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Re: [PATCH 6/8] xfs: Use generic writers counter instead of m_active_tra

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 6/8] xfs: Use generic writers counter instead of m_active_trans counter
From: Eric Sandeen <sandeen@xxxxxxxxxxx>
Date: Fri, 03 Feb 2012 20:13:12 -0600
Cc: Jan Kara <jack@xxxxxxx>, linux-fsdevel@xxxxxxxxxxxxxxx, Dave Chinner <dchinner@xxxxxxxxxx>, Surbhi Palande <csurbhi@xxxxxxxxx>, Kamal Mostafa <kamal@xxxxxxxxxxxxx>, Christoph Hellwig <hch@xxxxxxxxxxxxx>, LKML <linux-kernel@xxxxxxxxxxxxxxx>, xfs@xxxxxxxxxxx, linux-ext4@xxxxxxxxxxxxxxx
In-reply-to: <20120124080511.GN15102@dastard>
References: <1327091686-23177-1-git-send-email-jack@xxxxxxx> <1327091686-23177-7-git-send-email-jack@xxxxxxx> <20120124080511.GN15102@dastard>
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:10.0) Gecko/20120129 Thunderbird/10.0
On 1/24/12 2:05 AM, Dave Chinner wrote:
> On Fri, Jan 20, 2012 at 09:34:44PM +0100, Jan Kara wrote:
>> m_active_trans counter is racy wrt filesystem freezing. The patch replaces it
>> with generic counter of running transactions which is properly synchronized
>> with filesystem freezing. Things are a bit more complex because we need to 
>> log
>> a dummy transaction and free block counters after the filesystem is frozen so
>> we need to pass information to _xfs_trans_alloc() whether the transaction is
>> part of filesystem freezing or not.
>>
>> Signed-off-by: Jan Kara <jack@xxxxxxx>
>> ---
>>  fs/xfs/xfs_fsops.c |    5 +++--
>>  fs/xfs/xfs_fsops.h |    2 +-
>>  fs/xfs/xfs_iomap.c |    4 ++--
>>  fs/xfs/xfs_mount.c |    2 +-
>>  fs/xfs/xfs_mount.h |    2 --
>>  fs/xfs/xfs_super.c |    3 +--
>>  fs/xfs/xfs_sync.c  |   13 +++----------
>>  fs/xfs/xfs_trans.c |   19 ++++++++++++-------
>>  fs/xfs/xfs_trans.h |    3 ++-
>>  9 files changed, 25 insertions(+), 28 deletions(-)
>>
>> diff --git a/fs/xfs/xfs_fsops.c b/fs/xfs/xfs_fsops.c
>> index 1c6fdeb..503fdfa 100644
>> --- a/fs/xfs/xfs_fsops.c
>> +++ b/fs/xfs/xfs_fsops.c
>> @@ -645,12 +645,13 @@ out:
>>   */
>>  int
>>  xfs_fs_log_dummy(
>> -    xfs_mount_t     *mp)
>> +    xfs_mount_t     *mp,
>> +    bool            for_freeze)
> 
> What does "for_freeze" mean? If it is true, does it mean we are in a
> freeze or not in a freeze? I can't really tell from the code,
> because it just passed true or false, and in one case the code
> always passes false even though the code can be called after
> SB_FREEZE_WRITE is set (xfs_quiesce_data() via sync_filesystem())

Is that a problem?  This is all about the FREEZE_TRANS context right?
So whether we are under FREEZE_WRITE (during freeze_super) or not
(during i.e. sys_sync) I think it's ok, no?  See more below...

>>  #endif      /* __XFS_FSOPS_H__ */
>> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
>> index 9afa282..fd47f6e 100644
>> --- a/fs/xfs/xfs_iomap.c
>> +++ b/fs/xfs/xfs_iomap.c
>> @@ -679,8 +679,8 @@ xfs_iomap_write_unwritten(
>>               * the same inode that we complete here and might deadlock
>>               * on the iolock.
>>               */
>> -            xfs_wait_for_freeze(mp, SB_FREEZE_TRANS);
>> -            tp = _xfs_trans_alloc(mp, XFS_TRANS_STRAT_WRITE, KM_NOFS);
>> +            tp = _xfs_trans_alloc(mp, XFS_TRANS_STRAT_WRITE,
>> +                            KM_NOFS, false);
> 
> This is a documentation regression - the code was clearly self
> documenting w.r.t. frozen filesystem behaviour. It isn't anymore.
> 
> I'd suggest that we need:
> 
> #define XFS_WAIT_FOR_FREEZE   false
> #define XFS_IGNORE_FROZEN_SB  true

>From my reading those are confusing as well.  What the flag controls is:

false: do sb_start_write(SB_FREEZE_TRANS) in trans alloc (will call 
sb_stop_write in trans free)
true: skip sb_start_write & set flag to skip sb_stop_write(SB_FREEZE_TRANS) in 
trans free

so maybe:

#define XFS_HONOR_FREEZE_TRANS  false
#define XFS_IGNORE_FREEZE_TRANS true

would be a little clearer?  Or maybe:

#define XFS_INC_FREEZE_TRANS    false
#define XFS_NOINC_FREEZE_TRANS  true

or

#define XFS_TRANS_START_WRITE           false
#define XFS_NO_TRANS_START_WRITE        true

bleah, ok, step away from the bike shed....

> as the parameters here to makeit extremely clear when reading the
> code exactly what that last parameter means. i.e. it is self
> documenting. That will help clear up a lot of the confusion on what
> these magic boolean parameters are supposed to mean....
> 
>> @@ -312,7 +311,6 @@ void xfs_do_force_shutdown(struct xfs_mount *mp, int 
>> flags, char *fname,
>>  #define SHUTDOWN_DEVICE_REQ 0x0020  /* failed all paths to the device */
>>  
>>  #define xfs_test_for_freeze(mp)             ((mp)->m_super->s_frozen)
> 
> I'd remove this, too, and just open code it.
> 
>> diff --git a/fs/xfs/xfs_sync.c b/fs/xfs/xfs_sync.c
>> index aa3dc1a..24f4d7c 100644
>> --- a/fs/xfs/xfs_sync.c
>> +++ b/fs/xfs/xfs_sync.c
>> @@ -373,7 +373,7 @@ xfs_quiesce_data(
>>  
>>      /* mark the log as covered if needed */
>>      if (xfs_log_need_covered(mp))
>> -            error2 = xfs_fs_log_dummy(mp);
>> +            error2 = xfs_fs_log_dummy(mp, false);
> 
> This is the call that can occur inside SB_FREEZE_WRITE context as
> well as outside it.

Somehow I'm missing the problem here.  This basically means that we will
always increment the metadata writer count for the new transaction, and drop
it when done.  But I _think_ that's ok in both spots, no?  Neither is called
inside of a FREEZE_TRANS.

-Eric


>>  
>>      /* flush data-only devices */
>>      if (mp->m_rtdev_targp)
>> @@ -421,18 +421,11 @@ xfs_quiesce_attr(
>>      int     error = 0;
>>  
>>      /* wait for all modifications to complete */
>> -    while (atomic_read(&mp->m_active_trans) > 0)
>> -            delay(100);
>> +    sb_wait_write(mp->m_super, SB_FREEZE_TRANS);
>>  
>>      /* flush inodes and push all remaining buffers out to disk */
>>      xfs_quiesce_fs(mp);
>>  
>> -    /*
>> -     * Just warn here till VFS can correctly support
>> -     * read-only remount without racing.
>> -     */
>> -    WARN_ON(atomic_read(&mp->m_active_trans) != 0);
>> -
> 
> Now there's an interesting question. Does this break read-only
> remount?
> 
> /me checks the sb_wait_write() code
> 
> No, it looks like it should be fine.
> 
>>      /* Push the superblock and write an unmount record */
>>      error = xfs_log_sbcount(mp);
>>      if (error)
>> @@ -467,7 +460,7 @@ xfs_sync_worker(
>>              /* dgc: errors ignored here */
>>              if (mp->m_super->s_frozen == SB_UNFROZEN &&
>>                  xfs_log_need_covered(mp))
>> -                    error = xfs_fs_log_dummy(mp);
>> +                    error = xfs_fs_log_dummy(mp, false);
>>              else
>>                      xfs_log_force(mp, 0);
>>              error = xfs_qm_sync(mp, SYNC_TRYLOCK);
>> diff --git a/fs/xfs/xfs_trans.c b/fs/xfs/xfs_trans.c
>> index 1f35b2f..e97014b 100644
>> --- a/fs/xfs/xfs_trans.c
>> +++ b/fs/xfs/xfs_trans.c
>> @@ -577,24 +577,28 @@ xfs_trans_alloc(
>>      xfs_mount_t     *mp,
>>      uint            type)
>>  {
>> -    xfs_wait_for_freeze(mp, SB_FREEZE_TRANS);
>> -    return _xfs_trans_alloc(mp, type, KM_SLEEP);
>> +    return _xfs_trans_alloc(mp, type, KM_SLEEP, false);
>>  }
>>  
>>  xfs_trans_t *
>>  _xfs_trans_alloc(
>>      xfs_mount_t     *mp,
>>      uint            type,
>> -    uint            memflags)
>> +    uint            memflags,
>> +    bool            freezing)
>>  {
>>      xfs_trans_t     *tp;
>>  
>> -    atomic_inc(&mp->m_active_trans);
>> -
>> +    if (!freezing)
>> +            sb_start_write(mp->m_super, SB_FREEZE_TRANS);
>> +    else
>> +            WARN_ON(xfs_test_for_freeze(mp) != SB_FREEZE_TRANS);
> 
> Just open code xfs_test_for_freeze() and add a line of whitespace
> after this.
> 
>>      tp = kmem_zone_zalloc(xfs_trans_zone, memflags);
>>      tp->t_magic = XFS_TRANS_MAGIC;
>>      tp->t_type = type;
>>      tp->t_mountp = mp;
>> +    if (freezing)
>> +            tp->t_flags |= XFS_TRANS_FREEZING;
> 
> Simply assign the value - tp->t_flags is guaranteed to be 0 right
> now.
> 
>>      INIT_LIST_HEAD(&tp->t_items);
>>      INIT_LIST_HEAD(&tp->t_busy);
>>      return tp;
>> @@ -611,7 +615,8 @@ xfs_trans_free(
>>      xfs_alloc_busy_sort(&tp->t_busy);
>>      xfs_alloc_busy_clear(tp->t_mountp, &tp->t_busy, false);
>>  
>> -    atomic_dec(&tp->t_mountp->m_active_trans);
>> +    if (!(tp->t_flags & XFS_TRANS_FREEZING))
>> +            sb_end_write(tp->t_mountp->m_super, SB_FREEZE_TRANS);
>>      xfs_trans_free_dqinfo(tp);
>>      kmem_zone_free(xfs_trans_zone, tp);
>>  }
>> @@ -654,7 +659,7 @@ xfs_trans_dup(
>>  
>>      xfs_trans_dup_dqinfo(tp, ntp);
>>  
>> -    atomic_inc(&tp->t_mountp->m_active_trans);
>> +    sb_dup_write(tp->t_mountp->m_super, SB_FREEZE_TRANS);
> 
> That's strangly named. Isn't the normal thing to do here use a "__"
> prefix for operations that just need an extra reference because they
> already have one (i.e. __sb_start_write())?
> 
> This also looks broken with repsect to the new XFS_TRANS_FREEZING
> flag. If it is set on the parent, it needs to be set on the
> duplicated transaction. And if it is set, then no extra reference
> should be taken.
> 
> Cheers,
> 
> Dave.

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