Re: [PATCH v5 04/29] sched: Let a task boosted by a deadline waiter change its policy

[Juri Lelli]

Hello,

On 08/11/26 13:53, Mathieu Desnoyers wrote:
> A SCHED_OTHER or SCHED_FIFO task that holds an rt_mutex can be boosted
> into the deadline class by a deadline waiter. Then a sched_setattr() or
> sched_setscheduler() call on it fails with EBUSY, unless it asks for
> SCHED_DEADLINE. sysrq-n would not reset it either.
> 
> The admission test runs "if setscheduling to SCHED_DEADLINE (or
> changing the parameters of a SCHED_DEADLINE task)", as its comment
> says. sched_dl_overflow() then handles three cases, by the task's
> policy: the task enters, leaves or stays in SCHED_DEADLINE. But the
> call site tests dl_task(p). That tests the priority, so a boosted task
> passes it. For such a task and another policy, sched_dl_overflow()
> matches no case and returns -1.
> 
> Fix this by testing the policy, with task_has_dl_policy(). A boosted
> task has no bandwidth of its own to admit or release.
> 
> Now sysrq-n can take a boosted task out of the class.
> normalize_rt_tasks() calls __sched_setscheduler() without pi, so the
> task gets its normal priority. Nothing then drops the boost. At the
> unlock, rt_mutex_setprio() sees no change of priority. The pi_se of the
> task would keep pointing to the waiter, which can exit. So also drop the
> boost in __setscheduler_dl_pi() when the task leaves the class, as
> rt_mutex_setprio() does.
> 
> The boosted_owner_policy case of deadline_bw_test, added later in this
> series, fails with EBUSY on v7.4-rc1. It passes with this series. The
> sysrq_boosted_owner case of deadline_switch_test tests the sysrq-n path.
> 
> Fixes: 332ac17ef5bf ("sched/deadline: Add bandwidth management for SCHED_DEADLINE tasks")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> ---

Reviewed-by: Juri Lelli <juri.lelli@xxxxxxxxxx>

Thanks,
Juri