Linux Virtualization - Feb 2015 - [PATCH for stable] x86/spinlocks/paravirt: Fix memory corruption on unlock

On Tue, Feb 24, 2015 at 11:49:13PM +0530, Raghavendra K T
wrote:
> On 02/24/2015 08:17 PM, Ingo Molnar wrote:
> >
> >* Greg KH <gregkh at linuxfoundation.org> wrote:
> >
> >>On Tue, Feb 24, 2015 at 02:54:59PM +0530, Raghavendra K T wrote:
> >>>Paravirt spinlock clears slowpath flag after doing unlock.
> >>>As explained by Linus currently it does:
> >>>                 prev = *lock;
> >>>                 add_smp(&lock->tickets.head,
TICKET_LOCK_INC);
> >>>
> >>>                 /* add_smp() is a full mb() */
> >>>
> >>>                 if (unlikely(lock->tickets.tail &
TICKET_SLOWPATH_FLAG))
> >>>                         __ticket_unlock_slowpath(lock, prev);
> >>>
> >>>which is *exactly* the kind of things you cannot do with
spinlocks,
> >>>because after you've done the "add_smp()" and
released the spinlock
> >>>for the fast-path, you can't access the spinlock any more. 
Exactly
> >>>because a fast-path lock might come in, and release the whole
data
> >>>structure.
> >>>
> >>>Linus suggested that we should not do any writes to lock after
unlock(),
> >>>and we can move slowpath clearing to fastpath lock.
> >>>
> >>>So this patch implements the fix with:
> >>>1. Moving slowpath flag to head (Oleg):
> >>>Unlocked locks don't care about the slowpath flag;
therefore we can keep
> >>>it set after the last unlock, and clear it again on the first
(try)lock.
> >>>-- this removes the write after unlock. note that keeping
slowpath flag would
> >>>result in unnecessary kicks.
> >>>By moving the slowpath flag from the tail to the head ticket we
also avoid
> >>>the need to access both the head and tail tickets on unlock.
> >>>
> >>>2. use xadd to avoid read/write after unlock that checks the
need for
> >>>unlock_kick (Linus):
> >>>We further avoid the need for a read-after-release by using
xadd;
> >>>the prev head value will include the slowpath flag and indicate
if we
> >>>need to do PV kicking of suspended spinners -- on modern chips
xadd
> >>>isn't (much) more expensive than an add + load.
> >>>
> >>>Result:
> >>>  setup: 16core (32 cpu +ht sandy bridge 8GB 16vcpu guest)
> >>>  benchmark overcommit %improve
> >>>  kernbench  1x           -0.13
> >>>  kernbench  2x            0.02
> >>>  dbench     1x           -1.77
> >>>  dbench     2x           -0.63
> >>>
> >>>[Jeremy: hinted missing TICKET_LOCK_INC for kick]
> >>>[Oleg: Moving slowpath flag to head, ticket_equals idea]
> >>>[PeterZ: Detailed changelog]
> >>>
> >>>Reported-by: Sasha Levin <sasha.levin at oracle.com>
> >>>Suggested-by: Linus Torvalds <torvalds at
linux-foundation.org>
> >>>Signed-off-by: Raghavendra K T <raghavendra.kt at
linux.vnet.ibm.com>
> >>>Reviewed-by: Oleg Nesterov <oleg at redhat.com>
> >>>Acked-by: David Vrabel <david.vrabel at citrix.com>
> >>>---
> >>>  arch/x86/include/asm/spinlock.h | 94
++++++++++++++++++++---------------------
> >>>  arch/x86/kernel/kvm.c           |  7 ++-
> >>>  arch/x86/xen/spinlock.c         |  7 ++-
> >>>  3 files changed, 58 insertions(+), 50 deletions(-)
> >>>
> >>>Changes for stable:
> >>>   - Don't replace the ACCESS_ONCE to READ_ONCE which would
cause horraneous
> >>>     Compiler warnings (Linus, David Vbriel, PeterZ, Ingo)
> >>
> >>What is the git commit id of this in Linus's tree?  What
> >>stable tree(s) do you want this applied to?
> >
> >It's:
> >
> >  d6abfdb20223 x86/spinlocks/paravirt: Fix memory corruption on unlock
> 
> Yes, This is the original patch. Please note I have taken out the
> READ_ONCE changes from the original patch to avoid build warnings
> mentioned below.
> (Those READ_ONCE changes were cosmetic and was not present in the
> previous versions)
> 
> >
> >You'll also need this fix from Linus to avoid (harmless)
> >build warnings:
> >
> >  dd36929720f4 kernel: make READ_ONCE() valid on const arguments
> 
> So this may not be absolutely necessary with the current patch.
I'd prefer to be as close as possible to the upstream patch.  So if
applying both of these patches will work, I'd much rather do that.
Changing patches when backporting them to stable for no good reason than
to clean things up, just confuses everyone involved.

Let's keep our messy history :)

thanks,

greg k-h

* Greg KH <gregkh at linuxfoundation.org> wrote:
> > >It's:
> > >
> > >  d6abfdb20223 x86/spinlocks/paravirt: Fix memory corruption on
unlock
> > 
> > Yes, This is the original patch. Please note I have taken out the
> > READ_ONCE changes from the original patch to avoid build warnings
> > mentioned below.
> > (Those READ_ONCE changes were cosmetic and was not present in the
> > previous versions)
> > 
> > >
> > >You'll also need this fix from Linus to avoid (harmless)
> > >build warnings:
> > >
> > >  dd36929720f4 kernel: make READ_ONCE() valid on const arguments
> > 
> > So this may not be absolutely necessary with the current patch.
> 
> I'd prefer to be as close as possible to the upstream 
> patch.  So if applying both of these patches will work, 
> I'd much rather do that. Changing patches when 
> backporting them to stable for no good reason than to 
> clean things up, just confuses everyone involved.
> 
> Let's keep our messy history :)
By all means!

You'll first need to cherry-pick these commits:

 927609d622a3 kernel: tighten rules for ACCESS ONCE
 c5b19946eb76 kernel: Fix sparse warning for ACCESS_ONCE
 dd36929720f4 kernel: make READ_ONCE() valid on const arguments

That's the minimum set you will need for backporting, due 
to overlapping changes to the ACCESS_ONCE() definition.

and then apply this commit:

 d6abfdb20223 x86/spinlocks/paravirt: Fix memory corruption on unlock

I've double checked that these commits will cherry-pick 
fine on top of v3.19, in that order, and that an x86-64 
defconfig+kvmconfig+PARAVIRT_SPINLOCK=y kernel builds fine 
without warnings.

I've not boot tested the changes, so if anything breaks 
it's all your fault - while if it works just fine then
I'll be glad to take credit for that.

Thanks,

	Ingo

Am 25.02.2015 um 11:08 schrieb Ingo Molnar:
> 
> * Greg KH <gregkh at linuxfoundation.org> wrote:
> 
>>>> It's:
>>>>
>>>>  d6abfdb20223 x86/spinlocks/paravirt: Fix memory corruption on
unlock
>>>
>>> Yes, This is the original patch. Please note I have taken out the
>>> READ_ONCE changes from the original patch to avoid build warnings
>>> mentioned below.
>>> (Those READ_ONCE changes were cosmetic and was not present in the
>>> previous versions)
>>>
>>>>
>>>> You'll also need this fix from Linus to avoid (harmless)
>>>> build warnings:
>>>>
>>>>  dd36929720f4 kernel: make READ_ONCE() valid on const arguments
>>>
>>> So this may not be absolutely necessary with the current patch.
>>
>> I'd prefer to be as close as possible to the upstream 
>> patch.  So if applying both of these patches will work, 
>> I'd much rather do that. Changing patches when 
>> backporting them to stable for no good reason than to 
>> clean things up, just confuses everyone involved.
>>
>> Let's keep our messy history :)
> 
> By all means!
> 
> You'll first need to cherry-pick these commits:
> 
>  927609d622a3 kernel: tighten rules for ACCESS ONCE
>  c5b19946eb76 kernel: Fix sparse warning for ACCESS_ONCE
>  dd36929720f4 kernel: make READ_ONCE() valid on const arguments
If you go before 3.19, you will also need

   230fa253df63 kernel: Provide READ_ONCE and ASSIGN_ONCE
   43239cbe79fc kernel: Change ASSIGN_ONCE(val, x) to WRITE_ONCE(x, val)

> 
> That's the minimum set you will need for backporting, due 
> to overlapping changes to the ACCESS_ONCE() definition.
> 
> and then apply this commit:
> 
>  d6abfdb20223 x86/spinlocks/paravirt: Fix memory corruption on unlock
the alternative might be to replace READ_ONCE with ACCESS_ONCE when
doing the backport.
This depends on how important you consider backporting the ACCESS_ONCE fixes.

Christian