• Jan Kara's avatar
    jbd: Fix oops in journal_remove_journal_head() · bb189247
    Jan Kara authored
    journal_remove_journal_head() can oops when trying to access journal_head
    returned by bh2jh(). This is caused for example by the following race:
    
    	TASK1					TASK2
      journal_commit_transaction()
        ...
        processing t_forget list
          __journal_refile_buffer(jh);
          if (!jh->b_transaction) {
            jbd_unlock_bh_state(bh);
    					journal_try_to_free_buffers()
    					  journal_grab_journal_head(bh)
    					  jbd_lock_bh_state(bh)
    					  __journal_try_to_free_buffer()
    					  journal_put_journal_head(jh)
            journal_remove_journal_head(bh);
    
    journal_put_journal_head() in TASK2 sees that b_jcount == 0 and buffer is not
    part of any transaction and thus frees journal_head before TASK1 gets to doing
    so. Note that even buffer_head can be released by try_to_free_buffers() after
    journal_put_journal_head() which adds even larger opportunity for oops (but I
    didn't see this happen in reality).
    
    Fix the problem by making transactions hold their own journal_head reference
    (in b_jcount). That way we don't have to remove journal_head explicitely via
    journal_remove_journal_head() and instead just remove journal_head when
    b_jcount drops to zero. The result of this is that [__]journal_refile_buffer(),
    [__]journal_unfile_buffer(), and __journal_remove_checkpoint() can free
    journal_head which needs modification of a few callers. Also we have to be
    careful because once journal_head is removed, buffer_head might be freed as
    well. So we have to get our own buffer_head reference where it matters.
    Signed-off-by: default avatarJan Kara <jack@suse.cz>
    bb189247
commit.c 27.9 KB