_entry_release(), and I think
> that might put some of the raciness back?
>
> -Eric
>
>
>> Signed-off-by: Martin.Wilck@xxxxxxxxxxxxxx
>> ---
>> fs/ext4/xattr.c | 3 ++-
>> 1 files changed, 2 insertions(+), 1 deletions(-)
>>
>> diff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c
>> index 93a00d8..eb771ea 100644
>> --- a/fs/ext4/xattr.c
>> +++ b/fs/ext4/xattr.c
>> @@ -487,8 +487,10 @@ ext4_xattr_release_block(handle_t *handle, struct inode *inode,
>> ext4_free_blocks(handle, inode, bh, 0, 1,
>> EXT4_FREE_BLOCKS_METADATA |
>> EXT4_FREE_BLOCKS_FORGET);
>> + unlock_buffer(bh);
>> } else {
>> le32_add_cpu(&BHDR(bh)->h_refcount, -1);
>> + unlock_buffer(bh);
>> error = ext4_handle_dirty_metadata(handle, inode, bh);
>> if (IS_SYNC(inode))
>> ext4_handle_sync(handle);
>> @@ -498,7 +500,6 @@ ext4_xattr_release_block(handle_t *handle, struct inode *inode,
>> if (ce)
>> mb_cache_entry_release(ce);
>> }
>> - unlock_buffer(bh);
>> out:
>> ext4_std_error(inode->i_sb, error);
>> return;
>
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