From: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: jeremy@goop.org, kvm@vger.kernel.org, peterz@infradead.org,
virtualization@lists.linux-foundation.org,
paul.gortmaker@windriver.com, hpa@zytor.com, ak@linux.intel.com,
a.ryabinin@samsung.com, x86@kernel.org, borntraeger@de.ibm.com,
mingo@redhat.com, xen-devel@lists.xenproject.org,
paulmck@linux.vnet.ibm.com, riel@redhat.com,
konrad.wilk@oracle.com, dave@stgolabs.net,
sasha.levin@oracle.com, davej@redhat.com, tglx@linutronix.de,
waiman.long@hp.com, linux-kernel@vger.kernel.org,
pbonzini@redhat.com, akpm@linux-foundation.org,
torvalds@linux-foundation.org
Subject: Re: [PATCH V3] x86 spinlock: Fix memory corruption on completing completions
Date: Thu, 12 Feb 2015 19:53:29 +0530 [thread overview]
Message-ID: <54DCB761.4050504@linux.vnet.ibm.com> (raw)
In-Reply-To: <20150212135056.GA10646@redhat.com>
On 02/12/2015 07:20 PM, Oleg Nesterov wrote:
> On 02/12, Raghavendra K T wrote:
>>
>> @@ -191,8 +189,7 @@ static inline void arch_spin_unlock_wait(arch_spinlock_t *lock)
>> * We need to check "unlocked" in a loop, tmp.head == head
>> * can be false positive because of overflow.
>> */
>> - if (tmp.head == (tmp.tail & ~TICKET_SLOWPATH_FLAG) ||
>> - tmp.head != head)
>> + if (__tickets_equal(tmp.head, tmp.tail) || tmp.head != head)
>> break;
>
> Ah, it seems that "tmp.head != head" should be turned into
> !__tickets_equal(), no?
>
> Suppose that TICKET_SLOWPATH_FLAG is set after the first ACCESS_ONCE(head),
> then tmp.head != head will be true before the first unlock we are waiting
> for.
Good catch. othewise we would wrongly break out even when somebody
does halt wait.
>
> And perhaps you can turn these ACCESS_ONCE into READ_ONCE as well.
Yes again :)
next prev parent reply other threads:[~2015-02-12 14:23 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-02-12 11:47 [PATCH V3] x86 spinlock: Fix memory corruption on completing completions Raghavendra K T
2015-02-12 13:37 ` Oleg Nesterov
2015-02-12 14:21 ` Raghavendra K T
2015-02-12 13:50 ` Oleg Nesterov
2015-02-12 14:23 ` Raghavendra K T [this message]
2015-02-12 14:02 ` Oleg Nesterov
2015-02-12 14:25 ` Raghavendra K T
2015-02-12 15:00 ` Peter Zijlstra
2015-02-12 15:28 ` Raghavendra K T
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