Dave Chinner <david@xxxxxxxxxxxxx> 于2018年12月18日周二 上午7:33写道：
>
> On Sat, Dec 15, 2018 at 01:34:33PM +0800, 张本龙 wrote:
> > Hi Developpers and XFS,
> >
> > There seems to be a deadlock involving 3 threads: 1) the fsync thread
> > has acquired the project quota lock, and is trying to get the xfs_buf
> > (it's a an agf); 2) the xfs_buf is attached to a transaction, and
> > xfs_end_io is trying to get the xfs_inode ilock; 3) the write thread
> > has acquired the xfs_inode ilock, and tries to get the xfs_dquot.
> > Below are the traces.
>
> I don't see a deadlock here. What's holding the AGF lock and
> preventing progress from being made?
>
Oh, I was thinking the AGF is attached to a transaction. So between
xfs_trans_bjoin() and xfs_trans_commit(), a buf cannot be used by
others right? Then it should be released by xfs_end_io() in
xfs_trans_commit(), and the deadlock is like:
Thread 1 2
3
fsync()
dqlock P
agf lock
<blocks>
xfs_end_io
(agf locked by transaction)
ilock A
<blocks>
unlock agf in trans commit
write()
ilock A
dqlock P
<blocks>
> i.e. we have:
>
> process 1 2 3
> fsync()
> ilock A
> dqlock P
> agf lock
> <blocks>
> xfs_end_io
> ilock A
> <blocks>
> write()
> ilock B
> dqlock P
> <blocks>
>
> So, basically, everyhting is waiting for the AGF lock, which
> is held by something other than these three threads. When the AGF
> lock is relesaed, the fsync() will make progress, then release
> both dqlock P and ilock A and the other two threads will make
> progress again.
>
Absolutely possible. fsync() indeed acquired ilock in
xfs_iomap_write_allocate(). Yes the key is who holds AGF in this
scenario.
But in either guess it seems the AGF is being held by a transaction,
blocked in xfs_end_io() by ilock.
> > It's 3.10.0-514.16.1.el7.x86_64 kernel, met about 10-20 times a week
> > on several hundred of servers.
>
> That's not a mainline kernel and so we can't really help diagnose
> this much further. You should really report it to your distro
> support channels so they can help you with further diagnosis and
> fixes...
Oh oh sure, thank you for pointing out. We don't have support channels
since we use CentOS...
I really appreciate your reply!
>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@xxxxxxxxxxxxx
