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

[Ingo Molnar]

* 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

Thanks,

	Ingo
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