jbd: Fix oops in journal_remove_journal_head()

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: Jan Kara <jack@suse.cz>

1 files changed, 21 insertions, 25 deletions

diff --git a/fs/jbd/commit.c b/fs/jbd/commit.c
index eedd201374a8..8799207df058 100644
--- a/fs/jbd/commit.c
+++ b/fs/jbd/commit.c
@@ -258,10 +258,6 @@ write_out_data:
                         jbd_unlock_bh_state(bh);
                         if (locked)
                                 unlock_buffer(bh);
-                        journal_remove_journal_head(bh);
-                        /* One for our safety reference, other for
-                         * journal_remove_journal_head() */
-                        put_bh(bh);
                         release_data_buffer(bh);
                 }
 
@@ -455,14 +451,9 @@ void journal_commit_transaction(journal_t *journal)
                 }
                 if (buffer_jbd(bh) && bh2jh(bh) == jh &&
                     jh->b_transaction == commit_transaction &&
-                    jh->b_jlist == BJ_Locked) {
+                    jh->b_jlist == BJ_Locked)
                         __journal_unfile_buffer(jh);
-                        jbd_unlock_bh_state(bh);
-                        journal_remove_journal_head(bh);
-                        put_bh(bh);
-                } else {
-                        jbd_unlock_bh_state(bh);
-                }
+                jbd_unlock_bh_state(bh);
                 release_data_buffer(bh);
                 cond_resched_lock(&journal->j_list_lock);
         }
@@ -807,10 +798,16 @@ restart_loop:
         while (commit_transaction->t_forget) {
                 transaction_t *cp_transaction;
                 struct buffer_head *bh;
+                int try_to_free = 0;
 
                 jh = commit_transaction->t_forget;
                 spin_unlock(&journal->j_list_lock);
                 bh = jh2bh(jh);
+                /*
+                 * Get a reference so that bh cannot be freed before we are
+                 * done with it.
+                 */
+                get_bh(bh);
                 jbd_lock_bh_state(bh);
                 J_ASSERT_JH(jh, jh->b_transaction == commit_transaction ||
                         jh->b_transaction == journal->j_running_transaction);
@@ -868,28 +865,27 @@ restart_loop:
                         __journal_insert_checkpoint(jh, commit_transaction);
                         if (is_journal_aborted(journal))
                                 clear_buffer_jbddirty(bh);
-                        JBUFFER_TRACE(jh, "refile for checkpoint writeback");
-                        __journal_refile_buffer(jh);
-                        jbd_unlock_bh_state(bh);
                 } else {
                         J_ASSERT_BH(bh, !buffer_dirty(bh));
-                        /* The buffer on BJ_Forget list and not jbddirty means
+                        /*
+                         * The buffer on BJ_Forget list and not jbddirty means
                          * it has been freed by this transaction and hence it
                          * could not have been reallocated until this
                          * transaction has committed. *BUT* it could be
                          * reallocated once we have written all the data to
                          * disk and before we process the buffer on BJ_Forget
-                         * list. */
-                        JBUFFER_TRACE(jh, "refile or unfile freed buffer");
-                        __journal_refile_buffer(jh);
-                        if (!jh->b_transaction) {
-                                jbd_unlock_bh_state(bh);
-                                 /* needs a brelse */
-                                journal_remove_journal_head(bh);
-                                release_buffer_page(bh);
-                        } else
-                                jbd_unlock_bh_state(bh);
+                         * list.
+                         */
+                        if (!jh->b_next_transaction)
+                                try_to_free = 1;
                 }
+                JBUFFER_TRACE(jh, "refile or unfile freed buffer");
+                __journal_refile_buffer(jh);
+                jbd_unlock_bh_state(bh);
+                if (try_to_free)
+                        release_buffer_page(bh);
+                else
+                        __brelse(bh);
                 cond_resched_lock(&journal->j_list_lock);
         }
         spin_unlock(&journal->j_list_lock);