linux-kernel - Re: [PATCH -v1] acpi: Fix possible recursive locking in hwregs.c

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Date:	Sat, 5 Nov 2011 22:50:29 +0600
From:	Rakib Mullick <rakib.mullick@...il.com>
To:	Lin Ming <ming.m.lin@...el.com>
Cc:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-acpi <linux-acpi@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Brown, Len" <len.brown@...el.com>,
	"Moore, Robert" <robert.moore@...el.com>
Subject: Re: [PATCH -v1] acpi: Fix possible recursive locking in hwregs.c

On Sat, Nov 5, 2011 at 8:50 PM, Lin Ming <ming.m.lin@...el.com> wrote:
> On Fri, 2011-11-04 at 13:53 +0800, Rakib Mullick wrote:
>> On Thu, Nov 3, 2011 at 11:40 PM, Srivatsa S. Bhat
>> <srivatsa.bhat@...ux.vnet.ibm.com> wrote:
>> > Hi,
>> >
>> > On 11/03/2011 05:32 PM, Lin Ming wrote:
>> >> On Thu, 2011-11-03 at 18:48 +0800, Rakib Mullick wrote:
>> >>> Calling pm-suspend might trigger a recursive lock in it's code path. In function acpi_hw_clear_acpi_status,
>> >>
>> >> As I replied at https://lkml.org/lkml/2011/9/22/6, I still don't think
>> >> there is a recursive lock.
>> >>
>> >
>> > At first look, it definitely doesn't look like a recursive lock, as Lin said.
>> > But, quoting Documentation/lockdep-design.txt:
>> >
>> > "Multi-lock dependency rules:
>> > ----------------------------
>> >
>> > The same lock-class must not be acquired twice, because this could lead
>> > to lock recursion deadlocks."
>> >
>> > So, Rakib, do the 2 locks belong to the same lock-class? If yes, then I think
>> > that is the reason for the lockdep splat. Could you show the lockdep warning?
>> >
>> Yes, same lock-class. And as per "Multi-lock dependency rules:", it
>> leads to lock recursion deadlocks.
>> Lockdep warning attached.
>>
>> > By the way, another way to look at this patch is as an optimization..
>> > i.e., if acpi_gbl_hardware_lock doesn't need to be held to call
>> > acpi_ev_walk_gpe_list(), then we can move from the coarse-grained locking
>> > to finer-grained locking by releasing it earlier, as you did in your patch.
>> > [Note that you will have to update the goto label also, i.e., rename it as
>> > 'exit' or something like that]
>> >
>> I can do it, thanks for suggestions. But, what does Lin thinks? Lin
>> are you okay?
>
> I'm OK.
>
Thanks. Then, I'll come up with a follow up patch includes the
suggestion from Srivatsa.

> We need to figure out why the dead lock happens.

I think its pretty clear from the lockdep warning, the reason of the
possible (since not happened with mainline kernel) dead lock.

> Could you also paste the patch which trigger this dead lock?
>
It's basically a modification of kernel scheduler, it does load
distribution differently. Since it's different approach and needs to
introduce properly. I wish I'll show it some day, if I can make some
fair improvement. Hope you understand.

Thanks,
Rakib
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