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    SubjectRe: [RT PATCH v2] seqlock: serialize against writers
    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.
    > [
    > Changelog:
    > v2: only touch seqlock_t because raw_seqlock_t doesn't require
    > serialization and userspace cannot modify data during a read
    > v1: initial release
    > ]

    Hi Andi,
    As it turns out, my distcc backend was an x86_64 machine running the
    v1 patch and I started to notice sometime today that certain cc1 jobs
    were sometimes (albeit rarely) segfaulting on me. I noticed that before
    I even published the first patch, but I chalked it up to a corrupt .o on
    my NFS home. Plus I was forgetting that the distcc machine was running
    the patch, and I would probably have never made the userspace connection
    had you not mentioned it. In any case, I self-built this v2 patch with
    v2 applied, and the segfaults have gone away. So I think we know
    several things:

    1) You were right that this would cause an issue if the slow path is hit.
    2) Steven was right that userspace must use raw_seqlock_t because it no
    longer crashes with v2.
    3) I am satisfied that my primary concern is still properly addressed.

    Hopefully everyone is satisfied with this patch now.

    Thanks for the help. It's much appreciated!


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