linux-kernel - Re: [PATCH] x86 spinlock: Fix memory corruption on completing completions

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Date:	Fri, 06 Feb 2015 10:20:01 -0500
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	peterz@...radead.org, torvalds@...ux-foundation.org
CC:	konrad.wilk@...cle.com, pbonzini@...hat.com,
	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
Subject: Re: [PATCH] x86 spinlock: Fix memory corruption on completing completions

On 02/06/2015 09:49 AM, Raghavendra K T wrote:
>  static __always_inline void arch_spin_unlock(arch_spinlock_t *lock)
>  {
>  	if (TICKET_SLOWPATH_FLAG &&
> -	    static_key_false(&paravirt_ticketlocks_enabled)) {
> -		arch_spinlock_t prev;
> +		static_key_false(&paravirt_ticketlocks_enabled)) {
> +		__ticket_t prev_head;
>  
> -		prev = *lock;
> +		prev_head = lock->tickets.head;
>  		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);
> +		if (unlikely(lock->tickets.tail & TICKET_SLOWPATH_FLAG)) {
> +			BUILD_BUG_ON(((__ticket_t)NR_CPUS) != NR_CPUS);
> +			__ticket_unlock_kick(lock, prev_head);

Can we modify it slightly to avoid potentially accessing invalid memory:

diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index 5315887..cd22d73 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -144,13 +144,13 @@ static __always_inline void arch_spin_unlock(arch_spinlock_t *lock
        if (TICKET_SLOWPATH_FLAG &&
                static_key_false(&paravirt_ticketlocks_enabled)) {
                __ticket_t prev_head;
-
+               bool needs_kick = lock->tickets.tail & TICKET_SLOWPATH_FLAG;
                prev_head = lock->tickets.head;
                add_smp(&lock->tickets.head, TICKET_LOCK_INC);

                /* add_smp() is a full mb() */

-               if (unlikely(lock->tickets.tail & TICKET_SLOWPATH_FLAG)) {
+               if (unlikely(needs_kick)) {
                        BUILD_BUG_ON(((__ticket_t)NR_CPUS) != NR_CPUS);
                        __ticket_unlock_kick(lock, prev_head);
                }


Thanks,
Sasha
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