linux-kernel - Re: [PATCH] driver core / ACPI: Avoid device removal locking problems

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Date:	Wed, 28 Aug 2013 15:24:45 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Tejun Heo <tj@...nel.org>
Cc:	Gu Zheng <guz.fnst@...fujitsu.com>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Toshi Kani <toshi.kani@...com>,
	LKML <linux-kernel@...r.kernel.org>,
	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
Subject: Re: [PATCH] driver core / ACPI: Avoid device removal locking problems

On Wednesday, August 28, 2013 08:24:22 AM Tejun Heo wrote:
> Hello, Rafael.
> 
> On Tue, Aug 27, 2013 at 11:45:46PM +0200, Rafael J. Wysocki wrote:
> > I've thought about that a bit over the last several hours and I'm still
> > thinking that that patch is a bit overkill, because it will trigger the
> > restart_syscall() for all cases when device_hotplug_lock is locked, even
> > if they can't lead to any deadlocks.  The only deadlockish situation is
> > when device *removal* is in progress when store_online(), for example,
> > is called.
> > 
> > So to address that particular situation without adding too much overhead for
> > other cases, I've come up with the appended patch (untested for now).
> > 
> > This is how it is supposed to work.
> > 
> > There are three "lock levels" for device hotplug, "normal", "remove"
> > and "weak".  The difference is related to how __lock_device_hotplug()
> > works.  Namely, if device hotplug is currently locked, that function
> > will either block or return false, depending on the "current lock
> > level" and its argument (the "new lock level").  The rules here are
> > that false is returned immediately if the "current lock level" is
> > "remove" and the "new lock level" is "weak".  The function blocks
> > for all other combinations of the two.
> 
> I think this is going way too far and a massive overkill in terms of
> complexity, which is way more important than for doing some
> restart_syscall loops during some rare sysfs file access, which isn't
> gonna be that common anyway.  Fancy locking always sucks.  The root
> cause of the problem is file removal ref draining from sysfs side and
> a proper solution should be implemented there.  Adding all this
> complexity to device hotplug lock won't solve problems involving other
> locks while obfuscating what's going on through all the complexity.
> Also, when sysfs is updated with a proper solution, a simpler
> workaround from device hotplug side would be far easier to convert to
> the new solution than this, which over time is likely to grow
> interesting uses.
> 
> Let's *please* stick to something simple.

OK, I'll use restart_syscall() approach in the simplest form, then.

Thanks,
Rafael


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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