Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752604AbbFYAQW (ORCPT ); Wed, 24 Jun 2015 20:16:22 -0400 Received: from mga11.intel.com ([192.55.52.93]:61621 "EHLO mga11.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751724AbbFYAQH (ORCPT ); Wed, 24 Jun 2015 20:16:07 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.13,673,1427785200"; d="scan'208";a="734110603" Subject: [RFCv2][PATCH 2/7] fs: use RCU for free_super() vs. __sb_start_write() To: dave@sr71.net Cc: jack@suse.cz, viro@zeniv.linux.org.uk, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, paulmck@linux.vnet.ibm.com, tim.c.chen@linux.intel.com, ak@linux.intel.com, dave.hansen@linux.intel.com From: Dave Hansen Date: Wed, 24 Jun 2015 17:16:05 -0700 References: <20150625001605.72553909@viggo.jf.intel.com> In-Reply-To: <20150625001605.72553909@viggo.jf.intel.com> Message-Id: <20150625001605.50C9BE41@viggo.jf.intel.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5845 Lines: 180 From: Dave Hansen Currently, __sb_start_write() and freeze_super() can race with each other. __sb_start_write() uses a smp_mb() to ensure that freeze_super() can see its write to sb->s_writers.counter and that it can see freeze_super()'s update to sb->s_writers.frozen. This all seems to work fine. But, this smp_mb() makes __sb_start_write() the single hottest function in the kernel if I sit in a loop and do tiny write()s to tmpfs over and over. This is on a very small 2-core system, so it will only get worse on larger systems. This _seems_ like an ideal case for RCU. __sb_start_write() is the RCU read-side and is in a very fast, performance-sensitive path. freeze_super() is the RCU writer and is in an extremely rare non-performance-sensitive path. Instead of doing and smp_wmb() in __sb_start_write(), we do rcu_read_lock(). This ensures that a CPU doing freeze_super() can not proceed past its synchronize_rcu() until the grace period has ended and the 's_writers.frozen = SB_FREEZE_WRITE' is visible to __sb_start_write(). One question here: Does the work that __sb_start_write() does in a previous grace period becomes visible to freeze_super() after its call to synchronize_rcu()? It _seems_ like it should, but it seems backwards to me since __sb_start_write() is the "reader" in this case. This patch increases the number of writes/second that I can do by 5.6%. Does anybody see any holes with this? Cc: Jan Kara Cc: Alexander Viro Cc: linux-fsdevel@vger.kernel.org Cc: linux-kernel@vger.kernel.org Cc: Paul E. McKenney Cc: Tim Chen Cc: Andi Kleen Signed-off-by: Dave Hansen --- b/fs/super.c | 63 ++++++++++++++++++++++++++++++++++++----------------------- 1 file changed, 39 insertions(+), 24 deletions(-) diff -puN fs/super.c~rcu-__sb_start_write fs/super.c --- a/fs/super.c~rcu-__sb_start_write 2015-06-24 17:14:34.939125713 -0700 +++ b/fs/super.c 2015-06-24 17:14:34.942125847 -0700 @@ -1190,27 +1190,21 @@ static void acquire_freeze_lock(struct s */ int __sb_start_write(struct super_block *sb, int level, bool wait) { -retry: - if (unlikely(sb->s_writers.frozen >= level)) { + rcu_read_lock(); + while (unlikely(sb->s_writers.frozen >= level)) { + rcu_read_unlock(); if (!wait) return 0; wait_event(sb->s_writers.wait_unfrozen, sb->s_writers.frozen < level); + rcu_read_lock(); } #ifdef CONFIG_LOCKDEP acquire_freeze_lock(sb, level, !wait, _RET_IP_); #endif percpu_counter_inc(&sb->s_writers.counter[level-1]); - /* - * Make sure counter is updated before we check for frozen. - * freeze_super() first sets frozen and then checks the counter. - */ - smp_mb(); - if (unlikely(sb->s_writers.frozen >= level)) { - __sb_end_write(sb, level); - goto retry; - } + rcu_read_unlock(); return 1; } EXPORT_SYMBOL(__sb_start_write); @@ -1254,6 +1248,29 @@ static void sb_wait_write(struct super_b } while (writers); } +static void __thaw_super(struct super_block *sb) +{ + sb->s_writers.frozen = SB_UNFROZEN; + /* + * RCU protects us against races where we are taking + * s_writers.frozen in to a less permissive state. When + * that happens, __sb_start_write() might not yet have + * seen our write and might still increment + * s_writers.counter. + * + * Here, however, we are transitioning to a _more_ + * permissive state. The filesystem is frozen and no + * writes to s_writers.counter are being permitted. + * + * A smp_wmb() is sufficient here because we just need + * to ensure that new calls __sb_start_write() are + * allowed, not that _concurrent_ calls have finished. + */ + smp_wmb(); + wake_up(&sb->s_writers.wait_unfrozen); + deactivate_locked_super(sb); +} + /** * freeze_super - lock the filesystem and force it into a consistent state * @sb: the super to lock @@ -1312,7 +1329,13 @@ int freeze_super(struct super_block *sb) /* From now on, no new normal writers can start */ sb->s_writers.frozen = SB_FREEZE_WRITE; - smp_wmb(); + /* + * After we synchronize_rcu(), we have ensured that everyone + * who reads sb->s_writers.frozen under rcu_read_lock() can + * now see our update. This pretty much means that + * __sb_start_write() will not allow any new writers. + */ + synchronize_rcu(); /* Release s_umount to preserve sb_start_write -> s_umount ordering */ up_write(&sb->s_umount); @@ -1322,7 +1345,7 @@ int freeze_super(struct super_block *sb) /* Now we go and block page faults... */ down_write(&sb->s_umount); sb->s_writers.frozen = SB_FREEZE_PAGEFAULT; - smp_wmb(); + synchronize_rcu(); sb_wait_write(sb, SB_FREEZE_PAGEFAULT); @@ -1331,7 +1354,7 @@ int freeze_super(struct super_block *sb) /* Now wait for internal filesystem counter */ sb->s_writers.frozen = SB_FREEZE_FS; - smp_wmb(); + synchronize_rcu(); sb_wait_write(sb, SB_FREEZE_FS); if (sb->s_op->freeze_fs) { @@ -1339,11 +1362,7 @@ int freeze_super(struct super_block *sb) if (ret) { printk(KERN_ERR "VFS:Filesystem freeze failed\n"); - sb->s_writers.frozen = SB_UNFROZEN; - smp_wmb(); - wake_up(&sb->s_writers.wait_unfrozen); - deactivate_locked_super(sb); - return ret; + __thaw_super(sb); } } /* @@ -1386,11 +1405,7 @@ int thaw_super(struct super_block *sb) } out: - sb->s_writers.frozen = SB_UNFROZEN; - smp_wmb(); - wake_up(&sb->s_writers.wait_unfrozen); - deactivate_locked_super(sb); - + __thaw_super(sb); return 0; } EXPORT_SYMBOL(thaw_super); _ -- 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/