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

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Date:	Tue, 24 Feb 2015 06:17:31 -0800
From:	Greg KH <gregkh@...uxfoundation.org>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
Cc:	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	torvalds@...ux-foundation.org, konrad.wilk@...cle.com,
	pbonzini@...hat.com, peterz@...radead.org,
	paulmck@...ux.vnet.ibm.com, waiman.long@...com, davej@...hat.com,
	oleg@...hat.com, x86@...nel.org, jeremy@...p.org,
	paul.gortmaker@...driver.com, ak@...ux.intel.com,
	jasowang@...hat.com, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
	xen-devel@...ts.xenproject.org, riel@...hat.com,
	borntraeger@...ibm.com, akpm@...ux-foundation.org,
	a.ryabinin@...sung.com, sasha.levin@...cle.com, dave@...olabs.net,
	stable@...r.kernel.org
Subject: Re: [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@...cle.com>
> Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Signed-off-by: Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
> Reviewed-by: Oleg Nesterov <oleg@...hat.com>
> Acked-by: David Vrabel <david.vrabel@...rix.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?

thanks,

greg k-h
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