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    Subject[tip: sched/core] rseq: Optimise rseq_get_rseq_cs() and clear_rseq_cs()
    The following commit has been merged into the sched/core branch of tip:

    Commit-ID: 5e0ccd4a3b01c5a71732a13186ca110a138516ea
    Gitweb: https://git.kernel.org/tip/5e0ccd4a3b01c5a71732a13186ca110a138516ea
    Author: Eric Dumazet <edumazet@google.com>
    AuthorDate: Tue, 13 Apr 2021 13:33:52 -07:00
    Committer: Peter Zijlstra <peterz@infradead.org>
    CommitterDate: Wed, 14 Apr 2021 18:04:09 +02:00

    rseq: Optimise rseq_get_rseq_cs() and clear_rseq_cs()

    Commit ec9c82e03a74 ("rseq: uapi: Declare rseq_cs field as union,
    update includes") added regressions for our servers.

    Using copy_from_user() and clear_user() for 64bit values
    is suboptimal.

    We can use faster put_user() and get_user() on 64bit arches.

    Signed-off-by: Eric Dumazet <edumazet@google.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
    Link: https://lkml.kernel.org/r/20210413203352.71350-4-eric.dumazet@gmail.com
    ---
    kernel/rseq.c | 9 +++++++++
    1 file changed, 9 insertions(+)

    diff --git a/kernel/rseq.c b/kernel/rseq.c
    index cfe01ab..35f7bd0 100644
    --- a/kernel/rseq.c
    +++ b/kernel/rseq.c
    @@ -127,8 +127,13 @@ static int rseq_get_rseq_cs(struct task_struct *t, struct rseq_cs *rseq_cs)
    u32 sig;
    int ret;

    +#ifdef CONFIG_64BIT
    + if (get_user(ptr, &t->rseq->rseq_cs.ptr64))
    + return -EFAULT;
    +#else
    if (copy_from_user(&ptr, &t->rseq->rseq_cs.ptr64, sizeof(ptr)))
    return -EFAULT;
    +#endif
    if (!ptr) {
    memset(rseq_cs, 0, sizeof(*rseq_cs));
    return 0;
    @@ -211,9 +216,13 @@ static int clear_rseq_cs(struct task_struct *t)
    *
    * Set rseq_cs to NULL.
    */
    +#ifdef CONFIG_64BIT
    + return put_user(0UL, &t->rseq->rseq_cs.ptr64);
    +#else
    if (clear_user(&t->rseq->rseq_cs.ptr64, sizeof(t->rseq->rseq_cs.ptr64)))
    return -EFAULT;
    return 0;
    +#endif
    }

    /*
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