sched: Fix race against ptrace_freeze_trace() (d136122f) · Commits · 戴 / test

There is apparently one site that violates the rule that only current and ttwu() will modify task->state, namely ptrace_{,un}freeze_traced() will change task->state for a remote task. Oleg explains: "TASK_TRACED/TASK_STOPPED was always protected by siglock. In particular, ttwu(__TASK_TRACED) must be always called with siglock held. That is why ptrace_freeze_traced() assumes it can safely do s/TASK_TRACED/__TASK_TRACED/ under spin_lock(siglock)." This breaks the ordering scheme introduced by commit: ("sched: Fix loadavg accounting race") Specifically, the reload not matching no longer implies we don't have to block. Simply things by noting that what we need is a LOAD->STORE ordering and this can be provided by a control dependency. So replace: prev_state = prev->state; raw_spin_lock(&rq->lock); smp_mb__after_spinlock(); /* SMP-MB */ if (... && prev_state && prev_state == prev->state) deactivate_task(); with: prev_state = prev->state; if (... && prev_state) /* CTRL-DEP */ deactivate_task(); Since that already implies the 'prev->state' load must be complete before allowing the 'prev->on_rq = 0' store to become visible. Fixes: ("sched: Fix loadavg accounting race") Reported-by:

Jiri Slaby <jirislaby@kernel.org> Signed-off-by:

Peter Zijlstra (Intel) <peterz@infradead.org> Acked-by:

Oleg Nesterov <oleg@redhat.com> Tested-by:

Paul Gortmaker <paul.gortmaker@windriver.com> Tested-by:

Christian Brauner <christian.brauner@ubuntu.com>

kernel/sched/core.c

+14 −10

Original line number	Diff line number	Diff line
		@@ -4119,9 +4119,6 @@ static void __sched notrace __schedule(bool preempt)
		local_irq_disable();
		rcu_note_context_switch(preempt);

		/* See deactivate_task() below. */
		prev_state = prev->state;

		/*
		* Make sure that signal_pending_state()->signal_pending() below
		* can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)
		@@ -4145,11 +4142,16 @@ static void __sched notrace __schedule(bool preempt)
		update_rq_clock(rq);

		switch_count = &prev->nivcsw;

		/*
		* We must re-load prev->state in case ttwu_remote() changed it
		* before we acquired rq->lock.
		* We must load prev->state once (task_struct::state is volatile), such
		* that:
		*
		* - we form a control dependency vs deactivate_task() below.
		* - ptrace_{,un}freeze_traced() can change ->state underneath us.
		*/
		if (!preempt && prev_state && prev_state == prev->state) {
		prev_state = prev->state;
		if (!preempt && prev_state) {
		if (signal_pending_state(prev_state, prev)) {
		prev->state = TASK_RUNNING;
		} else {
		@@ -4163,10 +4165,12 @@ static void __sched notrace __schedule(bool preempt)

		/*
		* __schedule() ttwu()
		* prev_state = prev->state; if (READ_ONCE(p->on_rq) && ...)
		* LOCK rq->lock goto out;
		* smp_mb__after_spinlock(); smp_acquire__after_ctrl_dep();
		* p->on_rq = 0; p->state = TASK_WAKING;
		* prev_state = prev->state; if (p->on_rq && ...)
		* if (prev_state) goto out;
		* p->on_rq = 0; smp_acquire__after_ctrl_dep();
		* p->state = TASK_WAKING
		*
		* Where __schedule() and ttwu() have matching control dependencies.
		*
		* After this, schedule() must not care about p->state any more.
		*/

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