jbd2: Fix oops in jbd2_journal_remove_journal_head()

jbd2_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
  jbd2_journal_commit_transaction()
    ...
    processing t_forget list
      __jbd2_journal_refile_buffer(jh);
      if (!jh->b_transaction) {
        jbd_unlock_bh_state(bh);
					jbd2_journal_try_to_free_buffers()
					  jbd2_journal_grab_journal_head(bh)
					  jbd_lock_bh_state(bh)
					  __journal_try_to_free_buffer()
					  jbd2_journal_put_journal_head(jh)
        jbd2_journal_remove_journal_head(bh);

jbd2_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
jbd2_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 jbd2_journal_remove_journal_head() and instead just
remove journal_head when b_jcount drops to zero. The result of this is
that [__]jbd2_journal_refile_buffer(),
[__]jbd2_journal_unfile_buffer(), and
__jdb2_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>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

4 files changed, 99 insertions, 120 deletions

diff --git a/fs/jbd2/checkpoint.c b/fs/jbd2/checkpoint.c
index 6a79fd0a1a32..2c62c5aae82f 100644
--- a/fs/jbd2/checkpoint.c
+++ b/fs/jbd2/checkpoint.c
@@ -97,10 +97,14 @@ static int __try_to_free_cp_buf(struct journal_head *jh)
 
         if (jh->b_jlist == BJ_None && !buffer_locked(bh) &&
             !buffer_dirty(bh) && !buffer_write_io_error(bh)) {
+                /*
+                 * Get our reference so that bh cannot be freed before
+                 * we unlock it
+                 */
+                get_bh(bh);
                 JBUFFER_TRACE(jh, "remove from checkpoint list");
                 ret = __jbd2_journal_remove_checkpoint(jh) + 1;
                 jbd_unlock_bh_state(bh);
-                jbd2_journal_remove_journal_head(bh);
                 BUFFER_TRACE(bh, "release");
                 __brelse(bh);
         } else {
@@ -223,8 +227,8 @@ restart:
                         spin_lock(&journal->j_list_lock);
                         goto restart;
                 }
+                get_bh(bh);
                 if (buffer_locked(bh)) {
-                        atomic_inc(&bh->b_count);
                         spin_unlock(&journal->j_list_lock);
                         jbd_unlock_bh_state(bh);
                         wait_on_buffer(bh);
@@ -243,7 +247,6 @@ restart:
                  */
                 released = __jbd2_journal_remove_checkpoint(jh);
                 jbd_unlock_bh_state(bh);
-                jbd2_journal_remove_journal_head(bh);
                 __brelse(bh);
         }
 
@@ -284,7 +287,7 @@ static int __process_buffer(journal_t *journal, struct journal_head *jh,
         int ret = 0;
 
         if (buffer_locked(bh)) {
-                atomic_inc(&bh->b_count);
+                get_bh(bh);
                 spin_unlock(&journal->j_list_lock);
                 jbd_unlock_bh_state(bh);
                 wait_on_buffer(bh);
@@ -316,12 +319,12 @@ static int __process_buffer(journal_t *journal, struct journal_head *jh,
                 ret = 1;
                 if (unlikely(buffer_write_io_error(bh)))
                         ret = -EIO;
+                get_bh(bh);
                 J_ASSERT_JH(jh, !buffer_jbddirty(bh));
                 BUFFER_TRACE(bh, "remove from checkpoint");
                 __jbd2_journal_remove_checkpoint(jh);
                 spin_unlock(&journal->j_list_lock);
                 jbd_unlock_bh_state(bh);
-                jbd2_journal_remove_journal_head(bh);
                 __brelse(bh);
         } else {
                 /*
@@ -554,7 +557,8 @@ int jbd2_cleanup_journal_tail(journal_t *journal)
 /*
  * journal_clean_one_cp_list
  *
- * Find all the written-back checkpoint buffers in the given list and release them.
+ * Find all the written-back checkpoint buffers in the given list and
+ * release them.
  *
  * Called with the journal locked.
  * Called with j_list_lock held.
@@ -663,8 +667,8 @@ out:
  * checkpoint lists.
  *
  * The function returns 1 if it frees the transaction, 0 otherwise.
+ * The function can free jh and bh.
  *
- * This function is called with the journal locked.
  * This function is called with j_list_lock held.
  * This function is called with jbd_lock_bh_state(jh2bh(jh))
  */
@@ -684,13 +688,14 @@ int __jbd2_journal_remove_checkpoint(struct journal_head *jh)
         }
         journal = transaction->t_journal;
 
+        JBUFFER_TRACE(jh, "removing from transaction");
         __buffer_unlink(jh);
         jh->b_cp_transaction = NULL;
+        jbd2_journal_put_journal_head(jh);
 
         if (transaction->t_checkpoint_list != NULL ||
             transaction->t_checkpoint_io_list != NULL)
                 goto out;
-        JBUFFER_TRACE(jh, "transaction has no more buffers");
 
         /*
          * There is one special case to worry about: if we have just pulled the
@@ -701,10 +706,8 @@ int __jbd2_journal_remove_checkpoint(struct journal_head *jh)
          * The locking here around t_state is a bit sleazy.
          * See the comment at the end of jbd2_journal_commit_transaction().
          */
-        if (transaction->t_state != T_FINISHED) {
-                JBUFFER_TRACE(jh, "belongs to running/committing transaction");
+        if (transaction->t_state != T_FINISHED)
                 goto out;
-        }
 
         /* OK, that was the last buffer for the transaction: we can now
            safely remove this transaction from the log */
@@ -723,7 +726,6 @@ int __jbd2_journal_remove_checkpoint(struct journal_head *jh)
         wake_up(&journal->j_wait_logspace);
         ret = 1;
 out:
-        JBUFFER_TRACE(jh, "exit");
         return ret;
 }
 
@@ -742,6 +744,8 @@ void __jbd2_journal_insert_checkpoint(struct journal_head *jh,
         J_ASSERT_JH(jh, buffer_dirty(jh2bh(jh)) || buffer_jbddirty(jh2bh(jh)));
         J_ASSERT_JH(jh, jh->b_cp_transaction == NULL);
 
+        /* Get reference for checkpointing transaction */
+        jbd2_journal_grab_journal_head(jh2bh(jh));
         jh->b_cp_transaction = transaction;
 
         if (!transaction->t_checkpoint_list) {
diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
index 7f21cf3aaf92..eef6979821a4 100644
--- a/fs/jbd2/commit.c
+++ b/fs/jbd2/commit.c
@@ -848,10 +848,16 @@ 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);
 
@@ -914,28 +920,27 @@ restart_loop:
                         __jbd2_journal_insert_checkpoint(jh, commit_transaction);
                         if (is_journal_aborted(journal))
                                 clear_buffer_jbddirty(bh);
-                        JBUFFER_TRACE(jh, "refile for checkpoint writeback");
-                        __jbd2_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");
-                        __jbd2_journal_refile_buffer(jh);
-                        if (!jh->b_transaction) {
-                                jbd_unlock_bh_state(bh);
-                                 /* needs a brelse */
-                                jbd2_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 buffer");
+                __jbd2_journal_refile_buffer(jh);
+                jbd_unlock_bh_state(bh);
+                if (try_to_free)
+                        release_buffer_page(bh);        /* Drops bh reference */
+                else
+                        __brelse(bh);
                 cond_resched_lock(&journal->j_list_lock);
         }
         spin_unlock(&journal->j_list_lock);
diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
index 9a7826990304..0dfa5b598e68 100644
--- a/fs/jbd2/journal.c
+++ b/fs/jbd2/journal.c
@@ -2078,10 +2078,9 @@ static void journal_free_journal_head(struct journal_head *jh)
  * When a buffer has its BH_JBD bit set it is immune from being released by
  * core kernel code, mainly via ->b_count.
  *
- * A journal_head may be detached from its buffer_head when the journal_head's
- * b_transaction, b_cp_transaction and b_next_transaction pointers are NULL.
- * Various places in JBD call jbd2_journal_remove_journal_head() to indicate that the
- * journal_head can be dropped if needed.
+ * A journal_head is detached from its buffer_head when the journal_head's
+ * b_jcount reaches zero. Running transaction (b_transaction) and checkpoint
+ * transaction (b_cp_transaction) hold their references to b_jcount.
  *
  * Various places in the kernel want to attach a journal_head to a buffer_head
  * _before_ attaching the journal_head to a transaction.  To protect the
@@ -2094,17 +2093,16 @@ static void journal_free_journal_head(struct journal_head *jh)
  *      (Attach a journal_head if needed.  Increments b_jcount)
  *      struct journal_head *jh = jbd2_journal_add_journal_head(bh);
  *      ...
+ *      (Get another reference for transaction)
+ *      jbd2_journal_grab_journal_head(bh);
  *      jh->b_transaction = xxx;
+ *      (Put original reference)
  *      jbd2_journal_put_journal_head(jh);
- *
- * Now, the journal_head's b_jcount is zero, but it is safe from being released
- * because it has a non-zero b_transaction.
  */
 
 /*
  * Give a buffer_head a journal_head.
  *
- * Doesn't need the journal lock.
  * May sleep.
  */
 struct journal_head *jbd2_journal_add_journal_head(struct buffer_head *bh)
@@ -2168,61 +2166,29 @@ static void __journal_remove_journal_head(struct buffer_head *bh)
         struct journal_head *jh = bh2jh(bh);
 
         J_ASSERT_JH(jh, jh->b_jcount >= 0);
-
-        get_bh(bh);
-        if (jh->b_jcount == 0) {
-                if (jh->b_transaction == NULL &&
-                                jh->b_next_transaction == NULL &&
-                                jh->b_cp_transaction == NULL) {
-                        J_ASSERT_JH(jh, jh->b_jlist == BJ_None);
-                        J_ASSERT_BH(bh, buffer_jbd(bh));
-                        J_ASSERT_BH(bh, jh2bh(jh) == bh);
-                        BUFFER_TRACE(bh, "remove journal_head");
-                        if (jh->b_frozen_data) {
-                                printk(KERN_WARNING "%s: freeing "
-                                                "b_frozen_data\n",
-                                                __func__);
-                                jbd2_free(jh->b_frozen_data, bh->b_size);
-                        }
-                        if (jh->b_committed_data) {
-                                printk(KERN_WARNING "%s: freeing "
-                                                "b_committed_data\n",
-                                                __func__);
-                                jbd2_free(jh->b_committed_data, bh->b_size);
-                        }
-                        bh->b_private = NULL;
-                        jh->b_bh = NULL;        /* debug, really */
-                        clear_buffer_jbd(bh);
-                        __brelse(bh);
-                        journal_free_journal_head(jh);
-                } else {
-                        BUFFER_TRACE(bh, "journal_head was locked");
-                }
+        J_ASSERT_JH(jh, jh->b_transaction == NULL);
+        J_ASSERT_JH(jh, jh->b_next_transaction == NULL);
+        J_ASSERT_JH(jh, jh->b_cp_transaction == NULL);
+        J_ASSERT_JH(jh, jh->b_jlist == BJ_None);
+        J_ASSERT_BH(bh, buffer_jbd(bh));
+        J_ASSERT_BH(bh, jh2bh(jh) == bh);
+        BUFFER_TRACE(bh, "remove journal_head");
+        if (jh->b_frozen_data) {
+                printk(KERN_WARNING "%s: freeing b_frozen_data\n", __func__);
+                jbd2_free(jh->b_frozen_data, bh->b_size);
         }
+        if (jh->b_committed_data) {
+                printk(KERN_WARNING "%s: freeing b_committed_data\n", __func__);
+                jbd2_free(jh->b_committed_data, bh->b_size);
+        }
+        bh->b_private = NULL;
+        jh->b_bh = NULL;        /* debug, really */
+        clear_buffer_jbd(bh);
+        journal_free_journal_head(jh);
 }
 
 /*
- * jbd2_journal_remove_journal_head(): if the buffer isn't attached to a transaction
- * and has a zero b_jcount then remove and release its journal_head.   If we did
- * see that the buffer is not used by any transaction we also "logically"
- * decrement ->b_count.
- *
- * We in fact take an additional increment on ->b_count as a convenience,
- * because the caller usually wants to do additional things with the bh
- * after calling here.
- * The caller of jbd2_journal_remove_journal_head() *must* run __brelse(bh) at some
- * time.  Once the caller has run __brelse(), the buffer is eligible for
- * reaping by try_to_free_buffers().
- */
-void jbd2_journal_remove_journal_head(struct buffer_head *bh)
-{
-        jbd_lock_bh_journal_head(bh);
-        __journal_remove_journal_head(bh);
-        jbd_unlock_bh_journal_head(bh);
-}
-
-/*
- * Drop a reference on the passed journal_head.  If it fell to zero then try to
+ * Drop a reference on the passed journal_head.  If it fell to zero then
  * release the journal_head from the buffer_head.
  */
 void jbd2_journal_put_journal_head(struct journal_head *jh)
@@ -2232,11 +2198,12 @@ void jbd2_journal_put_journal_head(struct journal_head *jh)
         jbd_lock_bh_journal_head(bh);
         J_ASSERT_JH(jh, jh->b_jcount > 0);
         --jh->b_jcount;
-        if (!jh->b_jcount && !jh->b_transaction) {
+        if (!jh->b_jcount) {
                 __journal_remove_journal_head(bh);
+                jbd_unlock_bh_journal_head(bh);
                 __brelse(bh);
-        }
-        jbd_unlock_bh_journal_head(bh);
+        } else
+                jbd_unlock_bh_journal_head(bh);
 }
 
 /*
diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
index 547b101049e5..2d7109414cdd 100644
--- a/fs/jbd2/transaction.c
+++ b/fs/jbd2/transaction.c
@@ -30,6 +30,7 @@
 #include <linux/module.h>
 
 static void __jbd2_journal_temp_unlink_buffer(struct journal_head *jh);
+static void __jbd2_journal_unfile_buffer(struct journal_head *jh);
 
 /*
  * jbd2_get_transaction: obtain a new transaction_t object.
@@ -764,7 +765,6 @@ repeat:
         if (!jh->b_transaction) {
                 JBUFFER_TRACE(jh, "no transaction");
                 J_ASSERT_JH(jh, !jh->b_next_transaction);
-                jh->b_transaction = transaction;
                 JBUFFER_TRACE(jh, "file as BJ_Reserved");
                 spin_lock(&journal->j_list_lock);
                 __jbd2_journal_file_buffer(jh, transaction, BJ_Reserved);
@@ -895,8 +895,6 @@ int jbd2_journal_get_create_access(handle_t *handle, struct buffer_head *bh)
                  * committed and so it's safe to clear the dirty bit.
                  */
                 clear_buffer_dirty(jh2bh(jh));
-                jh->b_transaction = transaction;
-
                 /* first access by this transaction */
                 jh->b_modified = 0;
 
@@ -1230,8 +1228,6 @@ int jbd2_journal_forget (handle_t *handle, struct buffer_head *bh)
                         __jbd2_journal_file_buffer(jh, transaction, BJ_Forget);
                 } else {
                         __jbd2_journal_unfile_buffer(jh);
-                        jbd2_journal_remove_journal_head(bh);
-                        __brelse(bh);
                         if (!buffer_jbd(bh)) {
                                 spin_unlock(&journal->j_list_lock);
                                 jbd_unlock_bh_state(bh);
@@ -1554,19 +1550,32 @@ void __jbd2_journal_temp_unlink_buffer(struct journal_head *jh)
                 mark_buffer_dirty(bh);  /* Expose it to the VM */
 }
 
-void __jbd2_journal_unfile_buffer(struct journal_head *jh)
+/*
+ * Remove buffer from all transactions.
+ *
+ * Called with bh_state lock and j_list_lock
+ *
+ * jh and bh may be already freed when this function returns.
+ */
+static void __jbd2_journal_unfile_buffer(struct journal_head *jh)
 {
         __jbd2_journal_temp_unlink_buffer(jh);
         jh->b_transaction = NULL;
+        jbd2_journal_put_journal_head(jh);
 }
 
 void jbd2_journal_unfile_buffer(journal_t *journal, struct journal_head *jh)
 {
-        jbd_lock_bh_state(jh2bh(jh));
+        struct buffer_head *bh = jh2bh(jh);
+
+        /* Get reference so that buffer cannot be freed before we unlock it */
+        get_bh(bh);
+        jbd_lock_bh_state(bh);
         spin_lock(&journal->j_list_lock);
         __jbd2_journal_unfile_buffer(jh);
         spin_unlock(&journal->j_list_lock);
-        jbd_unlock_bh_state(jh2bh(jh));
+        jbd_unlock_bh_state(bh);
+        __brelse(bh);
 }
 
 /*
@@ -1593,8 +1602,6 @@ __journal_try_to_free_buffer(journal_t *journal, struct buffer_head *bh)
                 if (jh->b_jlist == BJ_None) {
                         JBUFFER_TRACE(jh, "remove from checkpoint list");
                         __jbd2_journal_remove_checkpoint(jh);
-                        jbd2_journal_remove_journal_head(bh);
-                        __brelse(bh);
                 }
         }
         spin_unlock(&journal->j_list_lock);
@@ -1657,7 +1664,6 @@ int jbd2_journal_try_to_free_buffers(journal_t *journal,
                 /*
                  * We take our own ref against the journal_head here to avoid
                  * having to add tons of locking around each instance of
-                 * jbd2_journal_remove_journal_head() and
                  * jbd2_journal_put_journal_head().
                  */
                 jh = jbd2_journal_grab_journal_head(bh);
@@ -1695,10 +1701,9 @@ static int __dispose_buffer(struct journal_head *jh, transaction_t *transaction)
         int may_free = 1;
         struct buffer_head *bh = jh2bh(jh);
 
-        __jbd2_journal_unfile_buffer(jh);
-
         if (jh->b_cp_transaction) {
                 JBUFFER_TRACE(jh, "on running+cp transaction");
+                __jbd2_journal_temp_unlink_buffer(jh);
                 /*
                  * We don't want to write the buffer anymore, clear the
                  * bit so that we don't confuse checks in
@@ -1709,8 +1714,7 @@ static int __dispose_buffer(struct journal_head *jh, transaction_t *transaction)
                 may_free = 0;
         } else {
                 JBUFFER_TRACE(jh, "on running transaction");
-                jbd2_journal_remove_journal_head(bh);
-                __brelse(bh);
+                __jbd2_journal_unfile_buffer(jh);
         }
         return may_free;
 }
@@ -1988,6 +1992,8 @@ void __jbd2_journal_file_buffer(struct journal_head *jh,
 
         if (jh->b_transaction)
                 __jbd2_journal_temp_unlink_buffer(jh);
+        else
+                jbd2_journal_grab_journal_head(bh);
         jh->b_transaction = transaction;
 
         switch (jlist) {
@@ -2039,9 +2045,10 @@ void jbd2_journal_file_buffer(struct journal_head *jh,
  * already started to be used by a subsequent transaction, refile the
  * buffer on that transaction's metadata list.
  *
- * Called under journal->j_list_lock
- *
+ * Called under j_list_lock
  * Called under jbd_lock_bh_state(jh2bh(jh))
+ *
+ * jh and bh may be already free when this function returns
  */
 void __jbd2_journal_refile_buffer(struct journal_head *jh)
 {
@@ -2065,6 +2072,11 @@ void __jbd2_journal_refile_buffer(struct journal_head *jh)
 
         was_dirty = test_clear_buffer_jbddirty(bh);
         __jbd2_journal_temp_unlink_buffer(jh);
+        /*
+         * We set b_transaction here because b_next_transaction will inherit
+         * our jh reference and thus __jbd2_journal_file_buffer() must not
+         * take a new one.
+         */
         jh->b_transaction = jh->b_next_transaction;
         jh->b_next_transaction = NULL;
         if (buffer_freed(bh))
@@ -2081,30 +2093,21 @@ void __jbd2_journal_refile_buffer(struct journal_head *jh)
 }
 
 /*
- * For the unlocked version of this call, also make sure that any
- * hanging journal_head is cleaned up if necessary.
- *
- * __jbd2_journal_refile_buffer is usually called as part of a single locked
- * operation on a buffer_head, in which the caller is probably going to
- * be hooking the journal_head onto other lists.  In that case it is up
- * to the caller to remove the journal_head if necessary.  For the
- * unlocked jbd2_journal_refile_buffer call, the caller isn't going to be
- * doing anything else to the buffer so we need to do the cleanup
- * ourselves to avoid a jh leak.
- *
- * *** The journal_head may be freed by this call! ***
+ * __jbd2_journal_refile_buffer() with necessary locking added. We take our
+ * bh reference so that we can safely unlock bh.
+ *
+ * The jh and bh may be freed by this call.
  */
 void jbd2_journal_refile_buffer(journal_t *journal, struct journal_head *jh)
 {
         struct buffer_head *bh = jh2bh(jh);
 
+        /* Get reference so that buffer cannot be freed before we unlock it */
+        get_bh(bh);
         jbd_lock_bh_state(bh);
         spin_lock(&journal->j_list_lock);
-
         __jbd2_journal_refile_buffer(jh);
         jbd_unlock_bh_state(bh);
-        jbd2_journal_remove_journal_head(bh);
-
         spin_unlock(&journal->j_list_lock);
         __brelse(bh);
 }