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@suse.de>
Reviewed-by: Ian Kent <raven@themaw.net>
Tested-by: Ian Kent <raven@themaw.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/fs/autofs4/autofs_i.h b/fs/autofs4/autofs_i.h
index 2f1032f..8e98cf9 100644
--- a/fs/autofs4/autofs_i.h
+++ b/fs/autofs4/autofs_i.h
@@ -79,6 +79,10 @@
};
#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 af141fc..683a5b9 100644
--- a/fs/autofs4/expire.c
+++ b/fs/autofs4/expire.c
@@ -321,10 +321,19 @@
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);
@@ -442,12 +451,29 @@
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;
@@ -455,8 +481,9 @@
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);
@@ -476,11 +503,14 @@
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);
