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Date: Wed, 16 Jul 2014 17:52:30 +0800
From: Ian Kent <raven@...maw.net>
To: NeilBrown <neilb@...e.de>
Cc: autofs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/6] autofs4: avoid taking fs_lock during rcu-walk
On Thu, 2014-07-10 at 09:41 +1000, NeilBrown wrote:
> ->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 AUTHFS_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 AUTHFS_INF_EXPIRING without
> taking the lock as along as we test AUTHFS_INF_NO_RCU too.
Couple of typos above, eeek!
> 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.
I believe the idea is sound and the patch looks good.
Nevertheless I think this is probably the tricky bit and if there is a
problem I'm not seeing it's probably in this patch.
The submount-test will probably help with that.
>
> 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 99dbb05d6148..469724d7568c 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 fb0b5003353f..98a6fd4957f8 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);
> @@ -445,12 +454,29 @@ 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;
> @@ -458,8 +484,9 @@ 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);
> @@ -479,11 +506,14 @@ 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;
Be nice to add a blank line here.
> 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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