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Re: [Xen-devel] [PATCH] x86 spinlock: Fix memory corruption on completing completions
- To: Oleg Nesterov <oleg@xxxxxxxxxx>
- From: Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>
- Date: Thu, 12 Feb 2015 00:08:17 +0530
- Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, KVM list <kvm@xxxxxxxxxxxxxxx>, Peter Zijlstra <peterz@xxxxxxxxxxxxx>, Jason Wang <jasowang@xxxxxxxxxx>, virtualization <virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx>, Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>, Peter Anvin <hpa@xxxxxxxxx>, Davidlohr Bueso <dave@xxxxxxxxxxxx>, Andrey Ryabinin <a.ryabinin@xxxxxxxxxxx>, the arch/x86 maintainers <x86@xxxxxxxxxx>, Christian Borntraeger <borntraeger@xxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, Sasha Levin <sasha.levin@xxxxxxxxxx>, Paul McKenney <paulmck@xxxxxxxxxxxxxxxxxx>, Andi Kleen <ak@xxxxxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, Dave Jones <davej@xxxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, Waiman Long <waiman.long@xxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Paolo Bonzini <pbonzini@xxxxxxxxxx>, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Wed, 11 Feb 2015 18:37:49 +0000
- List-id: Xen developer discussion <xen-devel.lists.xen.org>
On 02/11/2015 11:08 PM, Oleg Nesterov wrote:
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?
You are right.
I totally forgot we can get rid of tail operations :)
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;
}
Very nice idea. I was tired of ~TICKET_SLOWPATH_FLAG usage all over in
the current (complex :)) implementation. These two suggestions helps
alot.
and change kvm_lock_spinning() to use tickets_equal(tickets.head, want), plus
it can have more users in asm/spinlock.h.
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