Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756103AbWKRA1i (ORCPT ); Fri, 17 Nov 2006 19:27:38 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1756104AbWKRA1h (ORCPT ); Fri, 17 Nov 2006 19:27:37 -0500 Received: from e3.ny.us.ibm.com ([32.97.182.143]:47756 "EHLO e3.ny.us.ibm.com") by vger.kernel.org with ESMTP id S1756103AbWKRA1g (ORCPT ); Fri, 17 Nov 2006 19:27:36 -0500 Date: Fri, 17 Nov 2006 16:28:45 -0800 From: "Paul E. McKenney" To: Oleg Nesterov Cc: Jens Axboe , Alan Stern , Linus Torvalds , Thomas Gleixner , Ingo Molnar , LKML , john stultz , David Miller , Arjan van de Ven , Andrew Morton , Andi Kleen , manfred@colorfullife.com Subject: Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync Message-ID: <20061118002845.GF2632@us.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20061117065128.GA5452@us.ibm.com> <20061117092925.GT7164@kernel.dk> <20061117183945.GA367@oleg> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20061117183945.GA367@oleg> User-Agent: Mutt/1.4.1i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 11549 Lines: 352 On Fri, Nov 17, 2006 at 09:39:45PM +0300, Oleg Nesterov wrote: > Paul E. McKenney wrote: > > > > int srcu_read_lock(struct srcu_struct *sp) > > { > > @@ -112,11 +126,24 @@ int srcu_read_lock(struct srcu_struct *s > > > > 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. */ > > + if (likely(sp->per_cpu_ref != NULL)) { > > + barrier(); /* ensure compiler looks -once- at sp->completed. */ > > + per_cpu_ptr(rcu_dereference(sp->per_cpu_ref), > > + smp_processor_id())->c[idx]++; > > + smp_mb(); > > + preempt_enable(); > > + return idx; > > + } > > preempt_enable(); > > - return idx; > > + mutex_lock(&sp->mutex); > > + sp->per_cpu_ref = alloc_srcu_struct_percpu(); > > We should re-check sp->per_cpu_ref != NULL after taking sp->mutex, > it was probably allocated by another thread. Good catch!!! > > 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(sp->per_cpu_ref, smp_processor_id())->c[idx]--; > > + preempt_enable(); > > + return; > > + } > > + mutex_lock(&sp->mutex); > > + sp->hardluckref--; > > + mutex_unlock(&sp->mutex); > > } > > I think this is deadlockable, synchronize_srcu() does > > while (srcu_readers_active_idx(sp, idx)) > schedule_timeout_interruptible(1); > > under sp->mutex, so the loop above may spin forever while the reader > waits for sp->mutex in srcu_read_unlock(sp, -1). Indeed it is! This requires a nested reader, so that the outer reader blocks synchronize_srcu() and synchronize_srcu() blocks the inner reader -- but that is legal. So I made hardluckref be an atomic_t, and changed the mutex_lock() in srcu_read_lock() be a mutex_trylock() -- which cannot block, right? I also added the srcu_readers_active() declaration to srcu.h for Jens. Oleg, any thoughts about Jens's optimization? He would code something like: if (srcu_readers_active(&my_srcu)) synchronize_srcu(); else smp_mb(); However, he is doing ordered I/O requests rather than protecting data structures. Changes: o Make hardluckref be an atomic_t. o Put the now-needed rcu_dereference()s for per_cpu_ref (used to be constant...). o Moved to mutex_trylock() in srcu_read_lock() to avoid Oleg's deadlock scenario. o Added per_cpu_ref NULL rechecks to avoid the Oleg's memory leak (and worse). o Added srcu_readers_active() to srcu.h. Still untested (aside from Jens's runs). Signed-off-by: paulmck@linux.vnet.ibm.com (AKA paulmck@us.ibm.com) --- include/linux/srcu.h | 8 --- kernel/srcu.c | 130 +++++++++++++++++++++++++++------------------------ 2 files changed, 73 insertions(+), 65 deletions(-) diff -urpNa -X dontdiff linux-2.6.19-rc5/include/linux/srcu.h linux-2.6.19-rc5-dsrcu/include/linux/srcu.h --- linux-2.6.19-rc5/include/linux/srcu.h 2006-11-17 13:54:15.000000000 -0800 +++ linux-2.6.19-rc5-dsrcu/include/linux/srcu.h 2006-11-17 15:14:07.000000000 -0800 @@ -35,19 +35,15 @@ struct srcu_struct { int completed; struct srcu_struct_array *per_cpu_ref; struct mutex mutex; + atomic_t hardluckref; }; -#ifndef CONFIG_PREEMPT -#define srcu_barrier() barrier() -#else /* #ifndef CONFIG_PREEMPT */ -#define srcu_barrier() -#endif /* #else #ifndef CONFIG_PREEMPT */ - int init_srcu_struct(struct srcu_struct *sp); void cleanup_srcu_struct(struct srcu_struct *sp); int srcu_read_lock(struct srcu_struct *sp) __acquires(sp); void srcu_read_unlock(struct srcu_struct *sp, int idx) __releases(sp); void synchronize_srcu(struct srcu_struct *sp); long srcu_batches_completed(struct srcu_struct *sp); +int srcu_readers_active(struct srcu_struct *sp); #endif 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); +} + /** * init_srcu_struct - initialize a sleep-RCU structure * @sp: structure to initialize. @@ -46,7 +58,8 @@ int init_srcu_struct(struct srcu_struct { sp->completed = 0; mutex_init(&sp->mutex); - sp->per_cpu_ref = alloc_percpu(struct srcu_struct_array); + sp->per_cpu_ref = alloc_srcu_struct_percpu(); + atomic_set(&sp->hardluckref, 0); return (sp->per_cpu_ref ? 0 : -ENOMEM); } @@ -58,12 +71,15 @@ int init_srcu_struct(struct srcu_struct static int srcu_readers_active_idx(struct srcu_struct *sp, int idx) { int cpu; + struct srcu_struct_array *sap; int sum; sum = 0; - for_each_possible_cpu(cpu) - sum += per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx]; - return sum; + sap = rcu_dereference(sp->per_cpu_ref); + if (likely(sap != NULL)) + for_each_possible_cpu(cpu) + sum += per_cpu_ptr(sap, cpu)->c[idx]; + return sum + atomic_read(&sp->hardluckref); } /** @@ -76,7 +92,9 @@ static int srcu_readers_active_idx(struc */ int srcu_readers_active(struct srcu_struct *sp) { - return srcu_readers_active_idx(sp, 0) + srcu_readers_active_idx(sp, 1); + return srcu_readers_active_idx(sp, 0) + + srcu_readers_active_idx(sp, 1) - + atomic_read(&sp->hardluckref); /* No one will care, but... */ } /** @@ -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); 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. */ + per_cpu_ptr(rcu_dereference(sap), + smp_processor_id())->c[idx]++; + smp_mb(); + preempt_enable(); + return idx; + } + if (mutex_trylock(&sp->mutex)) { + preempt_enable(); + if (sp->per_cpu_ref == NULL) + sp->per_cpu_ref = alloc_srcu_struct_percpu(); + 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); } /** @@ -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(); + /* * 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); } - 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/