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    SubjectRe: [PATCH 4.19 625/639] packet: fix data-race in fanout_flow_is_huge()
    Hi!

    > From: Eric Dumazet <edumazet@google.com>
    >
    > [ Upstream commit b756ad928d98e5ef0b74af7546a6a31a8dadde00 ]
    >
    > KCSAN reported the following data-race [1]
    >
    > Adding a couple of READ_ONCE()/WRITE_ONCE() should silence it.
    >
    > Since the report hinted about multiple cpus using the history
    > concurrently, I added a test avoiding writing on it if the
    > victim slot already contains the desired value.

    > static bool fanout_flow_is_huge(struct packet_sock *po, struct sk_buff *skb)
    > {
    > - u32 rxhash;
    > + u32 *history = po->rollover->history;
    > + u32 victim, rxhash;
    > int i, count = 0;
    >
    > rxhash = skb_get_hash(skb);
    > for (i = 0; i < ROLLOVER_HLEN; i++)
    > - if (po->rollover->history[i] == rxhash)
    > + if (READ_ONCE(history[i]) == rxhash)
    > count++;
    >
    > - po->rollover->history[prandom_u32() % ROLLOVER_HLEN] = rxhash;
    > + victim = prandom_u32() % ROLLOVER_HLEN;
    > +
    > + /* Avoid dirtying the cache line if possible */
    > + if (READ_ONCE(history[victim]) != rxhash)
    > + WRITE_ONCE(history[victim], rxhash);
    > +

    Replacing simple asignment with if() is ... not nice and with all the
    "volatile" magic in _ONCE macros may not be win for
    everyone. [Actually, I don't think this is win here. This is not
    exactly hot path, is it? Is it likely that array aready contains
    required value?]

    If this is going to get more common, should we get
    WRITE_ONCE_NONDIRTY() macro hiding the uglyness?

    Best regards,
    Pavel

    --
    (english) http://www.livejournal.com/~pavelmachek
    (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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