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    SubjectRe: [RFC 0/2] srcu: Remove pre-flip memory barrier
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    On 2022-12-20 17:57, Frederic Weisbecker wrote:
    > On Tue, Dec 20, 2022 at 02:01:30PM -0500, Mathieu Desnoyers wrote:
    >> On 2022-12-20 13:29, Joel Fernandes wrote:
    >>>
    >>
    >>> I do want to finish my memory barrier studies of SRCU over the holidays since I have been deep in the hole with that already. Back to the post flip memory barrier here since I think now even that might not be needed…
    >>
    >> I strongly suspect the memory barrier after flip is useless for the same
    >> reasons I mentioned explaining why the barrier before the flip is useless.
    >>
    >> However, we need to double-check that we have memory barriers at the
    >> beginning and end of synchronize_srcu, and between load of "unlock" counters
    >> and load of "lock" counters.
    >>
    >> Where is the barrier at the beginning of synchronize_srcu ?
    >
    > rcu_seq_snap() ?

    The memory barrier in rcu_seq_snap is not at the very beginning of synchronize_srcu.

    For example we have:

    unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp)
    {
    // Any prior manipulation of SRCU-protected data must happen
    // before the load from ->srcu_gp_seq.
    smp_mb();
    return rcu_seq_snap(&ssp->srcu_gp_seq);

    which happens to have an explicit barrier before issuing rcu_seq_snap().

    So my question still stands: where is the memory barrier at the beginning of synchronize_srcu ?

    The memory ordering constraint I am concerned about here is:

    * [...] In addition,
    * each CPU having an SRCU read-side critical section that extends beyond
    * the return from synchronize_srcu() is guaranteed to have executed a
    * full memory barrier after the beginning of synchronize_srcu() and before
    * the beginning of that SRCU read-side critical section. [...]

    So if we have a SRCU read-side critical section that begins after the beginning
    of synchronize_srcu, but before its first memory barrier, it would miss the
    guarantee that the full memory barrier is issued before the beginning of that
    SRCU read-side critical section. IOW, that memory barrier needs to be at the
    very beginning of the grace period.

    I suspect that the memory barrier in srcu_read_lock() invoked by srcu_gp_start_if_needed
    may happen to be early enough, but I'm not sure, and it does not appear to be documented
    as such.

    Thanks,

    Mathieu

    >
    >>
    >> Thanks,
    >>
    >> Mathieu
    >>
    >> --
    >> Mathieu Desnoyers
    >> EfficiOS Inc.
    >> https://www.efficios.com
    >>

    --
    Mathieu Desnoyers
    EfficiOS Inc.
    https://www.efficios.com

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