From 5f5fa7ea89dc82d34ed458f4d7a8634e8e9eefce Mon Sep 17 00:00:00 2001 From: Lai Jiangshan Date: Sat, 15 Feb 2020 15:23:26 -0800 Subject: [PATCH] rcu: Don't use negative nesting depth in __rcu_read_unlock() Now that RCU flavors have been consolidated, an RCU-preempt rcu_read_unlock() in an interrupt or softirq handler cannot possibly end the RCU read-side critical section. Consider the old vulnerability involving rcu_read_unlock() being invoked within such a handler that interrupted an __rcu_read_unlock_special(), in which a wakeup might be invoked with a scheduler lock held. Because rcu_read_unlock_special() no longer does wakeups in such situations, it is no longer necessary for __rcu_read_unlock() to set the nesting level negative. This commit therefore removes this recursion-protection code from __rcu_read_unlock(). [ paulmck: Let rcu_exp_handler() continue to call rcu_report_exp_rdp(). ] [ paulmck: Adjust other checks given no more negative nesting. ] Signed-off-by: Lai Jiangshan Signed-off-by: Paul E. McKenney --- kernel/rcu/tree_exp.h | 31 +++++-------------------------- kernel/rcu/tree_plugin.h | 22 +++++++--------------- 2 files changed, 12 insertions(+), 41 deletions(-) diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h index 1a617b9dffb0..0e5ccb330f9d 100644 --- a/kernel/rcu/tree_exp.h +++ b/kernel/rcu/tree_exp.h @@ -639,6 +639,7 @@ static void wait_rcu_exp_gp(struct work_struct *wp) */ static void rcu_exp_handler(void *unused) { + int depth = rcu_preempt_depth(); unsigned long flags; struct rcu_data *rdp = this_cpu_ptr(&rcu_data); struct rcu_node *rnp = rdp->mynode; @@ -649,7 +650,7 @@ static void rcu_exp_handler(void *unused) * critical section. If also enabled or idle, immediately * report the quiescent state, otherwise defer. */ - if (!rcu_preempt_depth()) { + if (!depth) { if (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) || rcu_dynticks_curr_cpu_in_eqs()) { rcu_report_exp_rdp(rdp); @@ -673,7 +674,7 @@ static void rcu_exp_handler(void *unused) * can have caused this quiescent state to already have been * reported, so we really do need to check ->expmask. */ - if (rcu_preempt_depth() > 0) { + if (depth > 0) { raw_spin_lock_irqsave_rcu_node(rnp, flags); if (rnp->expmask & rdp->grpmask) { rdp->exp_deferred_qs = true; @@ -683,30 +684,8 @@ static void rcu_exp_handler(void *unused) return; } - /* - * The final and least likely case is where the interrupted - * code was just about to or just finished exiting the RCU-preempt - * read-side critical section, and no, we can't tell which. - * So either way, set ->deferred_qs to flag later code that - * a quiescent state is required. - * - * If the CPU is fully enabled (or if some buggy RCU-preempt - * read-side critical section is being used from idle), just - * invoke rcu_preempt_deferred_qs() to immediately report the - * quiescent state. We cannot use rcu_read_unlock_special() - * because we are in an interrupt handler, which will cause that - * function to take an early exit without doing anything. - * - * Otherwise, force a context switch after the CPU enables everything. - */ - rdp->exp_deferred_qs = true; - if (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) || - WARN_ON_ONCE(rcu_dynticks_curr_cpu_in_eqs())) { - rcu_preempt_deferred_qs(t); - } else { - set_tsk_need_resched(t); - set_preempt_need_resched(); - } + // Finally, negative nesting depth should not happen. + WARN_ON_ONCE(1); } /* PREEMPTION=y, so no PREEMPTION=n expedited grace period to clean up after. */ diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index f31c5992f842..088e84e4578f 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -345,9 +345,7 @@ static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp) return READ_ONCE(rnp->gp_tasks) != NULL; } -/* Bias and limit values for ->rcu_read_lock_nesting. */ -#define RCU_NEST_BIAS INT_MAX -#define RCU_NEST_NMAX (-INT_MAX / 2) +/* limit value for ->rcu_read_lock_nesting. */ #define RCU_NEST_PMAX (INT_MAX / 2) static void rcu_preempt_read_enter(void) @@ -355,9 +353,9 @@ static void rcu_preempt_read_enter(void) current->rcu_read_lock_nesting++; } -static void rcu_preempt_read_exit(void) +static int rcu_preempt_read_exit(void) { - current->rcu_read_lock_nesting--; + return --current->rcu_read_lock_nesting; } static void rcu_preempt_depth_set(int val) @@ -390,21 +388,15 @@ void __rcu_read_unlock(void) { struct task_struct *t = current; - if (rcu_preempt_depth() != 1) { - rcu_preempt_read_exit(); - } else { + if (rcu_preempt_read_exit() == 0) { barrier(); /* critical section before exit code. */ - rcu_preempt_depth_set(-RCU_NEST_BIAS); - barrier(); /* assign before ->rcu_read_unlock_special load */ if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s))) rcu_read_unlock_special(t); - barrier(); /* ->rcu_read_unlock_special load before assign */ - rcu_preempt_depth_set(0); } if (IS_ENABLED(CONFIG_PROVE_LOCKING)) { int rrln = rcu_preempt_depth(); - WARN_ON_ONCE(rrln < 0 && rrln > RCU_NEST_NMAX); + WARN_ON_ONCE(rrln < 0 || rrln > RCU_NEST_PMAX); } } EXPORT_SYMBOL_GPL(__rcu_read_unlock); @@ -556,7 +548,7 @@ static bool rcu_preempt_need_deferred_qs(struct task_struct *t) { return (__this_cpu_read(rcu_data.exp_deferred_qs) || READ_ONCE(t->rcu_read_unlock_special.s)) && - rcu_preempt_depth() <= 0; + rcu_preempt_depth() == 0; } /* @@ -692,7 +684,7 @@ static void rcu_flavor_sched_clock_irq(int user) } else if (rcu_preempt_need_deferred_qs(t)) { rcu_preempt_deferred_qs(t); /* Report deferred QS. */ return; - } else if (!rcu_preempt_depth()) { + } else if (!WARN_ON_ONCE(rcu_preempt_depth())) { rcu_qs(); /* Report immediate QS. */ return; } -- 2.20.1