linux-kernel - Re: processes stuck in llseek

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Date:	Tue, 16 Aug 2011 08:51:18 -0400
From:	Chris Mason <chris.mason@...cle.com>
To:	Dan Merillat <dan.merillat@...il.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	BTRFS Mailing list <linux-btrfs@...r.kernel.org>
Subject: Re: processes stuck in llseek

Excerpts from Dan Merillat's message of 2011-08-15 23:59:50 -0400:
> I noticed a series of hung_task notifications in dmesg, so I went
> poking at it.   Process is 'dropbox', and it's stuck trying to llseek
> it's library.zip file.
> 
> strace of dropbox:
> ...
> stat("/home/x/.dropbox-dist/library.zip", {st_mode=S_IFREG|0755,
> st_size=11575179, ...}) = 0
> open("/home/x/.dropbox-dist/library.zip", O_RDONLY) = 3
> fstat(3, {st_mode=S_IFREG|0755, st_size=11575179, ...}) = 0
> mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1,
> 0) = 0x7fa766ea8000
> fstat(3, {st_mode=S_IFREG|0755, st_size=11575179, ...}) = 0
> lseek(3, 11571200, SEEK_SET
> 
> SEEK_SET is less than st_size
> 
> strace of dd if=library.zip of=/dev/null bs=1 seek=11571200:
> 
> open("library.zip", O_RDONLY)           = 3
> dup2(3, 0)                              = 0
> close(3)                                = 0
> lseek(0, 0, SEEK_CUR
> 
> [72960.716080] INFO: task dropbox:1348 blocked for more than 120 seconds.
> [72960.716084] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs"
> disables this message.
> [72960.716087] dropbox         D ffff8800762d8a78     0  1348      1 0x00000004
> [72960.716092]  ffff880069cede18 0000000000000086 0000000000096000
> 0000000000000000
> [72960.716097]  ffff880069cec000 00000000000119c0 ffff8800762d8700
> 00000000000119c0
> [72960.716101]  ffff880069cedfd8 0000000000004000 ffff880069cedfd8
> 00000000000119c0
> [72960.716106] Call Trace:
> [72960.716113]  [<ffffffff81331f9e>] ? trace_hardirqs_on_thunk+0x3a/0x3c
> [72960.716119]  [<ffffffff8155710a>] ? retint_restore_args+0xe/0xe
> [72960.716124]  [<ffffffff810e8ef1>] ? noop_llseek+0xa/0xa
> [72960.716129]  [<ffffffff810340ff>] ? mutex_spin_on_owner+0x1c/0x45
> [72960.716133]  [<ffffffff81555be0>] __mutex_lock_slowpath+0xd2/0x116
> [72960.716137]  [<ffffffff81559edd>] ? do_page_fault+0x374/0x3e6
> [72960.716140]  [<ffffffff81555a87>] mutex_lock+0x16/0x27
> [72960.716146]  [<ffffffff812c8ad7>] btrfs_file_llseek+0x38/0x297
> [72960.716150]  [<ffffffff81331fda>] ? trace_hardirqs_off_thunk+0x3a/0x6c
> [72960.716153]  [<ffffffff810e8f2c>] vfs_llseek+0x2e/0x30
> [72960.716155]  [<ffffffff810e9311>] sys_lseek+0x3e/0x5d
> [72960.716159]  [<ffffffff8155d4fb>] system_call_fastpath+0x16/0x1b
> 
> Oddly enough, it's just llseek.  I can copy/read the file sequentially
> just fine, but llseek fails on a copy as well.   Once I get physically
> to the system I'll recompile to an unmodified kernel and try again.
> Other files of the same approximate age llseek correctly.
> 
> Kernel is 3.1-rc1  with the fglrx module from AMD and the following
> patch:  (I was playing with cross-subvolume reflinking, but not on
> this file)

Dan Carpenter sent a patch for this, I'll get it queued up for rc3.

-chris
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