[RFC][PATCH 1/2] fs: use RCU for free_super() vs. __sb_start_write()

[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 10.4%.

Does anybody see any holes with this?

Cc: Jan Kara <jack@xxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>

---

 b/fs/super.c |   38 +++++++++++++++++++++-----------------
 1 file changed, 21 insertions(+), 17 deletions(-)

diff -puN fs/super.c~rcu-__sb_start_write fs/super.c
--- a/fs/super.c~rcu-__sb_start_write	2015-06-19 14:50:53.081869092 -0700
+++ b/fs/super.c	2015-06-19 15:19:03.000473047 -0700
@@ -1190,27 +1190,25 @@ 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 keeps freeze_super() from proceeding
+	 * while we are in here.
+	 */
+	rcu_read_lock();
+	while (unlikely(sb->s_writers.frozen >= level)) {
+		rcu_read_unlock();
 		if (!wait)
-			return 0;
+	       		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);
@@ -1312,7 +1310,13 @@ 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 +1326,7 @@ 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 +1335,7 @@ 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) {
@@ -1340,7 +1344,7 @@ int freeze_super(struct super_block *sb)
 			printk(KERN_ERR
 				"VFS:Filesystem freeze failed\n");
 			sb->s_writers.frozen = SB_UNFROZEN;
-			smp_wmb();
+			synchronize_rcu();
 			wake_up(&sb->s_writers.wait_unfrozen);
 			deactivate_locked_super(sb);
 			return ret;
@@ -1387,7 +1391,7 @@ int thaw_super(struct super_block *sb)
 
 out:
 	sb->s_writers.frozen = SB_UNFROZEN;
-	smp_wmb();
+	synchronize_rcu();
 	wake_up(&sb->s_writers.wait_unfrozen);
 	deactivate_locked_super(sb);
 
_
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