linux-ext4 - Re: [BUG][dioread_nolock] blocked for more than 120s when we run xfstests #269

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Message-ID: <20130313105233.GB12012@gmail.com>
Date:	Wed, 13 Mar 2013 18:52:33 +0800
From:	Zheng Liu <gnehzuil.liu@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: [BUG][dioread_nolock] blocked for more than 120s when we run
 xfstests #269

On Wed, Mar 13, 2013 at 10:15:11AM +0100, Jan Kara wrote:
[snip]
> > > I post the sysrq-w output here.  But IMHO it is not very useful.  So I
> > > also post the sysrq-t output.
> >   Heh, curious. Thanks for the data. So worker thinks there's nothing to do
> > but inode has elevated i_ioend_count... Maybe we leaked ioend somewhere.
> > I'll check the code when I have time.
>   Ah, I think I see what's going on.
> a) Code in ext4_ext_direct_IO() is racy wrt iocb->private handling (that
>    can get cleared concurrently from ext4_end_io_dio()).

Thanks for tracing this problem.  But I am still confused that iocb is
allocated on stack in do_sync_write(), and is allocated from slab in
ioctx_alloc().  You mean iocb in ext4_ext_direct_IO and ext4_end_io_dio
is the same one?  Then this iocb could be changed concurrently, and we
are blocked for more than 120s.  I must miss something.

> b) ext4_end_io_dio() forgets to free the io_end if size == 0 (but this
>    shouldn't really happen looking into fs/direct_io.c).

Yes, we will return directly from do_blockdev_direct_IO().

Regards,
                                                - Zheng
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