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    SubjectRe: [PATCH 1/2] ipc/sem.c: Fix complex_count vs. simple op race
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    On 06/21/2016 01:04 AM, Andrew Morton wrote:
    > On Sat, 18 Jun 2016 22:02:21 +0200 Manfred Spraul <manfred@colorfullife.com> wrote:
    >
    >> Commit 6d07b68ce16a ("ipc/sem.c: optimize sem_lock()") introduced a race:
    >>
    >> sem_lock has a fast path that allows parallel simple operations.
    >> There are two reasons why a simple operation cannot run in parallel:
    >> - a non-simple operations is ongoing (sma->sem_perm.lock held)
    >> - a complex operation is sleeping (sma->complex_count != 0)
    >>
    >> As both facts are stored independently, a thread can bypass the current
    >> checks by sleeping in the right positions. See below for more details
    >> (or kernel bugzilla 105651).
    >>
    >> The patch fixes that by creating one variable (complex_mode)
    >> that tracks both reasons why parallel operations are not possible.
    >>
    >> The patch also updates stale documentation regarding the locking.
    >>
    >> With regards to stable kernels:
    >> The patch is required for all kernels that include the commit 6d07b68ce16a
    >> ("ipc/sem.c: optimize sem_lock()") (3.10?)
    > I've had this in -mm (and -next) since January 4, without issues. I
    > put it on hold because Davidlohr expressed concern about performance
    > regressions.
    I had several ideas how to fix it. The initial ideas probably had
    performance issue.

    The current one doesn't have any issues. It just took longer than
    expected to test it.
    > Your [2/2] should prevent those regressions (yes?) so I assume that any
    > kernel which has [1/2] really should have [2/2] as well. But without
    > any quantitative information, this is all mad guesswork.
    >
    > What to do?
    [2/2] is an improvement, it handles one case better than the current code.
    If you want:
    3.10 improved scalability, but it introduced a performance regression
    for one use case.
    [with 3.10, simple ops got parallel, but complex ops had to perform a
    "for_each_sem() {spin_unlock_wait()}"]
    The patch fixes that.


    --
    Manfred

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