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SubjectRe: general protection fault: from release_blocks_on_commit
Theodore Tso wrote:

> On Tue, Oct 21, 2008 at 02:03:01PM -0400, Eric Paris wrote:
>
>> I can consistently get the below backtrace any time I try to shutdown my
>> machine. This machine has ext4 as it's root FS. This is 100%
>> reproducible. I backed out commit
>> 3e624fc72fba09b6f999a9fbb87b64efccd38036 and it fixed the problem.
>>
>> This is a regression.
>>
>
> Can you send me your .config, please? I'm trying to duplicate it on
> my end.
>
> - Ted
>
Ted, you probably need some slab debugging on to hit it.

I think the problem is that jbd2_journal_commit_transaction may call
__jbd2_journal_drop_transaction(journal, commit_transaction) if the
checkpoint lists are NULL, and this frees the commit_transaction.

However, the call to ->j_commit_callback() tries to use it after that.

I'm out of time for now to be sure this is the right fix, but something
like this perhaps?

Index: linux-2.6/fs/jbd2/commit.c
===================================================================
--- linux-2.6.orig/fs/jbd2/commit.c 2008-10-27 11:24:42.000000000 -0500
+++ linux-2.6/fs/jbd2/commit.c 2008-10-27 17:19:22.771063324 -0500
@@ -992,15 +992,15 @@ restart_loop:
commit_transaction->t_cpprev->t_cpnext =
commit_transaction;
}
+ if (journal->j_commit_callback)
+ journal->j_commit_callback(journal, commit_transaction);
+
+ trace_mark(jbd2_end_commit, "dev %s transaction %d head %d",
+ journal->j_devname, commit_transaction->t_tid,
+ journal->j_tail_sequence);
}
spin_unlock(&journal->j_list_lock);

- if (journal->j_commit_callback)
- journal->j_commit_callback(journal, commit_transaction);
-
- trace_mark(jbd2_end_commit, "dev %s transaction %d head %d",
- journal->j_devname, commit_transaction->t_tid,
- journal->j_tail_sequence);
jbd_debug(1, "JBD: commit %d complete, head %d\n",
journal->j_commit_sequence, journal->j_tail_sequence);

Also, I'm not certain that it matters, but the loop in
release_blocks_on_commit() is kfreeing list entries w/o taking
them off the list; I suppose maybe this is safe if the whole thing
is getting discarded when we're done, but just to keep things sane,
would this make sense (also, I think we need to double-check use of
s_md_lock; it's taken when adding things to the list, but not when
freeing/removing ... if it's needed, isn't it needed on both ends...):


Index: linux-2.6/fs/ext4/mballoc.c
===================================================================
--- linux-2.6.orig/fs/ext4/mballoc.c 2008-10-27 11:24:41.000000000 -0500
+++ linux-2.6/fs/ext4/mballoc.c 2008-10-27 17:19:43.401064490 -0500
@@ -2644,6 +2644,7 @@ static void release_blocks_on_commit(jou
struct super_block *sb = journal->j_private;
struct ext4_buddy e4b;
struct ext4_group_info *db;
+ struct ext4_sb_info *sbi = EXT4_SB(sb);
int err, count = 0, count2 = 0;
struct ext4_free_data *entry;
ext4_fsblk_t discard_block;
@@ -2683,6 +2684,9 @@ static void release_blocks_on_commit(jou
(unsigned long long) discard_block, entry->count);
sb_issue_discard(sb, discard_block, entry->count);

+ spin_lock(&sbi->s_md_lock);
+ list_del(&entry->list);
+ spin_unlock(&sbi->s_md_lock);
kmem_cache_free(ext4_free_ext_cachep, entry);
ext4_mb_release_desc(&e4b);
}
-Eric



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