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    SubjectRe: [PATCH] seqlock: serialize against writers
    On Fri, 2008-08-29 at 11:44 -0400, Gregory Haskins wrote:
    > *Patch submitted for inclusion in PREEMPT_RT 26-rt4. Applies to*
    > Hi Ingo, Steven, Thomas,
    > Please consider for -rt4. This fixes a nasty deadlock on my systems under
    > heavy load.
    > -Greg
    > ----
    > Seqlocks have always advertised that readers do not "block", but this was
    > never really true. Readers have always logically blocked at the head of
    > the critical section under contention with writers, regardless of whether
    > they were allowed to run code or not.
    > Recent changes in this space (88a411c07b6fedcfc97b8dc51ae18540bd2beda0)
    > have turned this into a more explicit blocking operation in mainline.
    > However, this change highlights a short-coming in -rt because the
    > normal seqlock_ts are preemptible. This means that we can potentially
    > deadlock should a reader spin waiting for a write critical-section to end
    > while the writer is preempted.

    I think the technical term is livelock.

    So the problem is that the write side gets preempted, and the read side
    spins in a non-preemptive fashion?

    Looking at the code, __read_seqbegin() doesn't disable preemption, so
    even while highly inefficient when spinning against a preempted lock, it
    shouldn't livelock, since the spinner can get preempted giving the
    writer a chance to finish.

    > This patch changes the internal implementation to use a rwlock and forces
    > the readers to serialize with the writers under contention. This will
    > have the advantage that -rt seqlocks_t will sleep the reader if deadlock
    > were imminent, and it will pi-boost the writer to prevent inversion.
    > This fixes a deadlock discovered under testing where all high prioritiy
    > readers were hogging the cpus and preventing a writer from releasing the
    > lock.
    > Since seqlocks are designed to be used as rarely-write locks, this should
    > not affect the performance in the fast-path

    Not quite, seqlocks never suffered the cacheline bounce rwlocks have -
    which was they strongest point - so I very much not like this change.

    As to the x86_64 gtod-vsyscall, that uses a raw_seqlock_t on -rt, which
    is still non-preemptable and should thus not be affected by this
    livelock scenario.

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      Last update: 2008-08-30 13:11    [W:0.023 / U:23.508 seconds]
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