linux-kernel - [tip: sched/core] sched: Fix race between yield_to() and try_to_wake

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Message-ID: <173693263944.31546.17125599138962679235.tip-bot2@tip-bot2>
Date: Wed, 15 Jan 2025 09:17:19 -0000
From: "tip-bot2 for Tianchen Ding" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Tianchen Ding <dtcccc@...ux.alibaba.com>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject:
 [tip: sched/core] sched: Fix race between yield_to() and try_to_wake_up()

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     5d808c78d97251af1d3a3e4f253e7d6c39fd871e
Gitweb:        https://git.kernel.org/tip/5d808c78d97251af1d3a3e4f253e7d6c39fd871e
Author:        Tianchen Ding <dtcccc@...ux.alibaba.com>
AuthorDate:    Tue, 31 Dec 2024 13:50:20 +08:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Mon, 13 Jan 2025 14:10:22 +01:00

sched: Fix race between yield_to() and try_to_wake_up()

We met a SCHED_WARN in set_next_buddy():
  __warn_printk
  set_next_buddy
  yield_to_task_fair
  yield_to
  kvm_vcpu_yield_to [kvm]
  ...

After a short dig, we found the rq_lock held by yield_to() may not
be exactly the rq that the target task belongs to. There is a race
window against try_to_wake_up().

         CPU0                             target_task

                                        blocking on CPU1
   lock rq0 & rq1
   double check task_rq == p_rq, ok
                                        woken to CPU2 (lock task_pi & rq2)
                                        task_rq = rq2
   yield_to_task_fair (w/o lock rq2)

In this race window, yield_to() is operating the task w/o the correct
lock. Fix this by taking task pi_lock first.

Fixes: d95f41220065 ("sched: Add yield_to(task, preempt) functionality")
Signed-off-by: Tianchen Ding <dtcccc@...ux.alibaba.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lkml.kernel.org/r/20241231055020.6521-1-dtcccc@linux.alibaba.com
---
 kernel/sched/syscalls.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
index ff0e5ab..943406c 100644
--- a/kernel/sched/syscalls.c
+++ b/kernel/sched/syscalls.c
@@ -1433,7 +1433,7 @@ int __sched yield_to(struct task_struct *p, bool preempt)
 	struct rq *rq, *p_rq;
 	int yielded = 0;
 
-	scoped_guard (irqsave) {
+	scoped_guard (raw_spinlock_irqsave, &p->pi_lock) {
 		rq = this_rq();
 
 again: