jbd2: fix race in t_outstanding_credits update in jbd2_journal_extend()

jbd2_journal_extend() first checked whether transaction can accept
extending handle with more credits and then added credits to
t_outstanding_credits.  This can race with start_this_handle() adding
another handle to a transaction and thus overbooking a transaction.
Make jbd2_journal_extend() use atomic_add_return() to close the race.

Reviewed-by: Zheng Liu <wenqing.lz@taobao.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 4 insertions, 2 deletions

diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
index f9cd43190b43..f14288b04042 100644
--- a/fs/jbd2/transaction.c
+++ b/fs/jbd2/transaction.c
@@ -433,11 +433,13 @@ int jbd2_journal_extend(handle_t *handle, int nblocks)
         }
 
         spin_lock(&transaction->t_handle_lock);
-        wanted = atomic_read(&transaction->t_outstanding_credits) + nblocks;
+        wanted = atomic_add_return(nblocks,
+                                   &transaction->t_outstanding_credits);
 
         if (wanted > journal->j_max_transaction_buffers) {
                 jbd_debug(3, "denied handle %p %d blocks: "
                           "transaction too large\n", handle, nblocks);
+                atomic_sub(nblocks, &transaction->t_outstanding_credits);
                 goto unlock;
         }
 
@@ -445,6 +447,7 @@ int jbd2_journal_extend(handle_t *handle, int nblocks)
             jbd2_log_space_left(journal)) {
                 jbd_debug(3, "denied handle %p %d blocks: "
                           "insufficient log space\n", handle, nblocks);
+                atomic_sub(nblocks, &transaction->t_outstanding_credits);
                 goto unlock;
         }
 
@@ -456,7 +459,6 @@ int jbd2_journal_extend(handle_t *handle, int nblocks)
 
         handle->h_buffer_credits += nblocks;
         handle->h_requested_credits += nblocks;
-        atomic_add(nblocks, &transaction->t_outstanding_credits);
         result = 0;
 
         jbd_debug(3, "extended handle %p by %d\n", handle, nblocks);