sched/core: Fix DEBUG_SPINLOCK annotation for rq->lock

Mark noticed that he had sporadic "spinlock recursion" warnings from
the DEBUG_SPINLOCK code. Now rq->lock is special in that the owner
changes in the middle of a context switch.

It so happens that we fix up the lock.owner too late, @prev can run
(remotely) the moment prev->on_cpu is cleared, this then allows @prev
to again try and acquire this rq->lock and trigger this warning.

So we have to switch lock.owner before clearing prev->on_cpu.

Do this by moving the DEBUG_SPINLOCK annotation from after switch_to()
to before switch_to() and collect all lockdep annotations there into
prepare_lock_switch() to mirror the existing finish_lock_switch().

Debugged-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index bf724c1952eaca3b1eb7cb6bf83b2fa436068f4d..e7c535eee0a6d493a2a43eba210c08c6858b63d1 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2601,19 +2601,31 @@ static inline void finish_task(struct task_struct *prev)
 #endif
 }
 
-static inline void finish_lock_switch(struct rq *rq)
+static inline void
+prepare_lock_switch(struct rq *rq, struct task_struct *next, struct rq_flags *rf)
 {
+       /*
+        * Since the runqueue lock will be released by the next
+        * task (which is an invalid locking op but in the case
+        * of the scheduler it's an obvious special-case), so we
+        * do an early lockdep release here:
+        */
+       rq_unpin_lock(rq, rf);
+       spin_release(&rq->lock.dep_map, 1, _THIS_IP_);
 #ifdef CONFIG_DEBUG_SPINLOCK
        /* this is a valid case when another task releases the spinlock */
-       rq->lock.owner = current;
+       rq->lock.owner = next;
 #endif
+}
+
+static inline void finish_lock_switch(struct rq *rq)
+{
        /*
         * If we are tracking spinlock dependencies then we have to
         * fix up the runqueue lock - which gets 'carried over' from
         * prev into current:
         */
        spin_acquire(&rq->lock.dep_map, 0, 0, _THIS_IP_);
-
        raw_spin_unlock_irq(&rq->lock);
 }
 
@@ -2844,14 +2856,7 @@ context_switch(struct rq *rq, struct task_struct *prev,
 
        rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP);
 
-       /*
-        * Since the runqueue lock will be released by the next
-        * task (which is an invalid locking op but in the case
-        * of the scheduler it's an obvious special-case), so we
-        * do an early lockdep release here:
-        */
-       rq_unpin_lock(rq, rf);
-       spin_release(&rq->lock.dep_map, 1, _THIS_IP_);
+       prepare_lock_switch(rq, next, rf);
 
        /* Here we just switch the register state and the stack. */
        switch_to(prev, next, prev);