lkml.org 
[lkml]   [2005]   [Feb]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH] arp_queue: serializing unlink + kfree_skb
    On Fri, 4 Feb 2005 12:20:53 +1100
    Herbert Xu <herbert@gondor.apana.org.au> wrote:

    > This is true if CPU 0 reads the count before reading skb->list.
    > Without a memory barrier, atomic_read and reading skb->list can
    > be reordered. Put it another way, reading skb->list could return
    > a cached value that was read from the main memory prior to the
    > atomic_read.
    >
    > So in order for CPU 0 to always see an up-to-date value of skb->list,
    > it needs to do an smp_rmb() between the atomic_read and reading
    > skb->list.

    You're absolutely right. Ok, so we do need to change kfree_skb().
    I believe even with the memory barrier, the atomic_read() optimization
    is still worth it. atomic ops on sparc64 take a minimum of 40 some odd
    cycles on UltraSPARC-III and later, whereas the memory barrier will
    take up a single cycle most of the time.

    So it'll look something like:

    if (atomic_read(&skb->users) == 1)
    smb_rmb();
    else if (!atomic_dec_and_test(&skb->users))
    return;
    __kfree_skb(skb);
    -
    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to majordomo@vger.kernel.org
    More majordomo info at http://vger.kernel.org/majordomo-info.html
    Please read the FAQ at http://www.tux.org/lkml/

    \
     
     \ /
      Last update: 2005-03-22 14:10    [W:0.037 / U:2.236 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site