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    Subject[RFC PATCH for 5.8 1/4] sched: Fix unreliable rseq cpu_id for new tasks
    Date
    While integrating rseq into glibc and replacing glibc's sched_getcpu
    implementation with rseq, glibc's tests discovered an issue with
    incorrect __rseq_abi.cpu_id field value right after the first time
    a newly created process issues sched_setaffinity.

    For the records, it triggers after building glibc and running tests, and
    then issuing:

    for x in {1..2000} ; do posix/tst-affinity-static & done

    and shows up as:

    error: Unexpected CPU 2, expected 0
    error: Unexpected CPU 2, expected 0
    error: Unexpected CPU 2, expected 0
    error: Unexpected CPU 2, expected 0
    error: Unexpected CPU 138, expected 0
    error: Unexpected CPU 138, expected 0
    error: Unexpected CPU 138, expected 0
    error: Unexpected CPU 138, expected 0

    This is caused by the scheduler invoking __set_task_cpu() directly from
    sched_fork() and wake_up_new_task(), thus bypassing rseq_migrate() which
    is done by set_task_cpu().

    Add the missing rseq_migrate() to both functions. The only other direct
    use of __set_task_cpu() is done by init_idle(), which does not involve a
    user-space task.

    Based on my testing with the glibc test-case, just adding rseq_migrate()
    to wake_up_new_task() is sufficient to fix the observed issue. Also add
    it to sched_fork() to keep things consistent.

    The reason why this never triggered so far with the rseq/basic_test
    selftest is unclear.

    The current use of sched_getcpu(3) does not typically require it to be
    always accurate. However, use of the __rseq_abi.cpu_id field within rseq
    critical sections requires it to be accurate. If it is not accurate, it
    can cause corruption in the per-cpu data targeted by rseq critical
    sections in user-space.

    Link: https://sourceware.org/pipermail/libc-alpha/2020-July/115816.html
    Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
    Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Florian Weimer <fw@deneb.enyo.de>
    Cc: "Paul E. McKenney" <paulmck@linux.ibm.com>
    Cc: Boqun Feng <boqun.feng@gmail.com>
    Cc: "H . Peter Anvin" <hpa@zytor.com>
    Cc: Paul Turner <pjt@google.com>
    Cc: Dmitry Vyukov <dvyukov@google.com>
    Cc: Neel Natu <neelnatu@google.com>
    Cc: linux-api@vger.kernel.org
    Cc: stable@vger.kernel.org # v4.18+
    ---
    kernel/sched/core.c | 2 ++
    1 file changed, 2 insertions(+)

    diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    index ca5db40392d4..86a855bd4d90 100644
    --- a/kernel/sched/core.c
    +++ b/kernel/sched/core.c
    @@ -2962,6 +2962,7 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
    * Silence PROVE_RCU.
    */
    raw_spin_lock_irqsave(&p->pi_lock, flags);
    + rseq_migrate(p);
    /*
    * We're setting the CPU for the first time, we don't migrate,
    * so use __set_task_cpu().
    @@ -3026,6 +3027,7 @@ void wake_up_new_task(struct task_struct *p)
    * as we're not fully set-up yet.
    */
    p->recent_used_cpu = task_cpu(p);
    + rseq_migrate(p);
    __set_task_cpu(p, select_task_rq(p, task_cpu(p), SD_BALANCE_FORK, 0));
    #endif
    rq = __task_rq_lock(p, &rf);
    --
    2.17.1
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