[PATCH 3/4] jbd: drop checkpoint mutex when waiting in __log_wait_for_space()

[Paul Gortmaker <paul.gortmaker@windriver.com>]

Backport of jbd2 commit 0ef54180e0187117062939202b96faf04c8673bc
("jbd2: drop checkpoint mutex when waiting in __jbd2_log_wait_for_space()")

Quoting that commit:
 --------------
 While trying to debug an an issue under extreme I/O loading
 on preempt-rt kernels, the following backtrace was observed
 via SysRQ output:

 rm              D ffff8802203afbc0  4600  4878   4748 0x00000000
  ffff8802217bfb78 0000000000000082 ffff88021fc2bb80 ffff88021fc2bb80
  ffff88021fc2bb80 ffff8802217bffd8 ffff8802217bffd8 ffff8802217bffd8
  ffff88021f1d4c80 ffff88021fc2bb80 ffff8802217bfb88 ffff88022437b000
 Call Trace:
  [<ffffffff8172dc34>] schedule+0x24/0x70
  [<ffffffff81225b5d>] jbd2_log_wait_commit+0xbd/0x140
  [<ffffffff81060390>] ? __init_waitqueue_head+0x50/0x50
  [<ffffffff81223635>] jbd2_log_do_checkpoint+0xf5/0x520
  [<ffffffff81223b09>] __jbd2_log_wait_for_space+0xa9/0x1f0
  [<ffffffff8121dc40>] start_this_handle.isra.10+0x2e0/0x530
  [<ffffffff81060390>] ? __init_waitqueue_head+0x50/0x50
  [<ffffffff8121e0a3>] jbd2__journal_start+0xc3/0x110
  [<ffffffff811de7ce>] ? ext4_rmdir+0x6e/0x230
  [<ffffffff8121e0fe>] jbd2_journal_start+0xe/0x10
  [<ffffffff811f308b>] ext4_journal_start_sb+0x5b/0x160
  [<ffffffff811de7ce>] ext4_rmdir+0x6e/0x230
  [<ffffffff811435c5>] vfs_rmdir+0xd5/0x140
  [<ffffffff8114370f>] do_rmdir+0xdf/0x120
  [<ffffffff8105c6b4>] ? task_work_run+0x44/0x80
  [<ffffffff81002889>] ? do_notify_resume+0x89/0x100
  [<ffffffff817361ae>] ? int_signal+0x12/0x17
  [<ffffffff81145d85>] sys_unlinkat+0x25/0x40
  [<ffffffff81735f22>] system_call_fastpath+0x16/0x1b

 What is interesting here, is that we call log_wait_commit, from
 within wait_for_space, but we are still holding the checkpoint_mutex
 as it surrounds mostly the whole of wait_for_space.  And then, as we
 are waiting, journal_commit_transaction can run, and if the JBD2_FLUSHED
 bit is set, then we will also try to take the same checkpoint_mutex.

 It seems that we need to drop the checkpoint_mutex while sitting in
 jbd2_log_wait_commit, if we want to guarantee that progress can be made
 by jbd2_journal_commit_transaction().  There does not seem to be
 anything preempt-rt specific about this, other then perhaps increasing
 the odds of it happening.
 --------------

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
---
 fs/jbd/checkpoint.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/fs/jbd/checkpoint.c b/fs/jbd/checkpoint.c
index 08c0304..8e8e0ee 100644
--- a/fs/jbd/checkpoint.c
+++ b/fs/jbd/checkpoint.c
@@ -160,7 +160,15 @@ void __log_wait_for_space(journal_t *journal)
 				/* We were able to recover space; yay! */
 				;
 			} else if (tid) {
+				/*
+				 * jbd_journal_commit_transaction() may want
+				 * to take the checkpoint_mutex if JBD_FLUSHED
+				 * is set.  So we need to temporarily drop it.
+				 */
+				mutex_unlock(&journal->j_checkpoint_mutex);
 				log_wait_commit(journal, tid);
+				spin_lock(&journal->j_state_lock);
+				continue;
 			} else {
 				printk(KERN_ERR "%s: needed %d blocks and "
 				       "only had %d space available\n",
-- 
1.8.1.2