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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 22.01.14 at 11:50, Frediano Ziglio <frediano.ziglio@xxxxxxxxxx> 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.

Except you never test for all zeroes. All of the operations you use
(with the exception of find_first_set_bit(), which would need
replacing by find_first_bit()) are suitable to be used on bitmaps, so
I really don't see why this can't be a proper bitmap.

Jan


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