linux-kernel - Re: [PATCH] x86, efi: Don't recursively acquire rtc

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Message-Id: <4E426D4702000078000508B5@nat28.tlf.novell.com>
Date:	Wed, 10 Aug 2011 10:36:39 +0100
From:	"Jan Beulich" <JBeulich@...ell.com>
To:	"Ingo Molnar" <mingo@...e.hu>
Cc:	"Jeremy Fitzhardinge" <jeremy.fitzhardinge@...rix.com>,
	"Matt Fleming" <matt@...sole-pimps.org>,
	"Thomas Gleixner" <tglx@...utronix.de>, <hpa@...ux.intel.com>,
	"Avi Kivity" <avi@...hat.com>, <mjg@...hat.com>,
	"Marcelo Tosatti" <mtosatti@...hat.com>,
	"johnstultz" <johnstul@...ibm.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86, efi: Don't recursively acquire rtc_lock

>>> On 10.08.11 at 11:03, Ingo Molnar <mingo@...e.hu> wrote:

> * Jan Beulich <JBeulich@...ell.com> wrote:
> 
>> >>> On 08.08.11 at 15:40, Matt Fleming <matt@...sole-pimps.org> wrote:
>> > On Fri, 2011-08-05 at 18:04 +0100, Jan Beulich wrote:
>> >> 
>> >> Virtual platforms will have to take care of the serialization in the
>> >> host anyway, so the guest side implementation of getwallclock et al
>> >> is entirely unaffected.
>> > 
>> > Ah, OK, that's the important part. I didn't realise that rtc_lock isn't
>> > actually required by any other code. In which case, yes, it completely
>> > makes sense to push the locking of rtc_lock down into the
>> > implementations that actually need it.
>> > 
>> > It'd be great if I could get some ACK's from the virtualization guys.
>> > 
>> > --------8<--------
>> > 
>> > From a0a39dbb69f6ac675846bf00f30ad153506a4567 Mon Sep 17 00:00:00 2001
>> > From: Matt Fleming <matt.fleming@...el.com>
>> > Date: Mon, 8 Aug 2011 12:59:35 +0100
>> > Subject: [PATCH] x86, efi: Don't recursively acquire rtc_lock
>> > 
>> > A deadlock was introduced on x86 in commit ef68c8f87ed1 ("x86:
>> > Serialize EFI time accesses on rtc_lock") because efi_get_time() and
>> > friends can be called with rtc_lock already held by
>> > read_persistent_time(), e.g.
>> > 
>> > timekeeping_init()
>> >     read_persistent_clock()     <-- acquire rtc_lock
>> >         efi_get_time()
>> >             phys_efi_get_time() <-- acquire rtc_lock <DEADLOCK>
>> > 
>> > To fix this let's push the locking down into the get_wallclock() and
>> > set_wallclock() implementations. Only the clock implementations that
>> > access the x86 RTC directly need to acquire rtc_lock, so it makes
>> > sense to push the locking down into the rtc, vrtc and efi code.
>> > 
>> > The virtualization implementations don't require rtc_lock to be held
>> > because they provide their own serialization.
>> > 
>> > Signed-off-by: Matt Fleming <matt.fleming@...el.com>
>> 
>> Acked-by: Jan Beulich <jbeulich@...ell.com>
> 
> How urgent is thix fix - can it wait until Thomas comes back and 
> starts processing patches again?

Without it booting on EFI can't be expected to work (and while I was
really sure I tested this before submitting, I apparently must have
used a stale kernel then - I'm really sorry for that), so getting it
applied soon would be rather desirable.

But then again I don't seem to be able to boot 3.0.1 with both patches
applied on my only EFI box, so there must be something else that
broke, and hence maybe I really did test it (on 3.0-rc7) and it worked
then. Debugging...

Jan

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