[lustre-devel] [PATCH 17/27] lustre: mdc: start changelog thread upon first access

[James Simmons <jsimmons@infradead.org>]

From: Alex Zhuravlev <bzzz@whamcloud.com>

thus leaving the caller a chance to set CHANGELOG_FLAG_FOLLOW,
otherwise the thread (started from open()) can reach the end
of the changelog and exit early.

WC-bug-id: https://jira.whamcloud.com/browse/LU-14663
Lustre-commit: 72a08ea547dceb54 ("LU-14663 mdc: start changelog thread upon first access")
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/43513
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alexander Boyko <alexander.boyko@hpe.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
---
 fs/lustre/mdc/mdc_changelog.c | 54 +++++++++++++++++++++++++++++--------------
 1 file changed, 37 insertions(+), 17 deletions(-)

diff --git a/fs/lustre/mdc/mdc_changelog.c b/fs/lustre/mdc/mdc_changelog.c
index 31c6c8a..d366720 100644
--- a/fs/lustre/mdc/mdc_changelog.c
+++ b/fs/lustre/mdc/mdc_changelog.c
@@ -363,6 +363,33 @@ static int chlg_load(void *args)
 	return rc;
 }
 
+static int chlg_start_thread(struct file *file)
+{
+	struct chlg_reader_state *crs = file->private_data;
+	struct task_struct *task;
+	int rc = 0;
+
+	if (likely(crs->crs_prod_task))
+		return 0;
+	if (unlikely(file->f_mode & FMODE_READ) == 0)
+		return 0;
+
+	mutex_lock(&crs->crs_lock);
+	if (!crs->crs_prod_task) {
+		task = kthread_run(chlg_load, crs, "chlg_load_thread");
+		if (IS_ERR(task)) {
+			rc = PTR_ERR(task);
+			CERROR("%s: cannot start changelog thread: rc = %d\n",
+			       crs->crs_ced->ced_name, rc);
+			goto out;
+		}
+		crs->crs_prod_task = task;
+	}
+out:
+	mutex_unlock(&crs->crs_lock);
+	return rc;
+}
+
 /**
  * Read handler, dequeues records from the chlg_reader_state if any.
  * No partial records are copied to userland so this function can return less
@@ -396,6 +423,10 @@ static ssize_t chlg_read(struct file *file, char __user *buff, size_t count,
 			return -EAGAIN;
 	}
 
+	rc = chlg_start_thread(file);
+	if (rc)
+		return rc;
+
 	rc = wait_event_interruptible(crs->crs_waitq_cons,
 			crs->crs_rec_count > 0 || crs->crs_eof || crs->crs_err);
 
@@ -601,8 +632,6 @@ static int chlg_open(struct inode *inode, struct file *file)
 {
 	struct chlg_reader_state *crs;
 	struct chlg_registered_dev *dev;
-	struct task_struct *task;
-	int rc;
 
 	dev = container_of(inode->i_cdev, struct chlg_registered_dev, ced_cdev);
 
@@ -620,24 +649,10 @@ static int chlg_open(struct inode *inode, struct file *file)
 	init_waitqueue_head(&crs->crs_waitq_prod);
 	init_waitqueue_head(&crs->crs_waitq_cons);
 
-	if (file->f_mode & FMODE_READ) {
-		task = kthread_run(chlg_load, crs, "chlg_load_thread");
-		if (IS_ERR(task)) {
-			rc = PTR_ERR(task);
-			CERROR("%s: cannot start changelog thread: rc = %d\n",
-			       dev->ced_name, rc);
-			goto err_crs;
-		}
-		crs->crs_prod_task = task;
-	}
+	crs->crs_prod_task = NULL;
 
 	file->private_data = crs;
 	return 0;
-
-err_crs:
-	kref_put(&dev->ced_refs, chlg_dev_clear);
-	kfree(crs);
-	return rc;
 }
 
 /**
@@ -679,6 +694,11 @@ static unsigned int chlg_poll(struct file *file, poll_table *wait)
 {
 	struct chlg_reader_state *crs = file->private_data;
 	unsigned int mask = 0;
+	int rc;
+
+	rc = chlg_start_thread(file);
+	if (rc)
+		return rc;
 
 	mutex_lock(&crs->crs_lock);
 	poll_wait(file, &crs->crs_waitq_cons, wait);
-- 
1.8.3.1

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