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Date: Fri, 16 Dec 2011 20:22:15 +0800
From: Yongqiang Yang <xiaoqiangnk@...il.com>
To: Toshiyuki Okajima <toshi.okajima@...fujitsu.com>
Cc: tytso@....edu, adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org
Subject: Re: [PATCH][RFC] jbd2: jbd2_journal_stop needs an exclusive control
to synchronize around t_updates operations
On Fri, Dec 16, 2011 at 7:19 PM, Toshiyuki Okajima
<toshi.okajima@...fujitsu.com> 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@...fujitsu.com>
> ---
> 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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