jbd2: fix outstanding credits assert in jbd2_journal_commit_transaction()

We catch an assert problem in jbd2_journal_commit_transaction() when
doing fsstress and request falut injection tests. The problem is
happened in a race condition between jbd2_journal_commit_transaction()
and ext4_end_io_end(). Firstly, ext4_writepages() writeback dirty pages
and start reserved handle, and then the journal was aborted due to some
previous metadata IO error, jbd2_journal_abort() start to commit current
running transaction, the committing procedure could be raced by
ext4_end_io_end() and lead to subtract j_reserved_credits twice from
commit_transaction->t_outstanding_credits, finally the
t_outstanding_credits is mistakenly smaller than t_nr_buffers and
trigger assert.

kjournald2           kworker

jbd2_journal_commit_transaction()
 write_unlock(&journal->j_state_lock);
 atomic_sub(j_reserved_credits, t_outstanding_credits); //sub once

     	             jbd2_journal_start_reserved()
     	              start_this_handle()  //detect aborted journal
     	              jbd2_journal_free_reserved()  //get running transaction
                       read_lock(&journal->j_state_lock)
     	                __jbd2_journal_unreserve_handle()
     	               atomic_sub(j_reserved_credits, t_outstanding_credits);
                       //sub again
                       read_unlock(&journal->j_state_lock);

 journal->j_running_transaction = NULL;
 J_ASSERT(t_nr_buffers <= t_outstanding_credits) //bomb!!!

Fix this issue by using journal->j_state_lock to protect the subtraction
in jbd2_journal_commit_transaction().

Fixes: 96f1e0974575 ("jbd2: avoid long hold times of j_state_lock while committing a transaction")
Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220611130426.2013258-1-yi.zhang@huawei.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
index aa14f20241d7575d28216a5c459d7bbc30671828..7cb4f5de8155af30bef211a1afd1b6b5ec6df842 100644 (file)
--- a/fs/jbd2/commit.c
+++ b/fs/jbd2/commit.c
@@ -553,13 +553,13 @@ void jbd2_journal_commit_transaction(journal_t *journal)
         */
        jbd2_journal_switch_revoke_table(journal);
 
+       write_lock(&journal->j_state_lock);
        /*
         * Reserved credits cannot be claimed anymore, free them
         */
        atomic_sub(atomic_read(&journal->j_reserved_credits),
                   &commit_transaction->t_outstanding_credits);
 
-       write_lock(&journal->j_state_lock);
        trace_jbd2_commit_flushing(journal, commit_transaction);
        stats.run.rs_flushing = jiffies;
        stats.run.rs_locked = jbd2_time_diff(stats.run.rs_locked,