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    Subject[PATCH 1/1] futex: check PF_KTHREAD rather than !p->mm to filter out kthreads
    attach_to_pi_owner() checks p->mm to prevent attaching to kthreads and
    this looks doubly wrong:

    1. It should actually check PF_KTHREAD, kthread can do use_mm().

    2. If this task is not kthread and it is actually the lock owner we can
    wrongly return -EPERM instead of -ESRCH or retry-if-EAGAIN.

    And note that this wrong EPERM is the likely case unless the exiting
    task is (auto)reaped quickly, we check ->mm before PF_EXITING.

    Signed-off-by: Oleg Nesterov <oleg@redhat.com>
    ---
    kernel/futex.c | 2 +-
    1 files changed, 1 insertions(+), 1 deletions(-)

    diff --git a/kernel/futex.c b/kernel/futex.c
    index 63678b5..b101381 100644
    --- a/kernel/futex.c
    +++ b/kernel/futex.c
    @@ -900,7 +900,7 @@ static int attach_to_pi_owner(u32 uval, union futex_key *key,
    if (!p)
    return -ESRCH;

    - if (!p->mm) {
    + if (unlikely(p->flags & PF_KTHREAD)) {
    put_task_struct(p);
    return -EPERM;
    }
    --
    1.5.5.1



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