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

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Message-Id: <4E429FE30200007800050910@nat28.tlf.novell.com>
Date:	Wed, 10 Aug 2011 14:12:35 +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 11:51, Ingo Molnar <mingo@...e.hu> wrote:

> * 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.

So this is a combination of two recent events: In my patch set, I
recently removed efi.c's X86_32 conditional around setting
x86_platform.[gs]et_wallclock. Without that removal, the deadlock
wouldn't occur (because mach_get_cmos_time() would get called
instead) on 64-bits, which is the only thing I run.

With that conditional removed, Matthew's NX consolidation patch
gets into the way: For the physical call path it is simply wrong
(correct only for the phys_efi_set_virtual_address_map() call, as
there the memory map got virtual addresses assigned already) -
it switches from early_runtime_code_mapping_set_exec()'s using
of md->phys_addr to efi_set_executable() using md->virt_addr.
But the latter isn't set at that point yet. And it's unclear to me why
this works on 32-bit.

Bottom line - the fix here is needed for 32-bit booting only (and
hence isn't - afaic - as critical as I first thought). But the adjustment
I'm holding is a necessary one (and I don't even understand why,
when 32- and 64-bit EFI code got merged, this was left 32-bit only),
and the NX consolidation change needs to be fixed too (basically as
a prerequisite).

Jan
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