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

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Date:	Wed, 10 Aug 2011 11:03:30 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Jan Beulich <JBeulich@...ell.com>
Cc:	Matt Fleming <matt@...sole-pimps.org>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	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 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?

Thanks,

	Ingo
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