Messages in this thread Patch in this message | | | Date | Wed, 27 Jun 2018 10:33:35 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH tip/core/rcu 13/22] rcu: Fix grace-period hangs due to race with CPU offline |
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On Tue, Jun 26, 2018 at 04:40:04PM -0700, Paul E. McKenney wrote: > The options I have considered are as follows: > > 1. Stick with the no-failsafe approach, adding the lock as shown > in this patch. (I obviously prefer this approach.) > > 2. Stick with the no-failsafe approach, but rely on RCU's grace-period > kthread to wake up later due to its timed wait during the > force-quiescent-state process. This would be a bit obnoxious, > as it requires passing a don't-wake flag (or some such) up the > quiescent-state reporting mechanism. It would also needlessly > delay grace-period ends, especially on large systems (RCU scales > up the FQS delay on larger systems to maintain limited CPU > consumption per unit time). > > 3. Stick with the no-failsafe approach, but have the quiescent-state > reporting code hand back a value indicating that a wakeup is needed. > Also a bit obnoxious, as this value would need to be threaded up > the reporting code's return path. Simple in theory, but a bit > of an ugly change, especially for the many places in the code that > currently expect quiescent-state reporting to be an unconditional > fire-and-forget operation.
You can combine 2 and 3. Use a skip wakeup flag and ignore the return value most times. Let me do that just to see how horrible it is.
> > 4. Re-introduce the old fail-safe code, and don't report the > quiescent state at CPU-offline time, relying on the fail-safe > code to do so. This also potentially introduces delays and can > add extra FQS scans, which in turn increases lock contention. > > So what did you have in mind?
The thing I talked about last night before crashing is the patch below; it does however suffer from a little false-negative, much like the one you explained earlier. It allows @qsmaskinit to retain the bit set after offline.
I had hoped to be able to clear @qsmaskinit unconditionally, but that doesn't quite work.
The other approach is yet another mask @qsmaskofflinenext which the kthread will use to clear bits on @qsmaskinitnext.
In any case, aside from the above, the below contains a bunch of missing WRITE_ONCE()s. Since you read the various @qsmask variables using READ_ONCE() you must also consistently update them using WRITE_ONCE(), otherwise it's all still buggered.
--- diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 7832dd556490..8713048d5103 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -104,7 +104,6 @@ struct rcu_state sname##_state = { \ .abbr = sabbr, \ .exp_mutex = __MUTEX_INITIALIZER(sname##_state.exp_mutex), \ .exp_wake_mutex = __MUTEX_INITIALIZER(sname##_state.exp_wake_mutex), \ - .ofl_lock = __SPIN_LOCK_UNLOCKED(sname##_state.ofl_lock), \ } RCU_STATE_INITIALIZER(rcu_sched, 's', call_rcu_sched); @@ -209,7 +208,12 @@ EXPORT_SYMBOL_GPL(rcu_get_gp_kthreads_prio); */ unsigned long rcu_rnp_online_cpus(struct rcu_node *rnp) { - return READ_ONCE(rnp->qsmaskinitnext); + /* + * For both online and offline we first set/clear @qsmaskinitnext, + * and complete by propagating into @qsmaskinit. As long as the bit + * remains in either mask, RCU is still online. + */ + return READ_ONCE(rnp->qsmaskinit) | READ_ONCE(rnp->qsmaskinitnext); } /* @@ -1928,19 +1932,17 @@ static bool rcu_gp_init(struct rcu_state *rsp) */ rsp->gp_state = RCU_GP_ONOFF; rcu_for_each_leaf_node(rsp, rnp) { - spin_lock(&rsp->ofl_lock); raw_spin_lock_irq_rcu_node(rnp); if (rnp->qsmaskinit == rnp->qsmaskinitnext && !rnp->wait_blkd_tasks) { /* Nothing to do on this leaf rcu_node structure. */ raw_spin_unlock_irq_rcu_node(rnp); - spin_unlock(&rsp->ofl_lock); continue; } /* Record old state, apply changes to ->qsmaskinit field. */ oldmask = rnp->qsmaskinit; - rnp->qsmaskinit = rnp->qsmaskinitnext; + WRITE_ONCE(rnp->qsmaskinit, rnp->qsmaskinitnext); /* If zero-ness of ->qsmaskinit changed, propagate up tree. */ if (!oldmask != !rnp->qsmaskinit) { @@ -1970,7 +1972,6 @@ static bool rcu_gp_init(struct rcu_state *rsp) } raw_spin_unlock_irq_rcu_node(rnp); - spin_unlock(&rsp->ofl_lock); } rcu_gp_slow(rsp, gp_preinit_delay); /* Races with CPU hotplug. */ @@ -1992,7 +1993,7 @@ static bool rcu_gp_init(struct rcu_state *rsp) raw_spin_lock_irqsave_rcu_node(rnp, flags); rdp = this_cpu_ptr(rsp->rda); rcu_preempt_check_blocked_tasks(rsp, rnp); - rnp->qsmask = rnp->qsmaskinit; + WRITE_ONCE(rnp->qsmask, rnp->qsmaskinit); WRITE_ONCE(rnp->gp_seq, rsp->gp_seq); if (rnp == rdp->mynode) (void)__note_gp_changes(rsp, rnp, rdp); @@ -2295,7 +2296,7 @@ rcu_report_qs_rnp(unsigned long mask, struct rcu_state *rsp, WARN_ON_ONCE(oldmask); /* Any child must be all zeroed! */ WARN_ON_ONCE(!rcu_is_leaf_node(rnp) && rcu_preempt_blocked_readers_cgp(rnp)); - rnp->qsmask &= ~mask; + WRITE_ONCE(rnp->qsmask, rnp->qsmask & ~mask); trace_rcu_quiescent_state_report(rsp->name, rnp->gp_seq, mask, rnp->qsmask, rnp->level, rnp->grplo, rnp->grphi, @@ -2503,7 +2504,7 @@ static void rcu_cleanup_dead_rnp(struct rcu_node *rnp_leaf) if (!rnp) break; raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */ - rnp->qsmaskinit &= ~mask; + WRITE_ONCE(rnp->qsmaskinit, rnp->qsmaskinit & ~mask); /* Between grace periods, so better already be zero! */ WARN_ON_ONCE(rnp->qsmask); if (rnp->qsmaskinit) { @@ -3522,7 +3523,7 @@ static void rcu_init_new_rnp(struct rcu_node *rnp_leaf) return; raw_spin_lock_rcu_node(rnp); /* Interrupts already disabled. */ oldmask = rnp->qsmaskinit; - rnp->qsmaskinit |= mask; + WRITE_ONCE(rnp->qsmaskinit, rnp->qsmaskinit | mask); raw_spin_unlock_rcu_node(rnp); /* Interrupts remain disabled. */ if (oldmask) return; @@ -3733,7 +3734,7 @@ void rcu_cpu_starting(unsigned int cpu) rnp = rdp->mynode; mask = rdp->grpmask; raw_spin_lock_irqsave_rcu_node(rnp, flags); - rnp->qsmaskinitnext |= mask; + WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext | mask); oldmask = rnp->expmaskinitnext; rnp->expmaskinitnext |= mask; oldmask ^= rnp->expmaskinitnext; @@ -3768,18 +3769,36 @@ static void rcu_cleanup_dying_idle_cpu(int cpu, struct rcu_state *rsp) /* Remove outgoing CPU from mask in the leaf rcu_node structure. */ mask = rdp->grpmask; - spin_lock(&rsp->ofl_lock); raw_spin_lock_irqsave_rcu_node(rnp, flags); /* Enforce GP memory-order guarantee. */ rdp->rcu_ofl_gp_seq = READ_ONCE(rsp->gp_seq); rdp->rcu_ofl_gp_flags = READ_ONCE(rsp->gp_flags); + + /* + * First clear @qsmaskinitnext such that we'll not start a new GP + * on this outgoing CPU. + */ + WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext & ~mask); if (rnp->qsmask & mask) { /* RCU waiting on outgoing CPU? */ - /* Report quiescent state -before- changing ->qsmaskinitnext! */ + /* + * Report the QS on the outgoing CPU. This will propagate the + * cleared bit into @qsmaskinit and @qsmask. We rely on + * @qsmaskinit still containing this CPU such that + * rcu_rnp_online_cpus() will still consider RCU online. + * + * This allows us to wake the GP kthread, since wakeups rely on + * RCU. + */ + WARN_ON_ONCE(!(rnp->qsmaskinit & mask)); rcu_report_qs_rnp(mask, rsp, rnp, rnp->gp_seq, flags); raw_spin_lock_irqsave_rcu_node(rnp, flags); + } else { + /* + * If there was no QS required, clear @qsmaskinit now to + * finalize the offline. + */ + WRITE_ONCE(rnp->qsmaskinit, rnp->qsmaskinit & ~mask); } - rnp->qsmaskinitnext &= ~mask; raw_spin_unlock_irqrestore_rcu_node(rnp, flags); - spin_unlock(&rsp->ofl_lock); } /* diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h index 4e74df768c57..a1528b970257 100644 --- a/kernel/rcu/tree.h +++ b/kernel/rcu/tree.h @@ -84,19 +84,24 @@ struct rcu_node { unsigned long gp_seq; /* Track rsp->rcu_gp_seq. */ unsigned long gp_seq_needed; /* Track rsp->rcu_gp_seq_needed. */ unsigned long completedqs; /* All QSes done for this node. */ - unsigned long qsmask; /* CPUs or groups that need to switch in */ - /* order for current grace period to proceed.*/ - /* In leaf rcu_node, each bit corresponds to */ - /* an rcu_data structure, otherwise, each */ - /* bit corresponds to a child rcu_node */ - /* structure. */ - unsigned long rcu_gp_init_mask; /* Mask of offline CPUs at GP init. */ + + /* + * @qsmask - CPUs pending in this GP + * @qsmaskinit - CPUs we started this GP with + * @qsmaskinitnext - CPUs we'll start the next GP with + * + * online: we add the incoming CPU to @qsmaskinitnext which will then + * be propagated into @qsmaskinit and @qsmask by starting/joining a GP. + * + * offline: we remove the CPU from @qsmaskinitnext such that the + * outgoing CPU will not be part of a next GP, which will then be + * propagated into @qsmaskinit and @qsmask by completing/leaving a GP. + */ + unsigned long qsmask; unsigned long qsmaskinit; - /* Per-GP initial value for qsmask. */ - /* Initialized from ->qsmaskinitnext at the */ - /* beginning of each grace period. */ unsigned long qsmaskinitnext; - /* Online CPUs for next grace period. */ + + unsigned long rcu_gp_init_mask; /* Mask of offline CPUs at GP init. */ unsigned long expmask; /* CPUs or groups that need to check in */ /* to allow the current expedited GP */ /* to complete. */ @@ -367,10 +372,6 @@ struct rcu_state { const char *name; /* Name of structure. */ char abbr; /* Abbreviated name. */ struct list_head flavors; /* List of RCU flavors. */ - - spinlock_t ofl_lock ____cacheline_internodealigned_in_smp; - /* Synchronize offline with */ - /* GP pre-initialization. */ }; /* Values for rcu_state structure's gp_flags field. */
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