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

[Greg KH <gregkh@linuxfoundation.org>]

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?

thanks,

greg k-h