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

[Frediano Ziglio <frediano.ziglio@xxxxxxxxxx>]

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.

Actually I read the bitmap, check for all zeroes (some item free) and
then use find_first_set_bit to find the bit number set to 1.

Frediano



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