Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755173AbWKRQPa (ORCPT ); Sat, 18 Nov 2006 11:15:30 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755178AbWKRQPa (ORCPT ); Sat, 18 Nov 2006 11:15:30 -0500 Received: from mx2.rowland.org ([192.131.102.7]:2321 "HELO mx2.rowland.org") by vger.kernel.org with SMTP id S1755173AbWKRQP3 (ORCPT ); Sat, 18 Nov 2006 11:15:29 -0500 Date: Sat, 18 Nov 2006 11:15:27 -0500 (EST) From: Alan Stern X-X-Sender: stern@netrider.rowland.org To: "Paul E. McKenney" cc: Oleg Nesterov , Kernel development list Subject: Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync In-Reply-To: <20061118002845.GF2632@us.ibm.com> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 7496 Lines: 223 There are a few things I don't like about this patch. On Fri, 17 Nov 2006, Paul E. McKenney wrote: > diff -urpNa -X dontdiff linux-2.6.19-rc5/kernel/srcu.c linux-2.6.19-rc5-dsrcu/kernel/srcu.c > --- linux-2.6.19-rc5/kernel/srcu.c 2006-11-17 13:54:17.000000000 -0800 > +++ linux-2.6.19-rc5-dsrcu/kernel/srcu.c 2006-11-17 14:15:06.000000000 -0800 > @@ -34,6 +34,18 @@ > #include > #include > > +/* > + * Initialize the per-CPU array, returning the pointer. > + */ > +static inline struct srcu_struct_array *alloc_srcu_struct_percpu(void) > +{ > + struct srcu_struct_array *sap; > + > + sap = alloc_percpu(struct srcu_struct_array); > + smp_wmb(); > + return (sap); Style: Don't use () here. > +} > + > /** > * init_srcu_struct - initialize a sleep-RCU structure > * @sp: structure to initialize. > @@ -94,7 +112,8 @@ void cleanup_srcu_struct(struct srcu_str > WARN_ON(sum); /* Leakage unless caller handles error. */ > if (sum != 0) > return; > - free_percpu(sp->per_cpu_ref); > + if (sp->per_cpu_ref != NULL) > + free_percpu(sp->per_cpu_ref); Now that Andrew has accepted the "allow free_percpu(NULL)" change, you can remove the test here. > sp->per_cpu_ref = NULL; > } > > @@ -105,18 +124,39 @@ void cleanup_srcu_struct(struct srcu_str > * Counts the new reader in the appropriate per-CPU element of the > * srcu_struct. Must be called from process context. > * Returns an index that must be passed to the matching srcu_read_unlock(). > + * The index is -1 if the srcu_struct is not and cannot be initialized. > */ > int srcu_read_lock(struct srcu_struct *sp) > { > int idx; > + struct srcu_struct_array *sap; > > preempt_disable(); > idx = sp->completed & 0x1; > - barrier(); /* ensure compiler looks -once- at sp->completed. */ > - per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]++; > - srcu_barrier(); /* ensure compiler won't misorder critical section. */ > + sap = rcu_dereference(sp->per_cpu_ref); > + if (likely(sap != NULL)) { > + barrier(); /* ensure compiler looks -once- at sp->completed. */ Put this barrier() back where the old one was (outside the "if"). > + per_cpu_ptr(rcu_dereference(sap), You don't need the rcu_dereference here, you already have it above. > + smp_processor_id())->c[idx]++; > + smp_mb(); > + preempt_enable(); > + return idx; > + } > + if (mutex_trylock(&sp->mutex)) { > + preempt_enable(); Move the preempt_enable() before the "if", then get rid of the preempt_enable() after the "if" block. > + if (sp->per_cpu_ref == NULL) > + sp->per_cpu_ref = alloc_srcu_struct_percpu(); It would be cleaner to put the mutex_unlock() and closing '}' right here. > + if (sp->per_cpu_ref == NULL) { > + atomic_inc(&sp->hardluckref); > + mutex_unlock(&sp->mutex); > + return -1; > + } > + mutex_unlock(&sp->mutex); > + return srcu_read_lock(sp); > + } > preempt_enable(); > - return idx; > + atomic_inc(&sp->hardluckref); > + return -1; > } > > /** > @@ -131,10 +171,17 @@ int srcu_read_lock(struct srcu_struct *s > */ > void srcu_read_unlock(struct srcu_struct *sp, int idx) > { > - preempt_disable(); > - srcu_barrier(); /* ensure compiler won't misorder critical section. */ > - per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]--; > - preempt_enable(); > + if (likely(idx != -1)) { > + preempt_disable(); > + smp_mb(); > + per_cpu_ptr(rcu_dereference(sp->per_cpu_ref), > + smp_processor_id())->c[idx]--; > + preempt_enable(); > + return; > + } > + mutex_lock(&sp->mutex); > + atomic_dec(&sp->hardluckref); > + mutex_unlock(&sp->mutex); You don't need the mutex to protect an atomic_dec. > } > > /** > @@ -158,6 +205,11 @@ void synchronize_srcu(struct srcu_struct > idx = sp->completed; > mutex_lock(&sp->mutex); > > + /* Initialize if not already initialized. */ > + > + if (sp->per_cpu_ref == NULL) > + sp->per_cpu_ref = alloc_srcu_struct_percpu(); What happens if a prior reader failed to allocate the memory but this call succeeds? You need to check hardluckref before doing this. The same is true in srcu_read_lock(). > + > /* > * Check to see if someone else did the work for us while we were > * waiting to acquire the lock. We need -two- advances of > @@ -173,65 +225,25 @@ void synchronize_srcu(struct srcu_struct > return; > } > > - synchronize_sched(); /* Force memory barrier on all CPUs. */ > - > - /* > - * The preceding synchronize_sched() ensures that any CPU that > - * sees the new value of sp->completed will also see any preceding > - * changes to data structures made by this CPU. This prevents > - * some other CPU from reordering the accesses in its SRCU > - * read-side critical section to precede the corresponding > - * srcu_read_lock() -- ensuring that such references will in > - * fact be protected. > - * > - * So it is now safe to do the flip. > - */ > - > + smp_mb(); /* ensure srcu_read_lock() sees prior change first! */ > idx = sp->completed & 0x1; > sp->completed++; > > - synchronize_sched(); /* Force memory barrier on all CPUs. */ > + synchronize_sched(); > > /* > * At this point, because of the preceding synchronize_sched(), > * all srcu_read_lock() calls using the old counters have completed. > * Their corresponding critical sections might well be still > * executing, but the srcu_read_lock() primitives themselves > - * will have finished executing. > + * will have finished executing. The "old" rank of counters > + * can therefore only decrease, never increase in value. > */ > > while (srcu_readers_active_idx(sp, idx)) > schedule_timeout_interruptible(1); > > - synchronize_sched(); /* Force memory barrier on all CPUs. */ > - > - /* > - * The preceding synchronize_sched() forces all srcu_read_unlock() > - * primitives that were executing concurrently with the preceding > - * for_each_possible_cpu() loop to have completed by this point. > - * More importantly, it also forces the corresponding SRCU read-side > - * critical sections to have also completed, and the corresponding > - * references to SRCU-protected data items to be dropped. > - * > - * Note: > - * > - * Despite what you might think at first glance, the > - * preceding synchronize_sched() -must- be within the > - * critical section ended by the following mutex_unlock(). > - * Otherwise, a task taking the early exit can race > - * with a srcu_read_unlock(), which might have executed > - * just before the preceding srcu_readers_active() check, > - * and whose CPU might have reordered the srcu_read_unlock() > - * with the preceding critical section. In this case, there > - * is nothing preventing the synchronize_sched() task that is > - * taking the early exit from freeing a data structure that > - * is still being referenced (out of order) by the task > - * doing the srcu_read_unlock(). > - * > - * Alternatively, the comparison with "2" on the early exit > - * could be changed to "3", but this increases synchronize_srcu() > - * latency for bulk loads. So the current code is preferred. > - */ > + smp_mb(); /* must see critical section prior to srcu_read_unlock() */ > > mutex_unlock(&sp->mutex); > } > Alan Stern - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/