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Subject: Re: [PATCH 2/2] ceph: wait the first reply of inflight unlink/rmdir
To:     Jeff Layton <jlayton@kernel.org>, viro@zeniv.linux.org.uk
Cc:     idryomov@gmail.com, vshankar@redhat.com,
        ceph-devel@vger.kernel.org, mcgrof@kernel.org,
        akpm@linux-foundation.org, arnd@arndb.de,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20220516122046.40655-1-xiubli@redhat.com>
 <20220516122046.40655-3-xiubli@redhat.com>
 <ea6eef767ae6bcdf7aeae7bbc00c2dd89f8c7e5f.camel@kernel.org>
From:   Xiubo Li <xiubli@redhat.com>
Message-ID: <7a37f7a3-586b-5b75-3692-dc391f737a6b@redhat.com>
Date:   Mon, 16 May 2022 22:56:49 +0800
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On 5/16/22 9:23 PM, Jeff Layton wrote:
> On Mon, 2022-05-16 at 20:20 +0800, Xiubo Li wrote:
>> In async unlink case the kclient won't wait for the first reply
>> from MDS and just drop all the links and unhash the dentry and then
>> succeeds immediately.
>>
>> For any new create/link/rename,etc requests followed by using the
>> same file names we must wait for the first reply of the inflight
>> unlink request, or the MDS possibly will fail these following
>> requests with -EEXIST if the inflight async unlink request was
>> delayed for some reasons.
>>
>> And the worst case is that for the none async openc request it will
>> successfully open the file if the CDentry hasn't been unlinked yet,
>> but later the previous delayed async unlink request will remove the
>> CDenty. That means the just created file is possiblly deleted later
>> by accident.
>>
>> We need to wait for the inflight async unlink requests to finish
>> when creating new files/directories by using the same file names.
>>
>> URL: https://tracker.ceph.com/issues/55332
>> Signed-off-by: Xiubo Li <xiubli@redhat.com>
>> ---
>>   fs/ceph/dir.c        | 55 +++++++++++++++++++++++++++++++----
>>   fs/ceph/file.c       |  5 ++++
>>   fs/ceph/mds_client.c | 69 ++++++++++++++++++++++++++++++++++++++++++++
>>   fs/ceph/mds_client.h |  1 +
>>   fs/ceph/super.c      |  2 ++
>>   fs/ceph/super.h      | 18 ++++++++----
>>   6 files changed, 140 insertions(+), 10 deletions(-)
>>
>> diff --git a/fs/ceph/dir.c b/fs/ceph/dir.c
>> index eae417d71136..20c648406528 100644
>> --- a/fs/ceph/dir.c
>> +++ b/fs/ceph/dir.c
>> @@ -856,6 +856,10 @@ static int ceph_mknod(struct user_namespace *mnt_userns, struct inode *dir,
>>   	if (ceph_snap(dir) != CEPH_NOSNAP)
>>   		return -EROFS;
>>   
>> +	err = ceph_wait_on_conflict_unlink(dentry);
>> +	if (err)
>> +		return err;
>> +
>>   	if (ceph_quota_is_max_files_exceeded(dir)) {
>>   		err = -EDQUOT;
>>   		goto out;
>> @@ -918,6 +922,10 @@ static int ceph_symlink(struct user_namespace *mnt_userns, struct inode *dir,
>>   	if (ceph_snap(dir) != CEPH_NOSNAP)
>>   		return -EROFS;
>>   
>> +	err = ceph_wait_on_conflict_unlink(dentry);
>> +	if (err)
>> +		return err;
>> +
>>   	if (ceph_quota_is_max_files_exceeded(dir)) {
>>   		err = -EDQUOT;
>>   		goto out;
>> @@ -968,9 +976,13 @@ static int ceph_mkdir(struct user_namespace *mnt_userns, struct inode *dir,
>>   	struct ceph_mds_client *mdsc = ceph_sb_to_mdsc(dir->i_sb);
>>   	struct ceph_mds_request *req;
>>   	struct ceph_acl_sec_ctx as_ctx = {};
>> -	int err = -EROFS;
>> +	int err;
>>   	int op;
>>   
>> +	err = ceph_wait_on_conflict_unlink(dentry);
>> +	if (err)
>> +		return err;
>> +
>>   	if (ceph_snap(dir) == CEPH_SNAPDIR) {
>>   		/* mkdir .snap/foo is a MKSNAP */
>>   		op = CEPH_MDS_OP_MKSNAP;
>> @@ -980,6 +992,7 @@ static int ceph_mkdir(struct user_namespace *mnt_userns, struct inode *dir,
>>   		dout("mkdir dir %p dn %p mode 0%ho\n", dir, dentry, mode);
>>   		op = CEPH_MDS_OP_MKDIR;
>>   	} else {
>> +		err = -EROFS;
>>   		goto out;
>>   	}
>>   
>> @@ -1037,6 +1050,10 @@ static int ceph_link(struct dentry *old_dentry, struct inode *dir,
>>   	struct ceph_mds_request *req;
>>   	int err;
>>   
>> +	err = ceph_wait_on_conflict_unlink(dentry);
>> +	if (err)
>> +		return err;
>> +
>>   	if (ceph_snap(dir) != CEPH_NOSNAP)
>>   		return -EROFS;
>>   
>> @@ -1071,9 +1088,24 @@ static int ceph_link(struct dentry *old_dentry, struct inode *dir,
>>   static void ceph_async_unlink_cb(struct ceph_mds_client *mdsc,
>>   				 struct ceph_mds_request *req)
>>   {
>> +	struct dentry *dentry = req->r_dentry;
>> +	struct ceph_dentry_info *di = ceph_dentry(dentry);
>>   	int result = req->r_err ? req->r_err :
>>   			le32_to_cpu(req->r_reply_info.head->result);
>>   
>> +	if (test_bit(CEPH_DENTRY_ASYNC_UNLINK_BIT, &di->flags)) {
>> +		BUG_ON(req->r_op != CEPH_MDS_OP_UNLINK);
>> +
>> +		hash_del_rcu(&di->hnode);
>> +
>> +		spin_lock(&dentry->d_lock);
>> +		di->flags &= ~CEPH_DENTRY_ASYNC_UNLINK;
>> +		wake_up_bit(&di->flags, CEPH_DENTRY_ASYNC_UNLINK_BIT);
>> +		spin_unlock(&dentry->d_lock);
>> +
>> +		synchronize_rcu();
>> +	}
>> +
>>   	if (result == -EJUKEBOX)
>>   		goto out;
>>   
>> @@ -1081,7 +1113,7 @@ static void ceph_async_unlink_cb(struct ceph_mds_client *mdsc,
>>   	if (result) {
>>   		int pathlen = 0;
>>   		u64 base = 0;
>> -		char *path = ceph_mdsc_build_path(req->r_dentry, &pathlen,
>> +		char *path = ceph_mdsc_build_path(dentry, &pathlen,
>>   						  &base, 0);
>>   
>>   		/* mark error on parent + clear complete */
>> @@ -1089,13 +1121,13 @@ static void ceph_async_unlink_cb(struct ceph_mds_client *mdsc,
>>   		ceph_dir_clear_complete(req->r_parent);
>>   
>>   		/* drop the dentry -- we don't know its status */
>> -		if (!d_unhashed(req->r_dentry))
>> -			d_drop(req->r_dentry);
>> +		if (!d_unhashed(dentry))
>> +			d_drop(dentry);
>>   
>>   		/* mark inode itself for an error (since metadata is bogus) */
>>   		mapping_set_error(req->r_old_inode->i_mapping, result);
>>   
>> -		pr_warn("ceph: async unlink failure path=(%llx)%s result=%d!\n",
>> +		pr_warn("async unlink failure path=(%llx)%s result=%d!\n",
>>   			base, IS_ERR(path) ? "<<bad>>" : path, result);
>>   		ceph_mdsc_free_path(path, pathlen);
>>   	}
>> @@ -1189,12 +1221,21 @@ static int ceph_unlink(struct inode *dir, struct dentry *dentry)
>>   		ihold(req->r_old_inode);
>>   		err = ceph_mdsc_submit_request(mdsc, dir, req);
>>   		if (!err) {
>> +			struct ceph_dentry_info *di;
>> +
>>   			/*
>>   			 * We have enough caps, so we assume that the unlink
>>   			 * will succeed. Fix up the target inode and dcache.
>>   			 */
>>   			drop_nlink(inode);
>>   			d_delete(dentry);
>> +
>> +			spin_lock(&dentry->d_lock);
>> +			di = ceph_dentry(dentry);
>> +			di->flags |= CEPH_DENTRY_ASYNC_UNLINK;
>> +			hash_add_rcu(fsc->async_unlink_conflict, &di->hnode,
>> +				     dentry->d_name.hash);
>> +			spin_unlock(&dentry->d_lock);
> This looks racy. It's possible that the reply comes in before we get to
> the point of setting this flag. You probably want to do this before
> calling ceph_mdsc_submit_request, and just unwind it if the submission
> fails.

Ah, right. Will fix it.


>
> Also, you do still need some sort of lock to protect the
> hash_add/del/_rcu calls.

Sure, will fix it too.

>   Those don't do any locking on their own. The
> d_lock is insufficient here since it can't protect the whole list. You
> may be able to use the i_ceph_lock of the parent though?

The hashtable is a global one, so we couldn't use the i_ceph_lock here. 
I will add one dedicated spin lock for each sb.

>>   		} else if (err == -EJUKEBOX) {
>>   			try_async = false;
>>   			ceph_mdsc_put_request(req);
>> @@ -1237,6 +1278,10 @@ static int ceph_rename(struct user_namespace *mnt_userns, struct inode *old_dir,
>>   	    (!ceph_quota_is_same_realm(old_dir, new_dir)))
>>   		return -EXDEV;
>>   
>> +	err = ceph_wait_on_conflict_unlink(new_dentry);
>> +	if (err)
>> +		return err;
>> +
>>   	dout("rename dir %p dentry %p to dir %p dentry %p\n",
>>   	     old_dir, old_dentry, new_dir, new_dentry);
>>   	req = ceph_mdsc_create_request(mdsc, op, USE_AUTH_MDS);
>> diff --git a/fs/ceph/file.c b/fs/ceph/file.c
>> index 8c8226c0feac..47d068e6436a 100644
>> --- a/fs/ceph/file.c
>> +++ b/fs/ceph/file.c
>> @@ -740,6 +740,10 @@ int ceph_atomic_open(struct inode *dir, struct dentry *dentry,
>>   	if (dentry->d_name.len > NAME_MAX)
>>   		return -ENAMETOOLONG;
>>   
>> +	err = ceph_wait_on_conflict_unlink(dentry);
>> +	if (err)
>> +		return err;
>> +
> What might be nice here eventually is to not block an async create here,
> but instead queue the request so that it gets transmitted after the
> async unlink reply comes in.
>
> That'll be hard to get right though, so this is fine for now.

Sure.

>
>>   	if (flags & O_CREAT) {
>>   		if (ceph_quota_is_max_files_exceeded(dir))
>>   			return -EDQUOT;
>> @@ -757,6 +761,7 @@ int ceph_atomic_open(struct inode *dir, struct dentry *dentry,
>>   		/* If it's not being looked up, it's negative */
>>   		return -ENOENT;
>>   	}
>> +
>>   retry:
>>   	/* do the open */
>>   	req = prepare_open_request(dir->i_sb, flags, mode);
>> diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
>> index e8c87dea0551..0ae0e0110eb4 100644
>> --- a/fs/ceph/mds_client.c
>> +++ b/fs/ceph/mds_client.c
>> @@ -468,6 +468,75 @@ static int ceph_parse_deleg_inos(void **p, void *end,
>>   	return -EIO;
>>   }
>>   
>> +/*
>> + * In async unlink case the kclient won't wait for the first reply
>> + * from MDS and just drop all the links and unhash the dentry and then
>> + * succeeds immediately.
>> + *
>> + * For any new create/link/rename,etc requests followed by using the
>> + * same file names we must wait for the first reply of the inflight
>> + * unlink request, or the MDS possibly will fail these following
>> + * requests with -EEXIST if the inflight async unlink request was
>> + * delayed for some reasons.
>> + *
>> + * And the worst case is that for the none async openc request it will
>> + * successfully open the file if the CDentry hasn't been unlinked yet,
>> + * but later the previous delayed async unlink request will remove the
>> + * CDenty. That means the just created file is possiblly deleted later
>> + * by accident.
>> + *
>> + * We need to wait for the inflight async unlink requests to finish
>> + * when creating new files/directories by using the same file names.
>> + */
>> +int ceph_wait_on_conflict_unlink(struct dentry *dentry)
>> +{
>> +	struct ceph_fs_client *fsc = ceph_sb_to_client(dentry->d_sb);
>> +	struct dentry *pdentry = dentry->d_parent;
>> +	struct dentry *udentry, *found = NULL;
>> +	struct ceph_dentry_info *di;
>> +	struct qstr dname;
>> +	u32 hash = dentry->d_name.hash;
>> +	int err;
>> +
>> +	dname.name = dentry->d_name.name;
>> +	dname.len = dentry->d_name.len;
>> +
>> +	rcu_read_lock();
>> +	hash_for_each_possible_rcu(fsc->async_unlink_conflict, di,
>> +				   hnode, hash) {
>> +		udentry = di->dentry;
>> +
>> +		spin_lock(&udentry->d_lock);
>> +		if (udentry->d_name.hash != hash)
>> +			goto next;
>> +		if (unlikely(udentry->d_parent != pdentry))
>> +			goto next;
>> +		if (!hash_hashed(&di->hnode))
>> +			goto next;
>> +
>> +		if (!test_bit(CEPH_DENTRY_ASYNC_UNLINK_BIT, &di->flags))
>> +			goto next;
>> +
> Maybe this should be a warning? Will we ever have entries in this
> hashtable that don't have this bit set?

Just before we take "spin_lock(&udentry->d_lock)" the udentry could be 
already removed from hashtable and the bit was cleared ?

-- Xiubo

>
>> +		if (d_compare(pdentry, udentry, &dname))
>> +			goto next;
>> +
>> +		spin_unlock(&udentry->d_lock);
>> +		found = dget(udentry);
>> +		break;
>> +next:
>> +		spin_unlock(&udentry->d_lock);
>> +	}
>> +	rcu_read_unlock();
>> +
>> +	if (likely(!found))
>> +		return 0;
>> +
>> +	err = wait_on_bit(&di->flags, CEPH_DENTRY_ASYNC_UNLINK_BIT,
>> +			  TASK_INTERRUPTIBLE);
>> +	dput(found);
>> +	return err;
>> +}
>> +
>>   u64 ceph_get_deleg_ino(struct ceph_mds_session *s)
>>   {
>>   	unsigned long ino;
>> diff --git a/fs/ceph/mds_client.h b/fs/ceph/mds_client.h
>> index 33497846e47e..d1ae679c52c3 100644
>> --- a/fs/ceph/mds_client.h
>> +++ b/fs/ceph/mds_client.h
>> @@ -582,6 +582,7 @@ static inline int ceph_wait_on_async_create(struct inode *inode)
>>   			   TASK_INTERRUPTIBLE);
>>   }
>>   
>> +extern int ceph_wait_on_conflict_unlink(struct dentry *dentry);
>>   extern u64 ceph_get_deleg_ino(struct ceph_mds_session *session);
>>   extern int ceph_restore_deleg_ino(struct ceph_mds_session *session, u64 ino);
>>   #endif
>> diff --git a/fs/ceph/super.c b/fs/ceph/super.c
>> index b73b4f75462c..7ae65001f04c 100644
>> --- a/fs/ceph/super.c
>> +++ b/fs/ceph/super.c
>> @@ -816,6 +816,8 @@ static struct ceph_fs_client *create_fs_client(struct ceph_mount_options *fsopt,
>>   	if (!fsc->cap_wq)
>>   		goto fail_inode_wq;
>>   
>> +	hash_init(fsc->async_unlink_conflict);
>> +
>>   	spin_lock(&ceph_fsc_lock);
>>   	list_add_tail(&fsc->metric_wakeup, &ceph_fsc_list);
>>   	spin_unlock(&ceph_fsc_lock);
>> diff --git a/fs/ceph/super.h b/fs/ceph/super.h
>> index 506d52633627..58bbb5df42da 100644
>> --- a/fs/ceph/super.h
>> +++ b/fs/ceph/super.h
>> @@ -19,6 +19,7 @@
>>   #include <linux/security.h>
>>   #include <linux/netfs.h>
>>   #include <linux/fscache.h>
>> +#include <linux/hashtable.h>
>>   
>>   #include <linux/ceph/libceph.h>
>>   
>> @@ -99,6 +100,8 @@ struct ceph_mount_options {
>>   	char *mon_addr;
>>   };
>>   
>> +#define CEPH_ASYNC_CREATE_CONFLICT_BITS 12
>> +
>>   struct ceph_fs_client {
>>   	struct super_block *sb;
>>   
>> @@ -124,6 +127,8 @@ struct ceph_fs_client {
>>   	struct workqueue_struct *inode_wq;
>>   	struct workqueue_struct *cap_wq;
>>   
>> +	DECLARE_HASHTABLE(async_unlink_conflict, CEPH_ASYNC_CREATE_CONFLICT_BITS);
>> +
>>   #ifdef CONFIG_DEBUG_FS
>>   	struct dentry *debugfs_dentry_lru, *debugfs_caps;
>>   	struct dentry *debugfs_congestion_kb;
>> @@ -281,7 +286,8 @@ struct ceph_dentry_info {
>>   	struct dentry *dentry;
>>   	struct ceph_mds_session *lease_session;
>>   	struct list_head lease_list;
>> -	unsigned flags;
>> +	struct hlist_node hnode;
>> +	unsigned long flags;
>>   	int lease_shared_gen;
>>   	u32 lease_gen;
>>   	u32 lease_seq;
>> @@ -290,10 +296,12 @@ struct ceph_dentry_info {
>>   	u64 offset;
>>   };
>>   
>> -#define CEPH_DENTRY_REFERENCED		1
>> -#define CEPH_DENTRY_LEASE_LIST		2
>> -#define CEPH_DENTRY_SHRINK_LIST		4
>> -#define CEPH_DENTRY_PRIMARY_LINK	8
>> +#define CEPH_DENTRY_REFERENCED		(1 << 0)
>> +#define CEPH_DENTRY_LEASE_LIST		(1 << 1)
>> +#define CEPH_DENTRY_SHRINK_LIST		(1 << 2)
>> +#define CEPH_DENTRY_PRIMARY_LINK	(1 << 3)
>> +#define CEPH_DENTRY_ASYNC_UNLINK_BIT	(4)
>> +#define CEPH_DENTRY_ASYNC_UNLINK	(1 << CEPH_DENTRY_ASYNC_UNLINK_BIT)
>>   
>>   struct ceph_inode_xattrs_info {
>>   	/*