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    Subject[PATCH 5.7 146/204] sched/uclamp: Fix a deadlock when enabling uclamp static key
    Date
    From: Qais Yousef <qais.yousef@arm.com>

    [ Upstream commit e65855a52b479f98674998cb23b21ef5a8144b04 ]

    The following splat was caught when setting uclamp value of a task:

    BUG: sleeping function called from invalid context at ./include/linux/percpu-rwsem.h:49

    cpus_read_lock+0x68/0x130
    static_key_enable+0x1c/0x38
    __sched_setscheduler+0x900/0xad8

    Fix by ensuring we enable the key outside of the critical section in
    __sched_setscheduler()

    Fixes: 46609ce22703 ("sched/uclamp: Protect uclamp fast path code with static key")
    Signed-off-by: Qais Yousef <qais.yousef@arm.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Link: https://lkml.kernel.org/r/20200716110347.19553-4-qais.yousef@arm.com
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    kernel/sched/core.c | 11 +++++++++--
    1 file changed, 9 insertions(+), 2 deletions(-)

    diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    index 49b5c5cf9271d..ebecf1cc3b788 100644
    --- a/kernel/sched/core.c
    +++ b/kernel/sched/core.c
    @@ -1248,6 +1248,15 @@ static int uclamp_validate(struct task_struct *p,
    if (upper_bound > SCHED_CAPACITY_SCALE)
    return -EINVAL;

    + /*
    + * We have valid uclamp attributes; make sure uclamp is enabled.
    + *
    + * We need to do that here, because enabling static branches is a
    + * blocking operation which obviously cannot be done while holding
    + * scheduler locks.
    + */
    + static_branch_enable(&sched_uclamp_used);
    +
    return 0;
    }

    @@ -1278,8 +1287,6 @@ static void __setscheduler_uclamp(struct task_struct *p,
    if (likely(!(attr->sched_flags & SCHED_FLAG_UTIL_CLAMP)))
    return;

    - static_branch_enable(&sched_uclamp_used);
    -
    if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MIN) {
    uclamp_se_set(&p->uclamp_req[UCLAMP_MIN],
    attr->sched_util_min, true);
    --
    2.25.1


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