On Tue, Oct 13, 2020 at 12:44:50PM +0200, Peter Zijlstra wrote: > On Tue, Oct 13, 2020 at 12:34:06PM +0200, Peter Zijlstra wrote: > > On Mon, Oct 12, 2020 at 02:28:12PM -0700, Paul E. McKenney wrote: > > > It is certainly an accident waiting to happen. Would something like > > > the following make sense? > > > > Sadly no. > > > > > ------------------------------------------------------------------------ > > > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > > index bfd38f2..52a63bc 100644 > > > --- a/kernel/rcu/tree.c > > > +++ b/kernel/rcu/tree.c > > > @@ -4067,6 +4067,7 @@ void rcu_cpu_starting(unsigned int cpu) > > > > > > rnp = rdp->mynode; > > > mask = rdp->grpmask; > > > + lockdep_off(); > > > raw_spin_lock_irqsave_rcu_node(rnp, flags); > > > WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext | mask); > > > newcpu = !(rnp->expmaskinitnext & mask); > > > @@ -4086,6 +4087,7 @@ void rcu_cpu_starting(unsigned int cpu) > > > } else { > > > raw_spin_unlock_irqrestore_rcu_node(rnp, flags); > > > } > > > + lockdep_on(); > > > smp_mb(); /* Ensure RCU read-side usage follows above initialization. */ > > > } > > > > This will just shut it up, but will not fix the actual problem of that > > spin-lock ending up in trace_lock_acquire() which relies on RCU which > > isn't looking. > > > > What we need here is to supress tracing not lockdep. Let me consider. > > We appear to have a similar problem with rcu_report_dead(), it's > raw_spin_unlock()s can end up in trace_lock_release() while we just > killed RCU. So we can deal with the explicit trace_*() calls like the below, but I really don't like it much. It also doesn't help with function tracing. This is really early/late in the hotplug cycle and should be considered entry, we shouldn't be tracing anything here. Paul, would it be possible to use a scheme similar to IRQ/NMI for hotplug? That seems to mostly rely on atomic ops, not locks. --- diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h index d05db575f60f..22e3a3523ad3 100644 --- a/include/linux/lockdep.h +++ b/include/linux/lockdep.h @@ -159,7 +159,7 @@ extern void lockdep_init_task(struct task_struct *task); */ #define LOCKDEP_RECURSION_BITS 16 #define LOCKDEP_OFF (1U << LOCKDEP_RECURSION_BITS) -#define LOCKDEP_RECURSION_MASK (LOCKDEP_OFF - 1) +#define LOCKDEP_TRACE_MASK (LOCKDEP_OFF - 1) /* * lockdep_{off,on}() are macros to avoid tracing and kprobes; not inlines due @@ -176,6 +176,16 @@ do { \ current->lockdep_recursion -= LOCKDEP_OFF; \ } while (0) +#define lockdep_trace_off() \ +do { \ + current->lockdep_recursion++; \ +} while (0) + +#define lockdep_trace_on() \ +do { \ + current->lockdep_recursion-- \ +} while (0) + extern void lockdep_register_key(struct lock_class_key *key); extern void lockdep_unregister_key(struct lock_class_key *key); diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c index 3e99dfef8408..2df98abee82e 100644 --- a/kernel/locking/lockdep.c +++ b/kernel/locking/lockdep.c @@ -87,7 +87,7 @@ static inline bool lockdep_enabled(void) if (raw_cpu_read(lockdep_recursion)) return false; - if (current->lockdep_recursion) + if (current->lockdep_recursion >> LOCKDEP_RECURSION_BITS) return false; return true; @@ -5410,7 +5410,8 @@ void lock_acquire(struct lockdep_map *lock, unsigned int subclass, { unsigned long flags; - trace_lock_acquire(lock, subclass, trylock, read, check, nest_lock, ip); + if (!(current->lockdep_recursion & LOCKDEP_TRACE_MASK)) + trace_lock_acquire(lock, subclass, trylock, read, check, nest_lock, ip); if (!debug_locks) return; @@ -5450,7 +5451,8 @@ void lock_release(struct lockdep_map *lock, unsigned long ip) { unsigned long flags; - trace_lock_release(lock, ip); + if (!(current->lockdep_recursion & LOCKDEP_TRACE_MASK)) + trace_lock_release(lock, ip); if (unlikely(!lockdep_enabled())) return; @@ -5662,7 +5664,8 @@ void lock_contended(struct lockdep_map *lock, unsigned long ip) { unsigned long flags; - trace_lock_acquired(lock, ip); + if (!(current->lockdep_recursion & LOCKDEP_TRACE_MASK)) + trace_lock_acquired(lock, ip); if (unlikely(!lock_stat || !lockdep_enabled())) return; @@ -5680,7 +5683,8 @@ void lock_acquired(struct lockdep_map *lock, unsigned long ip) { unsigned long flags; - trace_lock_contended(lock, ip); + if (!(current->lockdep_recursion & LOCKDEP_TRACE_MASK)) + trace_lock_contended(lock, ip); if (unlikely(!lock_stat || !lockdep_enabled())) return; diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index edeabc232c21..dbd56603fc0a 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -4047,6 +4047,11 @@ void rcu_cpu_starting(unsigned int cpu) rnp = rdp->mynode; mask = rdp->grpmask; + + /* + * Lockdep will call tracing, which requires RCU, but RCU isn't on yet. + */ + lockdep_trace_off(); raw_spin_lock_irqsave_rcu_node(rnp, flags); WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext | mask); newcpu = !(rnp->expmaskinitnext & mask); @@ -4064,6 +4069,7 @@ void rcu_cpu_starting(unsigned int cpu) } else { raw_spin_unlock_irqrestore_rcu_node(rnp, flags); } + lockdep_trace_on(); smp_mb(); /* Ensure RCU read-side usage follows above initialization. */ } @@ -4091,6 +4097,11 @@ void rcu_report_dead(unsigned int cpu) /* Remove outgoing CPU from mask in the leaf rcu_node structure. */ mask = rdp->grpmask; + + /* + * Lockdep will call tracing, which requires RCU, but we're switching RCU off. + */ + lockdep_trace_off(); raw_spin_lock(&rcu_state.ofl_lock); raw_spin_lock_irqsave_rcu_node(rnp, flags); /* Enforce GP memory-order guarantee. */ rdp->rcu_ofl_gp_seq = READ_ONCE(rcu_state.gp_seq); @@ -4101,8 +4112,10 @@ void rcu_report_dead(unsigned int cpu) raw_spin_lock_irqsave_rcu_node(rnp, flags); } WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext & ~mask); + /* RCU is off, locks must not call into tracing */ raw_spin_unlock_irqrestore_rcu_node(rnp, flags); raw_spin_unlock(&rcu_state.ofl_lock); + lockdep_trace_on(); rdp->cpu_started = false; } diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c index 39334d2d2b37..403b138f7cd4 100644 --- a/kernel/rcu/update.c +++ b/kernel/rcu/update.c @@ -275,8 +275,8 @@ EXPORT_SYMBOL_GPL(rcu_callback_map); noinstr int notrace debug_lockdep_rcu_enabled(void) { - return rcu_scheduler_active != RCU_SCHEDULER_INACTIVE && debug_locks && - current->lockdep_recursion == 0; + return rcu_scheduler_active != RCU_SCHEDULER_INACTIVE && __lockdep_enabled; + } EXPORT_SYMBOL_GPL(debug_lockdep_rcu_enabled);