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    SubjectRe: Current mainline git (24e700e291d52bd2) hangs when building e.g. perf
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    > On Sep 8, 2017, at 6:05 PM, Linus Torvalds <torvalds@linux-foundation.org> wrote:
    >
    >> On Fri, Sep 8, 2017 at 5:00 PM, Andy Lutomirski <luto@kernel.org> wrote:
    >>
    >> I'm not convinced. The SDM says (Vol 3, 11.3, under WC):
    >>
    >> If the WC buffer is partially filled, the writes may be delayed until
    >> the next occurrence of a serializing event; such as, an SFENCE or
    >> MFENCE instruction, CPUID execution, a read or write to uncached
    >> memory, an interrupt occurrence, or a LOCK instruction execution.
    >>
    >> Thanks, Intel, for definiing "serializing event" differently here than
    >> anywhere else in the whole manual.
    >
    > Yeah, it's really badly defined. Ok, maybe a locked instruction does
    > actually wait for it.. It should be invisible to anything, regardless.
    >
    >> 1. The kernel wants to reclaim a page of normal memory, so it unmaps
    >> it and flushes. Another CPU has an entry for that page in its WC
    >> buffer. I don't think we care whether the flush causes the WC write
    >> to really hit RAM because it's unobservable -- we just need to make
    >> sure it is ordered, as seen by software, before the flush operation
    >> completes. From the quote above, I think we're okay here.
    >
    > Agreed.
    >
    >> 2. The kernel is unmapping some IO memory (e.g. a GPU command buffer).
    >> It wants a guarantee that, when flush_tlb_mm_range returns, all CPUs
    >> are really done writing to it. Here I'm less convinced. The SDM
    >> quote certainly suggests to me that we have a promise that the WC
    >> write has *started* before flush_tlb_mm_range returns, but I'm not
    >> sure I believe that it's guaranteed to have retired.
    >
    > If others have writable TLB entries, what keeps them from just
    > continuing to write for a long time afterwards?

    Whoever unmaps the resource by kicking out their drm fd? I admit I'm just trying to think of the worst case.

    >
    >> I'd prefer to leave it as is except on the buggy AMD CPUs, though,
    >> since the current code is nice and fast.
    >
    > So is there a patch to detect the 383 erratum and serialize for those?
    > I may have missed that part.
    >

    The patch is in my head. It's imaginarily attached to this email.


    > Linus

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