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

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?


greg k-h

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