Re: [PATCH for stable] x86/spinlocks/paravirt: Fix memory corruption on unlock

[Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>]

On 02/24/2015 08:50 PM, Greg KH wrote:
> On Tue, Feb 24, 2015 at 03:47:37PM +0100, Ingo Molnar wrote:
>>
>> * Greg KH <gregkh@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@oracle.com>
>>>> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
>>>> Signed-off-by: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
>>>> Reviewed-by: Oleg Nesterov <oleg@redhat.com>
>>>> Acked-by: David Vrabel <david.vrabel@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
>>
>> You'll also need this fix from Linus to avoid (harmless)
>> build warnings:
>>
>>   dd36929720f4 kernel: make READ_ONCE() valid on const arguments
>
> Great.  But what stable kernel trees should it be backported to?  Just
> 3.19?  Or anything older?

My patch was intended only for 3.19.

Though paravirt changes have gone in 3.12, the problem manifested
clearly after some of the completion related changes. but I leave that 
decision to experts here. (I 'll send necessary changes if patch is
needed for older versions because it may not apply cleanly).