sched: Fix migrate_disable_switch() locking

For some reason migrate_disable_switch() was more complicated than it
needs to be, resulting in mind bending locking of dubious quality.

Recognise that migrate_disable_switch() must be called before a
context switch, but any place before that switch is equally good.
Since the current place results in troubled locking, simply move the
thing before taking rq->lock.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Juri Lelli <juri.lelli@redhat.com>
Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Vincent Guittot <vincent.guittot@linaro.org>

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 4a4dbce77bd758b5f1679b726258cb50e6830e6f..f2d16d10516a76065b0137eda64c2434caa0004c 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2346,10 +2346,10 @@ static void migrate_disable_switch(struct rq *rq, struct task_struct *p)
        if (p->cpus_ptr != &p->cpus_mask)
                return;
 
-       /*
-        * Violates locking rules! See comment in __do_set_cpus_allowed().
-        */
-       __do_set_cpus_allowed(p, &ac);
+       scoped_guard (task_rq_lock, p) {
+               update_rq_clock(scope.rq);
+               __do_set_cpus_allowed(p, &ac);
+       }
 }
 
 void ___migrate_enable(void)
@@ -2667,22 +2667,7 @@ __do_set_cpus_allowed(struct task_struct *p, struct affinity_context *ctx)
        struct rq *rq = task_rq(p);
        bool queued, running;
 
-       /*
-        * This here violates the locking rules for affinity, since we're only
-        * supposed to change these variables while holding both rq->lock and
-        * p->pi_lock.
-        *
-        * HOWEVER, it magically works, because ttwu() is the only code that
-        * accesses these variables under p->pi_lock and only does so after
-        * smp_cond_load_acquire(&p->on_cpu, !VAL), and we're in __schedule()
-        * before finish_task().
-        *
-        * XXX do further audits, this smells like something putrid.
-        */
-       if (ctx->flags & SCA_MIGRATE_DISABLE)
-               WARN_ON_ONCE(!p->on_cpu);
-       else
-               lockdep_assert_held(&p->pi_lock);
+       lockdep_assert_held(&p->pi_lock);
 
        queued = task_on_rq_queued(p);
        running = task_current_donor(rq, p);
@@ -6781,6 +6766,7 @@ static void __sched notrace __schedule(int sched_mode)
 
        local_irq_disable();
        rcu_note_context_switch(preempt);
+       migrate_disable_switch(rq, prev);
 
        /*
         * Make sure that signal_pending_state()->signal_pending() below
@@ -6887,7 +6873,6 @@ keep_resched:
                 */
                ++*switch_count;
 
-               migrate_disable_switch(rq, prev);
                psi_account_irqtime(rq, prev, next);
                psi_sched_switch(prev, next, !task_on_rq_queued(prev) ||
                                             prev->se.sched_delayed);