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Date: Fri, 14 Oct 2011 08:49:49 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v2 1/2] Introduce QLIST_INSERT_HEAD_RCU and
 dummy RCU wrappers.
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To: Harsh Prateek Bora <harsh@linux.vnet.ibm.com>
Cc: jan.kiszka@web.de, qemu-devel@nongnu.org, aneesh.kumar@linux.vnet.ibm.com

On 10/13/2011 10:35 PM, Harsh Prateek Bora wrote:
>
> +#define QLIST_INSERT_HEAD_RCU(head, elm, field) do {                    \
> +        (elm)->field.le_prev =&(head)->lh_first;                       \
> +        smp_wmb();                                                      \
> +        if (((elm)->field.le_next = (head)->lh_first) != NULL)          \
> +                (head)->lh_first->field.le_prev =&(elm)->field.le_next;\
> +        smp_wmb();                                                      \
> +        (head)->lh_first = (elm);                                       \
> +        smp_wmb();                                                      \
> +} while (/* CONSTCOND*/0)

Actually, looking more at it it should be more like

        (elm)->field.le_prev =&(head)->lh_first;
        (elm)->field.le_next = (head)->lh_first;
        smb_wmb(); /* fill elm before linking it */
        if ((head)->lh_first != NULL)
           (head)->lh_first->field.le_prev =&(elm)->field.le_next;
        (head)->lh_first = (elm);
        smp_wmb();

... which even saves a memory barrier.

Paolo