Re: [Xen-devel] [PATCH] MCE: Fix race condition in mctelem_reserve

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 22/01/14 11:00, Frediano Ziglio wrote:
> On Wed, 2014-01-22 at 10:56 +0000, David Vrabel wrote:
>> On 22/01/14 10:50, Frediano Ziglio wrote:
>>> These lines (in mctelem_reserve)
>>>
>>>
>>>         newhead = oldhead->mcte_next;
>>>         if (cmpxchgptr(freelp, oldhead, newhead) == oldhead) {
>>>
>>> are racy. After you read the newhead pointer it can happen that another
>>> flow (thread or recursive invocation) change all the list but set head
>>> with same value. So oldhead is the same as *freelp but you are setting
>>> a new head that could point to whatever element (even already used).
>>>
>>> This patch use instead a bit array and atomic bit operations.
>>>
>>> Actually it use unsigned long instead of bitmap type as testing for
>>> all zeroes is easier.
>>
>> bitmap_zero() does what you want.
>>
>> David
> 
> No, bitmap_zero fills with zero, do not check for zeroes.

bitmap_empty() sorry.

David

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