Re: [PATCH v8 08/19] sched: Reject CPU affinity changes based on task_cpu_possible_mask()
From: Valentin Schneider <hidden>
Date: 2021-06-04 17:12:00
Also in:
linux-arch, lkml
On 02/06/21 17:47, Will Deacon wrote:
Reject explicit requests to change the affinity mask of a task via set_cpus_allowed_ptr() if the requested mask is not a subset of the mask returned by task_cpu_possible_mask(). This ensures that the 'cpus_mask' for a given task cannot contain CPUs which are incapable of executing it, except in cases where the affinity is forced. Reviewed-by: Quentin Perret <redacted> Signed-off-by: Will Deacon <will@kernel.org>
One comment/observation below, but regardless: Reviewed-by: Valentin Schneider <redacted>
quoted hunk ↗ jump to hunk
--- kernel/sched/core.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-)diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 0c1b6f1a6c91..b23c7f0ab31a 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c@@ -2347,15 +2347,17 @@ static int __set_cpus_allowed_ptr(struct task_struct *p, u32 flags) { const struct cpumask *cpu_valid_mask = cpu_active_mask; + const struct cpumask *cpu_allowed_mask = task_cpu_possible_mask(p); unsigned int dest_cpu; struct rq_flags rf; struct rq *rq; int ret = 0; + bool kthread = p->flags & PF_KTHREAD; rq = task_rq_lock(p, &rf); update_rq_clock(rq); - if (p->flags & PF_KTHREAD || is_migration_disabled(p)) { + if (kthread || is_migration_disabled(p)) { /* * Kernel threads are allowed on online && !active CPUs, * however, during cpu-hot-unplug, even these might get pushed@@ -2369,6 +2371,11 @@ static int __set_cpus_allowed_ptr(struct task_struct *p, cpu_valid_mask = cpu_online_mask; } + if (!kthread && !cpumask_subset(new_mask, cpu_allowed_mask)) { + ret = -EINVAL; + goto out; + } +
IIUC this wouldn't be required if guarantee_online_cpus() couldn't build a mask that extends beyond task_cpu_possible_mask(p): if the new mask doesn't intersect with that possible mask, it means we're carrying an empty cpumask and the cpumask_any_and_distribute() below would return nr_cpu_ids, so we'd bail with -EINVAL. I don't really see a way around it though due to the expectations behind guarantee_online_cpus() :/
/*
* Must re-check here, to close a race against __kthread_bind(),
* sched_setaffinity() is not guaranteed to observe the flag.
--
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