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Commit 4d885f90 authored by NeilBrown's avatar NeilBrown Committed by Linus Torvalds
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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: default avatarNeilBrown <neilb@suse.de>
Reviewed-by: default avatarIan Kent <raven@themaw.net>
Tested-by: default avatarIan Kent <raven@themaw.net>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 6ece08e6

fs/autofs4/autofs_i.h

+4 −0
Original line number Diff line number Diff line
@@ -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 {

fs/autofs4/expire.c

+38 −8
Original line number Diff line number Diff line
@@ -320,12 +320,21 @@ struct dentry *autofs4_expire_direct(struct super_block *sb, 
	/* No point expiring a pending mount */

	if (ino->flags & AUTOFS_INF_PENDING)

		goto out;

	if (!autofs4_direct_busy(mnt, root, timeout, do_now)) {

		ino->flags |= AUTOFS_INF_NO_RCU;

		spin_unlock(&sbi->fs_lock);

		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);

	dput(root);
@@ -442,12 +451,29 @@ struct dentry *autofs4_expire_indirect(struct super_block *sb, 
	dentry = NULL;

	while ((dentry = get_next_positive_subdir(dentry, root))) {

		spin_lock(&sbi->fs_lock);

		ino = autofs4_dentry_ino(dentry);

		if (ino->flags & AUTOFS_INF_NO_RCU)

			expired = NULL;

		else

			expired = should_expire(dentry, mnt, timeout, how);

		if (expired) {

		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 @@ 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);
@@ -476,11 +503,14 @@ 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);