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Date: Wed, 24 Jun 2015 17:16:05 -0700
From: Dave Hansen <dave@...1.net>
To: 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: [RFCv2][PATCH 2/7] fs: use RCU for free_super() vs. __sb_start_write()
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.
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);
_
--
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