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

[Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>]

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@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxx>
> > > > Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> > > > Signed-off-by: Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>
> > > > Reviewed-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> > > > Acked-by: David Vrabel <david.vrabel@xxxxxxxxxx>
> > > > ---
> > > >   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).



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