From: Zheng Liu <gnehzuil.liu@gmail.com>
Subject: Re: process hangs in ext4_sync_file
Date: Mon, 21 Oct 2013 20:57:58 +0800
Message-ID: <20131021125758.GA3253@gmail.com>
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To: Sandeep Joshi <sanjos100@gmail.com>
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Hi Sandeep,

On Mon, Oct 21, 2013 at 06:09:02PM +0530, Sandeep Joshi wrote:
> I am seeing a problem reported 4 years earlier
> https://lkml.org/lkml/2009/3/12/226
> (same stack as seen by Alexander)
> 
> The problem is reproducible.  Let me know if you need any info in
> addition to that seen below.
> 
> I have multiple threads in a process doing heavy IO on a ext4
> filesystem mounted with (discard, noatime) on a SSD or HDD.
> 
> This is on Linux 3.8.0-29-generic #42~precise1-Ubuntu SMP Wed Aug 14
> 16:19:23 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux
> 
> For upto minutes at a time, one of the threads seems to hang in sync to disk.
> 
> When I check the thread stack in /proc, I find that the stack is one
> of the following two
> 
> <ffffffff81134a4e>] sleep_on_page+0xe/0x20
> [<ffffffff81134c88>] wait_on_page_bit+0x78/0x80
> [<ffffffff81134d9c>] filemap_fdatawait_range+0x10c/0x1a0
> [<ffffffff811367d8>] filemap_write_and_wait_range+0x68/0x80
> [<ffffffff81236a4f>] ext4_sync_file+0x6f/0x2b0
> [<ffffffff811cba9b>] vfs_fsync+0x2b/0x40
> [<ffffffff81168fb3>] sys_msync+0x143/0x1d0
> [<ffffffff816fc8dd>] system_call_fastpath+0x1a/0x1f
> [<ffffffffffffffff>] 0xffffffffffffffff
> 
> 
> OR
> 
> 
> [<ffffffff812947f5>] jbd2_log_wait_commit+0xb5/0x130
> [<ffffffff81297213>] jbd2_complete_transaction+0x53/0x90
> [<ffffffff81236bcd>] ext4_sync_file+0x1ed/0x2b0
> [<ffffffff811cba9b>] vfs_fsync+0x2b/0x40
> [<ffffffff81168fb3>] sys_msync+0x143/0x1d0
> [<ffffffff816fc8dd>] system_call_fastpath+0x1a/0x1f
> [<ffffffffffffffff>] 0xffffffffffffffff
> 
> Any clues?

Thanks for reporting this.  Could you please try your test in latest
mainline kernel?  Further, could you please run the following command?
  'echo w >/proc/sysrq-trigger'
After running this command, system will dump all blocked tasks in dmesg.

Regards,
                                                - Zheng