[PATCH 0/2 take2][RFC] hangup in jbd2_journal_lock_updates

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Hi.

I found a bug by executing the following reproducer.  The reproducer is for a 
confirmation of 
 "[PATCH 1/5] ext4: allocate delalloc blocks before changing journal mode".
Therefore, without its patch, the other problem (panic) which the patch 
describes can happen more frequently:
-------------------------------------------------------------------------------
#!/bin/sh

date
LOOP=100000
dd if=/dev/zero of=/tmp/ext4.img bs=1k count=1000k
/sbin/mkfs.ext4 -Fq /tmp/ext4.img
mount -o loop /tmp/ext4.img /mnt
rm -f /mnt/file
echo "0" > /mnt/file
(while [ 1 ]; do dd if=/dev/zero of=/mnt/file2 bs=4k > /dev/null 2>&1; done) &
PID=$!
for ((i=0; i < LOOP; i++));
do
        DATE=$(date)
        echo -n "[LOOP $i] $DATE"
        if ((i%2 == 0));
        then
                chattr +j /mnt/file
        else
                chattr -j /mnt/file
        fi
        echo "0" >> /mnt/file
done
kill -9 $PID
rm -f /mnt/file*
umount /mnt
exit 0
-------------------------------------------------------------------------------
Though I ran it, it never ended forever.
(At one of my tries to reproduce the hangup, the reproducer stopped in 6
hours.)

Because jbd2_journal_lock_updates() never returns from schedule() after 
prepare_to_wait(). 
(Its filesystem becomes a hangup.)

The detailed explanation is as follows:
===============================================================================
Current implementation of jbd2_journal_stop() has a bug not to synchronize 
a critical code section of it with others,
 - jbd2_journal_lock_updates()
 - start_this_handle()
 - jbd2_journal_commit_transaction().

This bug was derived from commit: c35a56a090eacefca07afeb994029b57d8dd8025 and
 commit: 8dd420466c7bfc459fa04680bd5690bfc41a4553.

The source codes and critical code sections of them are as follows:
---------------------------
jbd2_journal_lock_updates()
---------------------------
 509         write_lock(&journal->j_state_lock);
--- critical code section ------------------------------------------------------
 510         ++journal->j_barrier_count;
...
 513         while (1) {
...
 519                 spin_lock(&transaction->t_handle_lock);
 520                 if (!atomic_read(&transaction->t_updates)) {
 521                         spin_unlock(&transaction->t_handle_lock);
 522                         break;
 523                 }
 524                 prepare_to_wait(&journal->j_wait_updates, &wait,
 525                                 TASK_UNINTERRUPTIBLE);
 526                 spin_unlock(&transaction->t_handle_lock);
 527                 write_unlock(&journal->j_state_lock);
 528                 schedule();
 529                 finish_wait(&journal->j_wait_updates, &wait);
 530                 write_lock(&journal->j_state_lock);
 531         }
-------------------------------------------------------------------------------
 532         write_unlock(&journal->j_state_lock);

* The critical code section is from line 510 to line 531
     referencer) protected by write_lock(j_state_lock) and 
	spin_lock(t_handle_lock)

-------------------
start_this_handle()
-------------------
 160         read_lock(&journal->j_state_lock);
--- critical code section ------------------------------------------------------
...
 278         atomic_inc(&transaction->t_updates);
...
-------------------------------------------------------------------------------
 284         read_unlock(&journal->j_state_lock);

* The critical code sections is in line 278 protected by read_lock(j_state_lock)

---------------------------------
jbd2_journal_commit_transaction()
---------------------------------
 358         write_lock(&journal->j_state_lock);
--- critical code section ------------------------------------------------------
...
 367         spin_lock(&commit_transaction->t_handle_lock);
 368         while (atomic_read(&commit_transaction->t_updates)) {
 369                 DEFINE_WAIT(wait);
 370 
 371                 prepare_to_wait(&journal->j_wait_updates, &wait,
 372                                         TASK_UNINTERRUPTIBLE);
 373                 if (atomic_read(&commit_transaction->t_updates)) {
 374                         spin_unlock(&commit_transaction->t_handle_lock);
 375                         write_unlock(&journal->j_state_lock);
 376                         schedule();
 377                         write_lock(&journal->j_state_lock);
 378                         spin_lock(&commit_transaction->t_handle_lock);
 379                 }
 380                 finish_wait(&journal->j_wait_updates, &wait);
 381         }
 382         spin_unlock(&commit_transaction->t_handle_lock);
...
 447         wake_up(&journal->j_wait_transaction_locked);
-------------------------------------------------------------------------------
 448         write_unlock(&journal->j_state_lock);

* The critical code sections is from line 367 to line 447 protected by 
  write_lock(&journal->j_state_lock) and spin_lock(&transaction->t_handle_lock).

-------------------
jbd2_journal_stop()
-------------------
...
--- critical code section ------------------------------------------------------
1452         if (atomic_dec_and_test(&transaction->t_updates)) {
1453                 wake_up(&journal->j_wait_updates);
1454                 if (journal->j_barrier_count)
1455                         wake_up(&journal->j_wait_transaction_locked);
1456         }
-------------------------------------------------------------------------------
1457 
1458         if (wait_for_commit)
1459                 err = jbd2_log_wait_commit(journal, tid);
1460 
1461         lock_map_release(&handle->h_lockdep_map);
1462 
1463         jbd2_free_handle(handle);
1464         return err;
1465 }

* The critical code sections is from line 1452 to line 1456. However,
 it isn't protected by no exclusive control.

Therefore, 
- jbd2_journal_lock_updates() sleeps if transaction->t_update is 0.
- jbd2_journal_stop() tries to wake someone up 
(wake_up(&journal->j_wait_updates)) after transaction->t_updates becomes 0. 
=> But jbd2_journal_lock_updates() isn't woken up and then sleeps forever,
   if it has not yet slept completely (before executing schedule()).

After we run the reproducer, we can see the processes which become hangup:
-------------------------------------------------------------------------------
crash> ps | grep UN
  16645  23737   1  ffff88007a592b20  UN   0.0  105136    508  dd
  16684  23727   1  ffff88007a2a9480  UN   0.0  107152    512  chattr
  23722      2   1  ffff88007b496080  UN   0.0       0      0  [flush-7:0]

[1]
crash> bt 16645
PID: 16645  TASK: ffff88007a592b20  CPU: 1   COMMAND: "dd"
 #0 [ffff88007b575878] __schedule at ffffffff814b9b77
 #1 [ffff88007b575920] schedule at ffffffff814ba3cf
 #2 [ffff88007b575930] start_this_handle at ffffffffa01acef5 [jbd2]
 #3 [ffff88007b5759f0] jbd2__journal_start at ffffffffa01ad4cb [jbd2]
 #4 [ffff88007b575a30] jbd2_journal_start at ffffffffa01ad523 [jbd2]
 #5 [ffff88007b575a40] ext4_journal_start_sb at ffffffffa03a7d83 [ext4]
 #6 [ffff88007b575ab0] ext4_da_write_begin at ffffffffa038ddce [ext4]
 #7 [ffff88007b575b50] generic_file_buffered_write at ffffffff811005ce
 #8 [ffff88007b575c10] __generic_file_aio_write at ffffffff81103418
 #9 [ffff88007b575cd0] generic_file_aio_write at ffffffff811036b6
#10 [ffff88007b575d50] ext4_file_write at ffffffffa0386ee9 [ext4]
#11 [ffff88007b575de0] do_sync_write at ffffffff8115de52
#12 [ffff88007b575f00] vfs_write at ffffffff8115e3e8
#13 [ffff88007b575f30] sys_write at ffffffff8115e5b1
#14 [ffff88007b575f80] system_call_fastpath at ffffffff814c3f42
    RIP: 00000034930d4230  RSP: 00007fffd8176980  RFLAGS: 00000206
    RAX: 0000000000000001  RBX: ffffffff814c3f42  RCX: 0000000000000010
    RDX: 0000000000001000  RSI: 0000000001ac6000  RDI: 0000000000000001
    RBP: 0000000001ac6000   R8: 0000000000000003   R9: 0000000000040000
    R10: 0000000000003003  R11: 0000000000000246  R12: 0000000001ac5fff
    R13: 0000000000000000  R14: 0000000000001000  R15: 0000000000000246
    ORIG_RAX: 0000000000000001  CS: 0033  SS: 002b

[2]
crash> bt 16684
PID: 16684  TASK: ffff88007a2a9480  CPU: 1   COMMAND: "chattr"
 #0 [ffff88007af87ca8] __schedule at ffffffff814b9b77
 #1 [ffff88007af87d50] schedule at ffffffff814ba3cf
 #2 [ffff88007af87d60] jbd2_journal_lock_updates at ffffffffa01acca6 [jbd2]
 #3 [ffff88007af87de0] ext4_change_inode_journal_flag at ffffffffa038faf1 [ext4]
 #4 [ffff88007af87e10] ext4_ioctl at ffffffffa0392f48 [ext4]
 #5 [ffff88007af87ea0] do_vfs_ioctl at ffffffff8116fb8a
 #6 [ffff88007af87f30] sys_ioctl at ffffffff811700d1
 #7 [ffff88007af87f80] system_call_fastpath at ffffffff814c3f42
    RIP: 00000034930d95f7  RSP: 00007fff8830e0d8  RFLAGS: 00010213
    RAX: 0000000000000010  RBX: ffffffff814c3f42  RCX: 00000034930d95f7
    RDX: 00007fff8830e17c  RSI: 0000000040086602  RDI: 0000000000000003
    RBP: 0000000000084000   R8: 0000000000000000   R9: 0000000000000001
    R10: 0000000000000000  R11: 0000000000000246  R12: 00007fff8830f8db
    R13: 0000000000000001  R14: 0000000000000000  R15: 0000000000000003
    ORIG_RAX: 0000000000000010  CS: 0033  SS: 002b

[3]
crash> bt 23722
PID: 23722  TASK: ffff88007b496080  CPU: 1   COMMAND: "flush-7:0"
 #0 [ffff88007b4f7840] __schedule at ffffffff814b9b77
 #1 [ffff88007b4f78e8] schedule at ffffffff814ba3cf
 #2 [ffff88007b4f78f8] start_this_handle at ffffffffa01acef5 [jbd2]
 #3 [ffff88007b4f79b8] jbd2__journal_start at ffffffffa01ad4cb [jbd2]
 #4 [ffff88007b4f79f8] jbd2_journal_start at ffffffffa01ad523 [jbd2]
 #5 [ffff88007b4f7a08] ext4_journal_start_sb at ffffffffa03a7d83 [ext4]
 #6 [ffff88007b4f7a78] ext4_da_writepages at ffffffffa0390b6d [ext4]
 #7 [ffff88007b4f7bc8] do_writepages at ffffffff8110c261
 #8 [ffff88007b4f7bd8] writeback_single_inode at ffffffff81185830
 #9 [ffff88007b4f7c38] writeback_sb_inodes at ffffffff81185e21
#10 [ffff88007b4f7ce8] __writeback_inodes_wb at ffffffff81185f7e
#11 [ffff88007b4f7d38] wb_writeback at ffffffff81186223
#12 [ffff88007b4f7db8] wb_do_writeback at ffffffff811864f9
#13 [ffff88007b4f7e58] bdi_writeback_thread at ffffffff811865fa
#14 [ffff88007b4f7ee8] kthread at ffffffff810828c6
#15 [ffff88007b4f7f48] kernel_thread_helper at ffffffff814c60b4

Step to reproduce a hangup(figure):
 =======================================+=======================================
 ( "chattr -j" process PID: 16684 )     |  ( a certain process )
t ======================================+=======================================
i                                       |
m                                       +---------------------------------------
e                                       |jbd2_journal_start()
|                                       +---------------------------------------
|                                       | . . .
|                                       | t_updates++ // t_updates = 1
|                                       + . . .
|                                       +---------------------------------------
|                                        ...
|---------------------------------------+---------------------------------------
|jbd2_journal_lock_updates()            |jbd2_journal_stop()
|---------------------------------------+---------------------------------------
| write_lock(&journal->j_state_lock)    |    .
| ++journal->j_barrier_count            |    .
| spin_lock(&tran->t_handle_lock)       |    .
| atomic_read(&tran->t_updates) //not 0 |
|                                       | atomic_dec_and_test(&tran->t_updates)
|                                       |    // t_updates = 0
|                                       | wake_up(&journal->j_wait_updates)
| prepare_to_wait()                     |    // no process is woken up.
| spin_unlock(&tran->t_handle_lock)     + wake_up(&journal->j_wait_tran_locked)
| write_unlock(&journal->j_state_lock)  |    // no process is woken up.
| schedule() // never return            +---------------------------------------
|                                       |  ( "dd" PID: 16645 or "flush-7:0" )
|                                       +=======================================
|                                       |start_this_handle()
|                                       +---------------------------------------
|                                       | . . .
|                                       | read_lock(&journal->j_state_lock)
|                                       | if (journal->j_barrier_count)
|                                       |  read_unlock(&journal->j_state_lock)
|                                       |  wait_event(jrnl->j_wait_tran_locked)
v                                       |  // never return

Therefore, jbd2_journal_stop() needs spin_lock(&tran->t_handle_lock) or 
read_lock(&journal->j_state_lock) to synchronize its critical code section with 
others.  Besides, I noticed that t_handle_lock exclusive control had already 
been unnecessary when I examined all the functions which used t_handle_lock. 
So, read_lock(&journal->j_state_lock) exclusive control is good for this fix.

Regards,
Toshiyuki Okajima
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