Commit 64e6fa76 authored by Pingfan Liu's avatar Pingfan Liu Committed by Peter Zijlstra
Browse files

sched/deadline: Fix potential race in dl_add_task_root_domain() 

The access rule for local_cpu_mask_dl requires it to be called on the
local CPU with preemption disabled. However, dl_add_task_root_domain()
currently violates this rule.

Without preemption disabled, the following race can occur:

1. ThreadA calls dl_add_task_root_domain() on CPU 0
2. Gets pointer to CPU 0's local_cpu_mask_dl
3. ThreadA is preempted and migrated to CPU 1
4. ThreadA continues using CPU 0's local_cpu_mask_dl
5. Meanwhile, the scheduler on CPU 0 calls find_later_rq() which also
   uses local_cpu_mask_dl (with preemption properly disabled)
6. Both contexts now corrupt the same per-CPU buffer concurrently

Fix this by moving the local_cpu_mask_dl access to the preemption
disabled section.

Closes: https://lore.kernel.org/lkml/aSBjm3mN_uIy64nz@jlelli-thinkpadt14gen4.remote.csb


Fixes: 318e18ed ("sched/deadline: Walk up cpuset hierarchy to decide root domain when hot-unplug")
Reported-by: default avatarJuri Lelli <juri.lelli@redhat.com>
Signed-off-by: default avatarPingfan Liu <piliu@redhat.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: default avatarJuri Lelli <juri.lelli@redhat.com>
Acked-by: default avatarWaiman Long <longman@redhat.com>
Link: https://patch.msgid.link/20251125032630.8746-3-piliu@redhat.com
parent 479972ef

kernel/sched/deadline.c

+2 −1
Original line number Diff line number Diff line
@@ -3154,7 +3154,7 @@ void dl_add_task_root_domain(struct task_struct *p) 
	struct rq *rq;

	struct dl_bw *dl_b;

	unsigned int cpu;

	struct cpumask *msk = this_cpu_cpumask_var_ptr(local_cpu_mask_dl);

	struct cpumask *msk;



	raw_spin_lock_irqsave(&p->pi_lock, rf.flags);

	if (!dl_task(p) || dl_entity_is_special(&p->dl)) {
@@ -3162,6 +3162,7 @@ void dl_add_task_root_domain(struct task_struct *p) 
		return;

	}



	msk = this_cpu_cpumask_var_ptr(local_cpu_mask_dl);

	dl_get_task_effective_cpus(p, msk);

	cpu = cpumask_first_and(cpu_active_mask, msk);

	BUG_ON(cpu >= nr_cpu_ids);