On Tue, Oct 13, 2020 at 01:25:44PM +0200, Peter Zijlstra wrote: > 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. The rest of the rcu_node tree and the various grace-period/hotplug races makes that question non-trivial. I will look into it, but I have no reason for optimism. But there is only one way to find out... ;-) Thanx, Paul > --- > 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); >
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