Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757963AbXEaP1P (ORCPT ); Thu, 31 May 2007 11:27:15 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752930AbXEaP1A (ORCPT ); Thu, 31 May 2007 11:27:00 -0400 Received: from mail.gmx.net ([213.165.64.20]:36401 "HELO mail.gmx.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1752728AbXEaP1A (ORCPT ); Thu, 31 May 2007 11:27:00 -0400 X-Authenticated: #5039886 X-Provags-ID: V01U2FsdGVkX18n8Bgs1M4ngIdqoD/4Rsk55tc+yGgeNdZIpeqdV/ KKJj7vGjUnAyU7 Date: Thu, 31 May 2007 17:27:09 +0200 From: =?iso-8859-1?Q?Bj=F6rn?= Steinbrink To: Eric Dumazet Cc: iank@bredband.net, Thomas Gleixner , Michal Piotrowski , Ingo Molnar , Arjan van de Ven , linux-kernel@vger.kernel.org Subject: Re: [BUG] Something goes wrong with timer statistics. Message-ID: <20070531152709.GA18775@atjola.homenet> Mail-Followup-To: =?iso-8859-1?Q?Bj=F6rn?= Steinbrink , Eric Dumazet , iank@bredband.net, Thomas Gleixner , Michal Piotrowski , Ingo Molnar , Arjan van de Ven , linux-kernel@vger.kernel.org References: <1180474738.22497.25.camel@pi.pomac.com> <6bffcb0e0705291524y2752d646p94b0bf6ca87af68f@mail.gmail.com> <1180478288.20546.39.camel@chaos> <20070530113825.GA5203@atjola.homenet> <20070530144449.36811a4d.dada1@cosmosbay.com> <20070530131458.GA5432@atjola.homenet> <20070531102047.GG19272@pomac.netswarm.net> <20070531142522.GA16690@atjola.homenet> <20070531171007.5780e5e6.dada1@cosmosbay.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20070531171007.5780e5e6.dada1@cosmosbay.com> User-Agent: Mutt/1.5.13 (2006-08-11) X-Y-GMX-Trusted: 0 Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2934 Lines: 104 On 2007.05.31 17:10:07 +0200, Eric Dumazet wrote: > Well... :) , there is still a memory barrier missing it seems. > > Another cpu might see a bad value if 'active=1' is set before tstat_hash_table is really cleared. Hm, that even makes the assumption in my first try valid ;-) Just for the record, this time I thought that the barrier from the spinlock in timer_stats_update_stats (right before the check for active) would be enough, but that's obviously running on the wrong cpu if we race... *sigh* Thanks, Bj?rn Fix two races in the timer stats lookup code. One by ensuring that the initialization of a new entry is finished upon insertion of that entry. The other by cleaning up the hash table when the entries array is cleared, so that we don't have "pre-inserted" entries. Thanks to Eric Dumazet for reminding me of the memory barriers. Signed-off-by: Bj?rn Steinbrink --- diff --git a/kernel/time/timer_stats.c b/kernel/time/timer_stats.c index 868f1bc..63d00f9 100644 --- a/kernel/time/timer_stats.c +++ b/kernel/time/timer_stats.c @@ -117,21 +117,6 @@ static struct entry entries[MAX_ENTRIES]; static atomic_t overflow_count; -static void reset_entries(void) -{ - nr_entries = 0; - memset(entries, 0, sizeof(entries)); - atomic_set(&overflow_count, 0); -} - -static struct entry *alloc_entry(void) -{ - if (nr_entries >= MAX_ENTRIES) - return NULL; - - return entries + nr_entries++; -} - /* * The entries are in a hash-table, for fast lookup: */ @@ -149,6 +134,22 @@ static struct entry *alloc_entry(void) static struct entry *tstat_hash_table[TSTAT_HASH_SIZE] __read_mostly; +static void reset_entries(void) +{ + nr_entries = 0; + memset(entries, 0, sizeof(entries)); + memset(tstat_hash_table, 0, sizeof(tstat_hash_table)); + atomic_set(&overflow_count, 0); +} + +static struct entry *alloc_entry(void) +{ + if (nr_entries >= MAX_ENTRIES) + return NULL; + + return entries + nr_entries++; +} + static int match_entries(struct entry *entry1, struct entry *entry2) { return entry1->timer == entry2->timer && @@ -202,12 +203,15 @@ static struct entry *tstat_lookup(struct entry *entry, char *comm) if (curr) { *curr = *entry; curr->count = 0; + curr->next = NULL; memcpy(curr->comm, comm, TASK_COMM_LEN); + + smp_mb(); /* Ensure that curr is initialized before insert */ + if (prev) prev->next = curr; else *head = curr; - curr->next = NULL; } out_unlock: spin_unlock(&table_lock); @@ -360,6 +364,7 @@ static ssize_t tstats_write(struct file *file, const char __user *buf, if (!active) { reset_entries(); time_start = ktime_get(); + smb_mb(); active = 1; } break; - 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/