Re: [PATCH][RFC] jbd2: jbd2_journal_stop needs an exclusive control to synchronize around t_updates operations

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On Fri, Dec 16, 2011 at 7:19 PM, Toshiyuki Okajima
<toshi.okajima@xxxxxxxxxxxxxx> wrote:
> Hi.
>
> I found a bug by executing the following reproducer (needs tuning).
> (The reproducer never ends.)
>
> I wrote it for a confirmation of
>  "[PATCH 1/5] ext4: allocate delalloc blocks before changing journal mode".
> Therefore, without
>  "[PATCH 1/5] ext4: allocate delalloc blocks before changing journal mode"
> 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 hang, the reproducer stopped in 6
> hours.)
>
> Because jbd2_journal_lock_updates() never returns from schedule()
> after prepare_to_wait().
>
> The detailed explanation is as follows:
> ===============================================================================
> Current implementation of jbd2_journal_stop() has a bug not to synchronize
> t_updates with other operators, jbd2_journal_lock_updates(), start_this_handle()
IMHO, the description is not very exactly, the problem is not due to
desync of t_updates, t_updates is a var of atomic_t.   The reason is
that we have a critical code section here:[operation on t_updates,
wait or wakeup], this section should be synchronized.

> and jbd2_journal_commit_transaction().
>
> This bug was derived from commit: c35a56a090eacefca07afeb994029b57d8dd8025 and
>  commit: 8dd420466c7bfc459fa04680bd5690bfc41a4553.
>
> ---------------------------
> jbd2_journal_lock_updates()
> ---------------------------
>  509         write_lock(&journal->j_state_lock);
>  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);
It seems that t_handle_lock is used to protect the critical section
here.   I think t_handle is not needed here at all.   t_handle_lock
should be used to protect transaction and j_state_lock should be used
to protect journal.  prepare_to_wait operates on journal, so we should
remove t_handle_lock here.

>  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 operation of t_updates is protected by write_lock(&journal->j_state_lock)
>  and spin_lock(&transaction->t_handle_lock).
>
> -------------------
> start_this_handle()
> -------------------
>  160         read_lock(&journal->j_state_lock);
> ...
>  278         atomic_inc(&transaction->t_updates);
> ...
>  284         read_unlock(&journal->j_state_lock);
>
> * The operation of t_updates is protected by read_lock(&journal->j_state_lock).
>
> ---------------------------------
> jbd2_journal_commit_transaction()
> ---------------------------------
>  358         write_lock(&journal->j_state_lock);
> ...
>  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);
Also, t_handle_lock is used to protect the critical section.  I think
t_handle_lock is not needed here at all.

>  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 operation of t_updates is protected by write_lock(&journal->j_state_lock)
>  and spin_lock(&transaction->t_handle_lock).
>
> -------------------
> jbd2_journal_stop()
> -------------------
> ...
> 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 operation of t_updates is not protected.
>
> 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.
>
> After we run the reproducer, we can see the processes which become hang-up.
>
> -------------------------------------------------------------------------------
> 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 hang-up(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(journal->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 trans->t_updates with other
> operators.
> ===============================================================================
>
> Signed-off-by: Toshiyuki Okajima <toshi.okajima@xxxxxxxxxxxxxx>
> ---
>  fs/jbd2/transaction.c |    2 ++
>  1 files changed, 2 insertions(+), 0 deletions(-)
>
> diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
> index a0e41a4..d48f395 100644
> --- a/fs/jbd2/transaction.c
> +++ b/fs/jbd2/transaction.c
> @@ -1448,12 +1448,14 @@ int jbd2_journal_stop(handle_t *handle)
>         * once we do this, we must not dereference transaction
>         * pointer again.
>         */
> +       read_lock(&journal->j_state_lock);
>        tid = transaction->t_tid;
>        if (atomic_dec_and_test(&transaction->t_updates)) {
>                wake_up(&journal->j_wait_updates);
>                if (journal->j_barrier_count)
oops, j_barrier_count is not protected too in original code.  The
problem does not exists in patched code any more:-)


Yongqiang.
>                        wake_up(&journal->j_wait_transaction_locked);
>        }
> +       read_unlock(&journal->j_state_lock);
>
>        if (wait_for_commit)
>                err = jbd2_log_wait_commit(journal, tid);
> --
> 1.5.5.6
> --
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-- 
Best Wishes
Yongqiang Yang
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