Date: Wed, 11 Feb 2015 18:38:38 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Jeremy Fitzhardinge <jeremy@goop.org>,
        Sasha Levin <sasha.levin@oracle.com>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        Peter Anvin <hpa@zytor.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Waiman Long <waiman.long@hp.com>, Dave Jones <davej@redhat.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        Paul Gortmaker <paul.gortmaker@windriver.com>,
        Andi Kleen <ak@linux.intel.com>, Jason Wang <jasowang@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        KVM list <kvm@vger.kernel.org>,
        virtualization <virtualization@lists.linux-foundation.org>,
        xen-devel@lists.xenproject.org, Rik van Riel <riel@redhat.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrey Ryabinin <a.ryabinin@samsung.com>
Subject: Re: [PATCH] x86 spinlock: Fix memory corruption on completing
	completions
Message-ID: <20150211173838.GB28689@redhat.com>
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On 02/11, Raghavendra K T wrote:
>
> On 02/10/2015 06:56 PM, Oleg Nesterov wrote:
>
>> In this case __ticket_check_and_clear_slowpath() really needs to cmpxchg
>> the whole .head_tail. Plus obviously more boring changes. This needs a
>> separate patch even _if_ this can work.
>
> Correct, but apart from this, before doing xadd in unlock,
> we would have to make sure lsb bit is cleared so that we can live with 1
> bit overflow to tail which is unused. now either or both of head,tail
> lsb bit may be set after unlock.

Sorry, can't understand... could you spell?

If TICKET_SLOWPATH_FLAG lives in .head arch_spin_unlock() could simply do

	head = xadd(&lock->tickets.head, TICKET_LOCK_INC);

	if (head & TICKET_SLOWPATH_FLAG)
		__ticket_unlock_kick(head);

so it can't overflow to .tail?

But probably I missed your concern.


And we we do this, probably it makes sense to add something like

	bool tickets_equal(__ticket_t one, __ticket_t two)
	{
		return (one ^ two) & ~TICKET_SLOWPATH_FLAG;
	}

and change kvm_lock_spinning() to use tickets_equal(tickets.head, want), plus
it can have more users in asm/spinlock.h.

Oleg.

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