2 files changed, 149 insertions, 83 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 7166023e07d2..8ab91880a0ad 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -14,6 +14,7 @@
 #include <linux/rcupdate.h>
 #include <linux/cgroupstats.h>
 #include <linux/prio_heap.h>
+#include <linux/rwsem.h>
 
 #ifdef CONFIG_CGROUPS
 
@@ -136,6 +137,15 @@ struct cgroup {
          * release_list_lock
          */
         struct list_head release_list;
+
+        /* pids_mutex protects the fields below */
+        struct rw_semaphore pids_mutex;
+        /* Array of process ids in the cgroup */
+        pid_t *tasks_pids;
+        /* How many files are using the current tasks_pids array */
+        int pids_use_count;
+        /* Length of the current tasks_pids array */
+        int pids_length;
 };
 
 /* A css_set is a structure holding pointers to a set of
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 1e49218457e0..046c1609606b 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -868,6 +868,14 @@ static struct super_operations cgroup_ops = {
         .remount_fs = cgroup_remount,
 };
 
+static void init_cgroup_housekeeping(struct cgroup *cgrp)
+{
+        INIT_LIST_HEAD(&cgrp->sibling);
+        INIT_LIST_HEAD(&cgrp->children);
+        INIT_LIST_HEAD(&cgrp->css_sets);
+        INIT_LIST_HEAD(&cgrp->release_list);
+        init_rwsem(&cgrp->pids_mutex);
+}
 static void init_cgroup_root(struct cgroupfs_root *root)
 {
         struct cgroup *cgrp = &root->top_cgroup;
@@ -876,10 +884,7 @@ static void init_cgroup_root(struct cgroupfs_root *root)
         root->number_of_cgroups = 1;
         cgrp->root = root;
         cgrp->top_cgroup = cgrp;
-        INIT_LIST_HEAD(&cgrp->sibling);
-        INIT_LIST_HEAD(&cgrp->children);
-        INIT_LIST_HEAD(&cgrp->css_sets);
-        INIT_LIST_HEAD(&cgrp->release_list);
+        init_cgroup_housekeeping(cgrp);
 }
 
 static int cgroup_test_super(struct super_block *sb, void *data)
@@ -1995,16 +2000,7 @@ int cgroup_scan_tasks(struct cgroup_scanner *scan)
  * but we cannot guarantee that the information we produce is correct
  * unless we produce it entirely atomically.
  *
- * Upon tasks file open(), a struct ctr_struct is allocated, that
- * will have a pointer to an array (also allocated here).  The struct
- * ctr_struct * is stored in file->private_data.  Its resources will
- * be freed by release() when the file is closed.  The array is used
- * to sprintf the PIDs and then used by read().
  */
-struct ctr_struct {
-        char *buf;
-        int bufsz;
-};
 
 /*
  * Load into 'pidarray' up to 'npids' of the tasks using cgroup
@@ -2086,42 +2082,132 @@ static int cmppid(const void *a, const void *b)
         return *(pid_t *)a - *(pid_t *)b;
 }
 
+
 /*
- * Convert array 'a' of 'npids' pid_t's to a string of newline separated
- * decimal pids in 'buf'.  Don't write more than 'sz' chars, but return
- * count 'cnt' of how many chars would be written if buf were large enough.
+ * seq_file methods for the "tasks" file. The seq_file position is the
+ * next pid to display; the seq_file iterator is a pointer to the pid
+ * in the cgroup->tasks_pids array.
  */
-static int pid_array_to_buf(char *buf, int sz, pid_t *a, int npids)
+
+static void *cgroup_tasks_start(struct seq_file *s, loff_t *pos)
 {
-        int cnt = 0;
-        int i;
+        /*
+         * Initially we receive a position value that corresponds to
+         * one more than the last pid shown (or 0 on the first call or
+         * after a seek to the start). Use a binary-search to find the
+         * next pid to display, if any
+         */
+        struct cgroup *cgrp = s->private;
+        int index = 0, pid = *pos;
+        int *iter;
 
-        for (i = 0; i < npids; i++)
-                cnt += snprintf(buf + cnt, max(sz - cnt, 0), "%d\n", a[i]);
-        return cnt;
+        down_read(&cgrp->pids_mutex);
+        if (pid) {
+                int end = cgrp->pids_length;
+                int i;
+                while (index < end) {
+                        int mid = (index + end) / 2;
+                        if (cgrp->tasks_pids[mid] == pid) {
+                                index = mid;
+                                break;
+                        } else if (cgrp->tasks_pids[mid] <= pid)
+                                index = mid + 1;
+                        else
+                                end = mid;
+                }
+        }
+        /* If we're off the end of the array, we're done */
+        if (index >= cgrp->pids_length)
+                return NULL;
+        /* Update the abstract position to be the actual pid that we found */
+        iter = cgrp->tasks_pids + index;
+        *pos = *iter;
+        return iter;
+}
+
+static void cgroup_tasks_stop(struct seq_file *s, void *v)
+{
+        struct cgroup *cgrp = s->private;
+        up_read(&cgrp->pids_mutex);
 }
 
+static void *cgroup_tasks_next(struct seq_file *s, void *v, loff_t *pos)
+{
+        struct cgroup *cgrp = s->private;
+        int *p = v;
+        int *end = cgrp->tasks_pids + cgrp->pids_length;
+
+        /*
+         * Advance to the next pid in the array. If this goes off the
+         * end, we're done
+         */
+        p++;
+        if (p >= end) {
+                return NULL;
+        } else {
+                *pos = *p;
+                return p;
+        }
+}
+
+static int cgroup_tasks_show(struct seq_file *s, void *v)
+{
+        return seq_printf(s, "%d\n", *(int *)v);
+}
+
+static struct seq_operations cgroup_tasks_seq_operations = {
+        .start = cgroup_tasks_start,
+        .stop = cgroup_tasks_stop,
+        .next = cgroup_tasks_next,
+        .show = cgroup_tasks_show,
+};
+
+static void release_cgroup_pid_array(struct cgroup *cgrp)
+{
+        down_write(&cgrp->pids_mutex);
+        BUG_ON(!cgrp->pids_use_count);
+        if (!--cgrp->pids_use_count) {
+                kfree(cgrp->tasks_pids);
+                cgrp->tasks_pids = NULL;
+                cgrp->pids_length = 0;
+        }
+        up_write(&cgrp->pids_mutex);
+}
+
+static int cgroup_tasks_release(struct inode *inode, struct file *file)
+{
+        struct cgroup *cgrp = __d_cgrp(file->f_dentry->d_parent);
+
+        if (!(file->f_mode & FMODE_READ))
+                return 0;
+
+        release_cgroup_pid_array(cgrp);
+        return seq_release(inode, file);
+}
+
+static struct file_operations cgroup_tasks_operations = {
+        .read = seq_read,
+        .llseek = seq_lseek,
+        .write = cgroup_file_write,
+        .release = cgroup_tasks_release,
+};
+
 /*
- * Handle an open on 'tasks' file.  Prepare a buffer listing the
+ * Handle an open on 'tasks' file.  Prepare an array containing the
  * process id's of tasks currently attached to the cgroup being opened.
- *
- * Does not require any specific cgroup mutexes, and does not take any.
  */
+
 static int cgroup_tasks_open(struct inode *unused, struct file *file)
 {
         struct cgroup *cgrp = __d_cgrp(file->f_dentry->d_parent);
-        struct ctr_struct *ctr;
         pid_t *pidarray;
         int npids;
-        char c;
+        int retval;
 
+        /* Nothing to do for write-only files */
         if (!(file->f_mode & FMODE_READ))
                 return 0;
 
-        ctr = kmalloc(sizeof(*ctr), GFP_KERNEL);
-        if (!ctr)
-                goto err0;
-
         /*
          * If cgroup gets more users after we read count, we won't have
          * enough space - tough.  This race is indistinguishable to the
@@ -2129,57 +2215,31 @@ static int cgroup_tasks_open(struct inode *unused, struct file *file)
          * show up until sometime later on.
          */
         npids = cgroup_task_count(cgrp);
-        if (npids) {
-                pidarray = kmalloc(npids * sizeof(pid_t), GFP_KERNEL);
-                if (!pidarray)
-                        goto err1;
-
-                npids = pid_array_load(pidarray, npids, cgrp);
-                sort(pidarray, npids, sizeof(pid_t), cmppid, NULL);
-
-                /* Call pid_array_to_buf() twice, first just to get bufsz */
-                ctr->bufsz = pid_array_to_buf(&c, sizeof(c), pidarray, npids) + 1;
-                ctr->buf = kmalloc(ctr->bufsz, GFP_KERNEL);
-                if (!ctr->buf)
-                        goto err2;
-                ctr->bufsz = pid_array_to_buf(ctr->buf, ctr->bufsz, pidarray, npids);
-
-                kfree(pidarray);
-        } else {
-                ctr->buf = NULL;
-                ctr->bufsz = 0;
-        }
-        file->private_data = ctr;
-        return 0;
-
-err2:
-        kfree(pidarray);
-err1:
-        kfree(ctr);
-err0:
-        return -ENOMEM;
-}
-
-static ssize_t cgroup_tasks_read(struct cgroup *cgrp,
-                                    struct cftype *cft,
-                                    struct file *file, char __user *buf,
-                                    size_t nbytes, loff_t *ppos)
-{
-        struct ctr_struct *ctr = file->private_data;
+        pidarray = kmalloc(npids * sizeof(pid_t), GFP_KERNEL);
+        if (!pidarray)
+                return -ENOMEM;
+        npids = pid_array_load(pidarray, npids, cgrp);
+        sort(pidarray, npids, sizeof(pid_t), cmppid, NULL);
 
-        return simple_read_from_buffer(buf, nbytes, ppos, ctr->buf, ctr->bufsz);
-}
+        /*
+         * Store the array in the cgroup, freeing the old
+         * array if necessary
+         */
+        down_write(&cgrp->pids_mutex);
+        kfree(cgrp->tasks_pids);
+        cgrp->tasks_pids = pidarray;
+        cgrp->pids_length = npids;
+        cgrp->pids_use_count++;
+        up_write(&cgrp->pids_mutex);
 
-static int cgroup_tasks_release(struct inode *unused_inode,
-                                        struct file *file)
-{
-        struct ctr_struct *ctr;
+        file->f_op = &cgroup_tasks_operations;
 
-        if (file->f_mode & FMODE_READ) {
-                ctr = file->private_data;
-                kfree(ctr->buf);
-                kfree(ctr);
+        retval = seq_open(file, &cgroup_tasks_seq_operations);
+        if (retval) {
+                release_cgroup_pid_array(cgrp);
+                return retval;
         }
+        ((struct seq_file *)file->private_data)->private = cgrp;
         return 0;
 }
 
@@ -2208,7 +2268,6 @@ static struct cftype files[] = {
         {
                 .name = "tasks",
                 .open = cgroup_tasks_open,
-                .read = cgroup_tasks_read,
                 .write_u64 = cgroup_tasks_write,
                 .release = cgroup_tasks_release,
                 .private = FILE_TASKLIST,
@@ -2298,10 +2357,7 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
 
         mutex_lock(&cgroup_mutex);
 
-        INIT_LIST_HEAD(&cgrp->sibling);
-        INIT_LIST_HEAD(&cgrp->children);
-        INIT_LIST_HEAD(&cgrp->css_sets);
-        INIT_LIST_HEAD(&cgrp->release_list);
+        init_cgroup_housekeeping(cgrp);
 
         cgrp->parent = parent;
         cgrp->root = parent->root;