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    SubjectRe: [PATCH] synchronize_irq needs a barrier
    Date
    Nick Piggin <nickpiggin@yahoo.com.au> wrote:
    >
    >> First of all let's agree on some basic assumptions:
    >>
    >> * A pair of spin lock/unlock subsumes the effect of a full mb.
    >
    > Not unless you mean a pair of spin lock/unlock as in
    > 2 spin lock/unlock pairs (4 operations).
    >
    > *X = 10;
    > spin_lock(&lock);
    > /* *Y speculatively loaded here */
    > /* store to *X leaves CPU store queue here */
    > spin_unlock(&lock);
    > y = *Y;

    Good point.

    Although in this case we're still safe because in the worst
    cases:

    CPU0 CPU1
    irq_sync = 1
    synchronize_irq
    spin lock
    load IRQ_INPROGRESS
    irq_sync sync is visible
    spin unlock
    spin lock
    load irq_sync
    while (IRQ_INPROGRESS)
    wait
    return
    set IRQ_INPROGRESS
    spin unlock
    tg3_msi
    ack IRQ
    if (irq_sync)
    return
    spin lock
    clear IRQ_INPROGRESS
    spin unlock

    ------------------------------------------------------------

    CPU0 CPU1
    spin lock
    load irq_sync
    irq_sync = 1
    synchronize_irq
    set IRQ_INPROGRESS
    spin unlock
    spin lock
    load IRQ_INPROGRESS
    irq_sync sync is visible
    spin unlock
    while (IRQ_INPROGRESS)
    wait
    tg3_msi
    ack IRQ
    if (irq_sync)
    return
    do work
    spin lock
    clear IRQ_INPROGRESS
    spin unlock
    return

    So because we're using the same lock on both sides, it does
    do the right thing even without the memory barrier.

    Cheers,
    --
    Visit Openswan at http://www.openswan.org/
    Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au>
    Home Page: http://gondor.apana.org.au/~herbert/
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