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

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Date:	Wed, 10 Aug 2011 11:51:40 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Jan Beulich <JBeulich@...ell.com>
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


* Jan Beulich <JBeulich@...ell.com> wrote:

> >>> 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...

Ok, once you figure it out we can merge all fixes upstream ASAP.

Thanks,

	Ingo
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