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Message-ID: <20140818063301.9860.11381.stgit@notabene.brown>
Date: Mon, 18 Aug 2014 16:33:01 +1000
From: NeilBrown <neilb@...e.de>
To: Ian Kent <raven@...maw.net>
Cc: autofs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH 3/5] autofs4: avoid taking fs_lock during rcu-walk
->fs_lock protects AUTOFS_INF_EXPIRING. We need to be sure
that once the flag is set, no new references beneath the dentry
are taken. So rcu-walk currently needs to take fs_lock before
checking the flag. This hurts performance.
Change the expiry to a two-stage process.
First set AUTOFS_INF_NO_RCU which forces any path walk into
ref-walk mode, then drop the lock and call synchronize_rcu().
Once that returns we can be sure no rcu-walk is active beneath
the dentry and we can check reference counts again.
Now during an RCU-walk we can test AUTOFS_INF_EXPIRING without
taking the lock as along as we test AUTOFS_INF_NO_RCU too.
If either are set, we must abort the RCU-walk
If neither are set, we know that refcounts will be tested again
after we finish the RCU-walk so we are safe to continue.
->fs_lock is still taken in d_manage() to check for a non-trap
directory. That will be resolved in the next patch.
Signed-off-by: NeilBrown <neilb@...e.de>
---
fs/autofs4/autofs_i.h | 4 ++++
fs/autofs4/expire.c | 46 ++++++++++++++++++++++++++++++++++++++--------
2 files changed, 42 insertions(+), 8 deletions(-)
diff --git a/fs/autofs4/autofs_i.h b/fs/autofs4/autofs_i.h
index 2f1032f12d91..8e98cf954bab 100644
--- a/fs/autofs4/autofs_i.h
+++ b/fs/autofs4/autofs_i.h
@@ -79,6 +79,10 @@ struct autofs_info {
};
#define AUTOFS_INF_EXPIRING (1<<0) /* dentry is in the process of expiring */
+#define AUTOFS_INF_NO_RCU (1<<1) /* the dentry is being considered
+ * for expiry, so RCU_walk is
+ * not permitted
+ */
#define AUTOFS_INF_PENDING (1<<2) /* dentry pending mount */
struct autofs_wait_queue {
diff --git a/fs/autofs4/expire.c b/fs/autofs4/expire.c
index bee939efca2b..eb4b770a4bf6 100644
--- a/fs/autofs4/expire.c
+++ b/fs/autofs4/expire.c
@@ -333,10 +333,19 @@ struct dentry *autofs4_expire_direct(struct super_block *sb,
if (ino->flags & AUTOFS_INF_PENDING)
goto out;
if (!autofs4_direct_busy(mnt, root, timeout, do_now)) {
- ino->flags |= AUTOFS_INF_EXPIRING;
- init_completion(&ino->expire_complete);
+ ino->flags |= AUTOFS_INF_NO_RCU;
spin_unlock(&sbi->fs_lock);
- return root;
+ synchronize_rcu();
+ spin_lock(&sbi->fs_lock);
+ if (!autofs4_direct_busy(mnt, root, timeout, do_now)) {
+ ino->flags |= AUTOFS_INF_EXPIRING;
+ smp_mb();
+ ino->flags &= ~AUTOFS_INF_NO_RCU;
+ init_completion(&ino->expire_complete);
+ spin_unlock(&sbi->fs_lock);
+ return root;
+ }
+ ino->flags &= ~AUTOFS_INF_NO_RCU;
}
out:
spin_unlock(&sbi->fs_lock);
@@ -454,12 +463,29 @@ struct dentry *autofs4_expire_indirect(struct super_block *sb,
dentry = NULL;
while ((dentry = get_next_positive_subdir(dentry, root))) {
spin_lock(&sbi->fs_lock);
- expired = should_expire(dentry, mnt, timeout, how);
- if (expired) {
+ ino = autofs4_dentry_ino(dentry);
+ if (ino->flags & AUTOFS_INF_NO_RCU)
+ expired = NULL;
+ else
+ expired = should_expire(dentry, mnt, timeout, how);
+ if (!expired) {
+ spin_unlock(&sbi->fs_lock);
+ continue;
+ }
+ ino = autofs4_dentry_ino(expired);
+ ino->flags |= AUTOFS_INF_NO_RCU;
+ spin_unlock(&sbi->fs_lock);
+ synchronize_rcu();
+ spin_lock(&sbi->fs_lock);
+ if (should_expire(expired, mnt, timeout, how)) {
if (expired != dentry)
dput(dentry);
goto found;
}
+
+ ino->flags &= ~AUTOFS_INF_NO_RCU;
+ if (expired != dentry)
+ dput(expired);
spin_unlock(&sbi->fs_lock);
}
return NULL;
@@ -467,8 +493,9 @@ struct dentry *autofs4_expire_indirect(struct super_block *sb,
found:
DPRINTK("returning %p %.*s",
expired, (int)expired->d_name.len, expired->d_name.name);
- ino = autofs4_dentry_ino(expired);
ino->flags |= AUTOFS_INF_EXPIRING;
+ smp_mb();
+ ino->flags &= ~AUTOFS_INF_NO_RCU;
init_completion(&ino->expire_complete);
spin_unlock(&sbi->fs_lock);
spin_lock(&sbi->lookup_lock);
@@ -488,11 +515,14 @@ int autofs4_expire_wait(struct dentry *dentry, int rcu_walk)
int status;
/* Block on any pending expire */
+ if (!(ino->flags & (AUTOFS_INF_EXPIRING | AUTOFS_INF_NO_RCU)))
+ return 0;
+ if (rcu_walk)
+ return -ECHILD;
+
spin_lock(&sbi->fs_lock);
if (ino->flags & AUTOFS_INF_EXPIRING) {
spin_unlock(&sbi->fs_lock);
- if (rcu_walk)
- return -ECHILD;
DPRINTK("waiting for expire %p name=%.*s",
dentry, dentry->d_name.len, dentry->d_name.name);
--
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