linux-kernel - [tip:sched/core] sched/core: Allow __sched_setscheduler() in interrupts when PI is not used

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Date:   Tue, 23 May 2017 01:46:00 -0700
From:   "tip-bot for Steven Rostedt (VMware)" <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     rostedt@...dmis.org, ldufour@...ux.vnet.ibm.com, hpa@...or.com,
        peterz@...radead.org, torvalds@...ux-foundation.org,
        mingo@...nel.org, tglx@...utronix.de, linux-kernel@...r.kernel.org,
        akpm@...l.org
Subject: [tip:sched/core] sched/core: Allow __sched_setscheduler() in
 interrupts when PI is not used

Commit-ID:  896bbb2522587e3b8eb2a0d204d43ccc1042a00d
Gitweb:     http://git.kernel.org/tip/896bbb2522587e3b8eb2a0d204d43ccc1042a00d
Author:     Steven Rostedt (VMware) <rostedt@...dmis.org>
AuthorDate: Thu, 9 Mar 2017 10:18:42 -0500
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Tue, 23 May 2017 10:01:34 +0200

sched/core: Allow __sched_setscheduler() in interrupts when PI is not used

When priority inheritance was added back in 2.6.18 to sched_setscheduler(), it
added a path to taking an rt-mutex wait_lock, which is not IRQ safe. As PI
is not a common occurrence, lockdep will likely never trigger if
sched_setscheduler was called from interrupt context. A BUG_ON() was added
to trigger if __sched_setscheduler() was ever called from interrupt context
because there was a possibility to take the wait_lock.

Today the wait_lock is irq safe, but the path to taking it in
sched_setscheduler() is the same as the path to taking it from normal
context. The wait_lock is taken with raw_spin_lock_irq() and released with
raw_spin_unlock_irq() which will indiscriminately enable interrupts,
which would be bad in interrupt context.

The problem is that normalize_rt_tasks, which is called by triggering the
sysrq nice-all-RT-tasks was changed to call __sched_setscheduler(), and this
is done from interrupt context!

Now __sched_setscheduler() takes a "pi" parameter that is used to know if
the priority inheritance should be called or not. As the BUG_ON() only cares
about calling the PI code, it should only bug if called from interrupt
context with the "pi" parameter set to true.

Reported-by: Laurent Dufour <ldufour@...ux.vnet.ibm.com>
Tested-by: Laurent Dufour <ldufour@...ux.vnet.ibm.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Andrew Morton <akpm@...l.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Fixes: dbc7f069b93a ("sched: Use replace normalize_task() with __sched_setscheduler()")
Link: http://lkml.kernel.org/r/20170308124654.10e598f2@gandalf.local.home
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/sched/core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 4a31239..877241e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4188,8 +4188,8 @@ static int __sched_setscheduler(struct task_struct *p,
 	int queue_flags = DEQUEUE_SAVE | DEQUEUE_MOVE | DEQUEUE_NOCLOCK;
 	struct rq *rq;

-	/* May grab non-irq protected spin_locks: */
-	BUG_ON(in_interrupt());
+	/* The pi code expects interrupts enabled */
+	BUG_ON(pi && in_interrupt());
 recheck:
 	/* Double check policy once rq lock held: */
 	if (policy < 0) {