linux-kernel - Re: [REGRESSION] [PATCH] block: uninitialized ioc->nr_tasks triggers WARN

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Date:	Wed, 01 Aug 2012 12:16:04 +0200
From:	Jens Axboe <axboe@...nel.dk>
To:	Olof Johansson <olof@...om.net>
CC:	tj@...nel.org, linux-kernel@...r.kernel.org, vgoyal@...hat.com,
	torvalds@...ux-foundation.org
Subject: Re: [REGRESSION] [PATCH] block: uninitialized ioc->nr_tasks triggers
 WARN_ON

On 07/17/2012 11:36 PM, Olof Johansson wrote:
> Hi,
> 
> I'm using the old-fashioned 'dump' backup tool, and I noticed that it spews the
> below warning as of 3.5-rc1 and later (3.4 is fine):
> 
> [   10.886893] ------------[ cut here ]------------
> [   10.886904] WARNING: at include/linux/iocontext.h:140 copy_process+0x1488/0x1560()
> [   10.886905] Hardware name: Bochs
> [   10.886906] Modules linked in:
> [   10.886908] Pid: 2430, comm: dump Not tainted 3.5.0-rc7+ #27
> [   10.886908] Call Trace:
> [   10.886911]  [<ffffffff8107ce8a>] warn_slowpath_common+0x7a/0xb0
> [   10.886912]  [<ffffffff8107ced5>] warn_slowpath_null+0x15/0x20
> [   10.886913]  [<ffffffff8107c088>] copy_process+0x1488/0x1560
> [   10.886914]  [<ffffffff8107c244>] do_fork+0xb4/0x340
> [   10.886918]  [<ffffffff8108effa>] ? recalc_sigpending+0x1a/0x50
> [   10.886919]  [<ffffffff8108f6b2>] ? __set_task_blocked+0x32/0x80
> [   10.886920]  [<ffffffff81091afa>] ? __set_current_blocked+0x3a/0x60
> [   10.886923]  [<ffffffff81051db3>] sys_clone+0x23/0x30
> [   10.886925]  [<ffffffff8179bd73>] stub_clone+0x13/0x20
> [   10.886927]  [<ffffffff8179baa2>] ? system_call_fastpath+0x16/0x1b
> [   10.886928] ---[ end trace 32a14af7ee6a590b ]---
> 
> Reproducing is easy, I can hit it on a KVM system with a very basic
> config (x86_64 make defconfig + enable the drivers needed). To hit it,
> just install dump (on debian/ubuntu, not sure what the package might be
> called on Fedora), and:
> 
> dump -o -f /tmp/foo /
> 
> You'll see the warning in dmesg once it forks off the I/O process and
> starts dumping filesystem contents.
> 
> I bisected it down to the following commit:
> 
> commit f6e8d01bee036460e03bd4f6a79d014f98ba712e
> Author: Tejun Heo <tj@...nel.org>
> Date:   Mon Mar 5 13:15:26 2012 -0800
> 
>     block: add io_context->active_ref
>     
>     Currently ioc->nr_tasks is used to decide two things - whether an ioc
>     is done issuing IOs and whether it's shared by multiple tasks.  This
>     patch separate out the first into ioc->active_ref, which is acquired
>     and released using {get|put}_io_context_active() respectively.
>     
>     This will be used to associate bio's with a given task.  This patch
>     doesn't introduce any visible behavior change.
>     
>     Signed-off-by: Tejun Heo <tj@...nel.org>
>     Cc: Vivek Goyal <vgoyal@...hat.com>
>     Signed-off-by: Jens Axboe <axboe@...nel.dk>
> 
> 
> It seems like the init of ioc->nr_tasks was removed in that patch,
> so it starts out at 0 instead of 1.
> 
> Tejun, is the right thing here to add back the init, or should something else
> be done?
> 
> The below patch removes the warning, but I haven't done any more extensive
> testing on it.
> 
> 
> Signed-off-by: Olof Johansson <olof@...om.net>
> 
> diff --git a/block/blk-ioc.c b/block/blk-ioc.c
> index 893b800..fab4cdd 100644
> --- a/block/blk-ioc.c
> +++ b/block/blk-ioc.c
> @@ -244,6 +244,7 @@ int create_task_io_context(struct task_struct *task, gfp_t gfp_flags, int node)
>  
>  	/* initialize */
>  	atomic_long_set(&ioc->refcount, 1);
> +	atomic_set(&ioc->nr_tasks, 1);
>  	atomic_set(&ioc->active_ref, 1);
>  	spin_lock_init(&ioc->lock);
>  	INIT_RADIX_TREE(&ioc->icq_tree, GFP_ATOMIC | __GFP_HIGH);

Thanks Olof, analysis (and patch) look correct. Applied for 3.6.

-- 
Jens Axboe

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