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    Subject[PATCH AUTOSEL 4.14 46/56] sched/vtime: Fix guest/system mis-accounting on task switch
    Date
    From: Frederic Weisbecker <frederic@kernel.org>

    [ Upstream commit 68e7a4d66b0ce04bf18ff2ffded5596ab3618585 ]

    vtime_account_system() assumes that the target task to account cputime
    to is always the current task. This is most often true indeed except on
    task switch where we call:

    vtime_common_task_switch(prev)
    vtime_account_system(prev)

    Here prev is the scheduling-out task where we account the cputime to. It
    doesn't match current that is already the scheduling-in task at this
    stage of the context switch.

    So we end up checking the wrong task flags to determine if we are
    accounting guest or system time to the previous task.

    As a result the wrong task is used to check if the target is running in
    guest mode. We may then spuriously account or leak either system or
    guest time on task switch.

    Fix this assumption and also turn vtime_guest_enter/exit() to use the
    task passed in parameter as well to avoid future similar issues.

    Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Rik van Riel <riel@redhat.com>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Wanpeng Li <wanpengli@tencent.com>
    Fixes: 2a42eb9594a1 ("sched/cputime: Accumulate vtime on top of nsec clocksource")
    Link: https://lkml.kernel.org/r/20190925214242.21873-1-frederic@kernel.org
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    kernel/sched/cputime.c | 6 +++---
    1 file changed, 3 insertions(+), 3 deletions(-)

    diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
    index 14d2dbf97c531..45c2cd37fe6b0 100644
    --- a/kernel/sched/cputime.c
    +++ b/kernel/sched/cputime.c
    @@ -738,7 +738,7 @@ void vtime_account_system(struct task_struct *tsk)

    write_seqcount_begin(&vtime->seqcount);
    /* We might have scheduled out from guest path */
    - if (current->flags & PF_VCPU)
    + if (tsk->flags & PF_VCPU)
    vtime_account_guest(tsk, vtime);
    else
    __vtime_account_system(tsk, vtime);
    @@ -781,7 +781,7 @@ void vtime_guest_enter(struct task_struct *tsk)
    */
    write_seqcount_begin(&vtime->seqcount);
    __vtime_account_system(tsk, vtime);
    - current->flags |= PF_VCPU;
    + tsk->flags |= PF_VCPU;
    write_seqcount_end(&vtime->seqcount);
    }
    EXPORT_SYMBOL_GPL(vtime_guest_enter);
    @@ -792,7 +792,7 @@ void vtime_guest_exit(struct task_struct *tsk)

    write_seqcount_begin(&vtime->seqcount);
    vtime_account_guest(tsk, vtime);
    - current->flags &= ~PF_VCPU;
    + tsk->flags &= ~PF_VCPU;
    write_seqcount_end(&vtime->seqcount);
    }
    EXPORT_SYMBOL_GPL(vtime_guest_exit);
    --
    2.20.1
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