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Date: Fri, 26 Jun 2015 14:59:47 +0200
From: Jan Kara <jack@...e.cz>
To: Dave Hansen <dave@...1.net>
Cc: jack@...e.cz, viro@...iv.linux.org.uk,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
paulmck@...ux.vnet.ibm.com, tim.c.chen@...ux.intel.com,
ak@...ux.intel.com, dave.hansen@...ux.intel.com
Subject: Re: [RFCv2][PATCH 2/7] fs: use RCU for free_super() vs.
__sb_start_write()
On Wed 24-06-15 17:16:05, Dave Hansen wrote:
>
> From: Dave Hansen <dave.hansen@...ux.intel.com>
>
> 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.
I believe yes. Because all accesses (be it reads or writes) must finish
before the current RCU period finishes. And synchronize_rcu() must make
sure that any code (loads / stores) after it execute only after the RCU
period has finished...
The patch looks good to me. You can add:
Reviewed-by: Jan Kara <jack@...e.com>
Honza
>
> 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 <jack@...e.cz>
> Cc: Alexander Viro <viro@...iv.linux.org.uk>
> Cc: linux-fsdevel@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: Tim Chen <tim.c.chen@...ux.intel.com>
> Cc: Andi Kleen <ak@...ux.intel.com>
> Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
> ---
>
> 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);
> _
--
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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