Messages in this thread | | | Date | Thu, 8 Mar 2018 16:30:06 +0800 | From | Boqun Feng <> | Subject | Re: [PATCH] rcu: exp: Fix "must hold exp_mutex" comments for QS reporting functions |
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On Thu, Mar 08, 2018 at 12:54:29PM +0800, Boqun Feng wrote: > On Wed, Mar 07, 2018 at 08:30:17PM -0800, Paul E. McKenney wrote: > [...] > > > > > > +/* > > > + * Like sync_rcu_preempt_exp_done(), but this function assumes the caller > > > + * doesn't hold the rcu_node's ->lock, and will acquire and release the lock > > > + * itself > > > + */ > > > +static bool sync_rcu_preempt_exp_done_unlocked(struct rcu_node *rnp) > > > +{ > > > + unsigned long flags; > > > + bool ret; > > > + > > > + raw_spin_lock_irqsave_rcu_node(rnp, flags); > > > + ret = sync_rcu_preempt_exp_done(rnp); > > > > Let's see... The sync_rcu_preempt_exp_done() function checks the > > ->exp_tasks pointer and the ->expmask bitmask. The number of bits in the > > mask can only decrease, and the ->exp_tasks pointer can only transition > > from NULL to non-NULL when there is at least one bit set. However, > > there is no ordering in sync_rcu_preempt_exp_done(), so it is possible > > that it could be fooled without the lock: > > > > o CPU 0 in sync_rcu_preempt_exp_done() reads ->exp_tasks and > > sees that it is NULL. > > > > o CPU 1 blocks within an RCU read-side critical section, so > > it enqueues the task and points ->exp_tasks at it and > > clears CPU 1's bit in ->expmask. > > > > o All other CPUs clear their bits in ->expmask. > > > > o CPU 0 reads ->expmask, sees that it is zero, so incorrectly > > concludes that all quiescent states have completed, despite > > the fact that ->exp_tasks is non-NULL. > > > > So it seems to me that the lock is needed. Good catch!!! The problem > > would occur only if the task running on CPU 0 received a spurious > > wakeup, but that could potentially happen. > > > > Thanks for the analysis ;-) > > > If lock contention becomes a problem, memory-ordering tricks could be > > applied, but the lock is of course simpler. > > > > Agreed. > > > I am guessing that this is a prototype patch, and that you are planning > > Yes, this is a prototype. And I'm preparing a proper patch to send > later. > > > to add lockdep annotations in more places, but either way please let > > me know. > > > > Give it's a bug as per your analysis, I'd like to defer other lockdep > annotations and send this first. However, I'm currently getting other > lockdep splats after applying this, so I need to get that sorted first. >
Hmm.. the other lockdep splat seems irrelevant with my patch, I could observe it on mainline using rcutorture with CONFIG_PROVE_LOCKING=y. I'd spend some more time on it, in the meanwhile, send a proper patch for this sync_rcu_preempt_exp_done().
Regards, Boqun
> Regards, > Boqun > > > Thanx, Paul > > > > > + raw_spin_unlock_irqrestore_rcu_node(rnp, flags); > > > + > > > + return ret; > > > +} > > > + > > > + > > > /* > > > * Report the exit from RCU read-side critical section for the last task > > > * that queued itself during or before the current expedited preemptible-RCU > > > @@ -490,6 +512,7 @@ static void synchronize_sched_expedited_wait(struct rcu_state *rsp) > > > struct rcu_node *rnp; > > > struct rcu_node *rnp_root = rcu_get_root(rsp); > > > int ret; > > > + unsigned long flags; > > > > > > trace_rcu_exp_grace_period(rsp->name, rcu_exp_gp_seq_endval(rsp), TPS("startwait")); > > > jiffies_stall = rcu_jiffies_till_stall_check(); > > > @@ -498,9 +521,9 @@ static void synchronize_sched_expedited_wait(struct rcu_state *rsp) > > > for (;;) { > > > ret = swait_event_timeout( > > > rsp->expedited_wq, > > > - sync_rcu_preempt_exp_done(rnp_root), > > > + sync_rcu_preempt_exp_done_unlocked(rnp_root), > > > jiffies_stall); > > > - if (ret > 0 || sync_rcu_preempt_exp_done(rnp_root)) > > > + if (ret > 0 || sync_rcu_preempt_exp_done_unlocked(rnp_root)) > > > return; > > > WARN_ON(ret < 0); /* workqueues should not be signaled. */ > > > if (rcu_cpu_stall_suppress) > > > @@ -533,8 +556,14 @@ static void synchronize_sched_expedited_wait(struct rcu_state *rsp) > > > rcu_for_each_node_breadth_first(rsp, rnp) { > > > if (rnp == rnp_root) > > > continue; /* printed unconditionally */ > > > - if (sync_rcu_preempt_exp_done(rnp)) > > > + > > > + raw_spin_lock_irqsave_rcu_node(rnp, flags); > > > + if (sync_rcu_preempt_exp_done(rnp)) { > > > + raw_spin_unlock_irqrestore_rcu_node(rnp, flags); > > > continue; > > > + } > > > + raw_spin_unlock_irqrestore_rcu_node(rnp, flags); > > > + > > > pr_cont(" l=%u:%d-%d:%#lx/%c", > > > rnp->level, rnp->grplo, rnp->grphi, > > > rnp->expmask, > > > -- > > > 2.16.2 > > > > > > >
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