[PATCH] proc: Rewrite the proc dentry flush on exit optimization

To keep the dcache from filling up with dead /proc entries we flush them on
process exit.  However over the years that code has gotten hairy with a
dentry_pointer and a lock in task_struct and misdocumented as a correctness
feature.

I have rewritten this code to look and see if we have a corresponding entry in
the dcache and if so flush it on process exit.  This removes the extra fields
in the task_struct and allows me to trivially handle the case of a
/proc/<tgid>/task/<pid> entry as well as the current /proc/<pid> entries.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 61 insertions, 73 deletions

diff --git a/fs/proc/base.c b/fs/proc/base.c
index c8636841bbc..f435932e643 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1352,16 +1352,6 @@ static int tid_fd_revalidate(struct dentry *dentry, struct nameidata *nd)
         return 0;
 }
 
-static void pid_base_iput(struct dentry *dentry, struct inode *inode)
-{
-        struct task_struct *task = proc_task(inode);
-        spin_lock(&task->proc_lock);
-        if (task->proc_dentry == dentry)
-                task->proc_dentry = NULL;
-        spin_unlock(&task->proc_lock);
-        iput(inode);
-}
-
 static int pid_delete_dentry(struct dentry * dentry)
 {
         /* Is the task we represent dead?
@@ -1383,13 +1373,6 @@ static struct dentry_operations pid_dentry_operations =
         .d_delete       = pid_delete_dentry,
 };
 
-static struct dentry_operations pid_base_dentry_operations =
-{
-        .d_revalidate   = pid_revalidate,
-        .d_iput         = pid_base_iput,
-        .d_delete       = pid_delete_dentry,
-};
-
 /* Lookups */
 
 static unsigned name_to_int(struct dentry *dentry)
@@ -1859,57 +1842,70 @@ static struct inode_operations proc_self_inode_operations = {
 };
 
 /**
- * proc_pid_unhash -  Unhash /proc/@pid entry from the dcache.
- * @p: task that should be flushed.
+ * proc_flush_task -  Remove dcache entries for @task from the /proc dcache.
+ *
+ * @task: task that should be flushed.
+ *
+ * Looks in the dcache for
+ * /proc/@pid
+ * /proc/@tgid/task/@pid
+ * if either directory is present flushes it and all of it'ts children
+ * from the dcache.
  *
- * Drops the /proc/@pid dcache entry from the hash chains.
+ * It is safe and reasonable to cache /proc entries for a task until
+ * that task exits.  After that they just clog up the dcache with
+ * useless entries, possibly causing useful dcache entries to be
+ * flushed instead.  This routine is proved to flush those useless
+ * dcache entries at process exit time.
  *
- * Dropping /proc/@pid entries and detach_pid must be synchroneous,
- * otherwise e.g. /proc/@pid/exe might point to the wrong executable,
- * if the pid value is immediately reused. This is enforced by
- * - caller must acquire spin_lock(p->proc_lock)
- * - must be called before detach_pid()
- * - proc_pid_lookup acquires proc_lock, and checks that
- *   the target is not dead by looking at the attach count
- *   of PIDTYPE_PID.
+ * NOTE: This routine is just an optimization so it does not guarantee
+ *       that no dcache entries will exist at process exit time it
+ *       just makes it very unlikely that any will persist.
  */
-
-struct dentry *proc_pid_unhash(struct task_struct *p)
+void proc_flush_task(struct task_struct *task)
 {
-        struct dentry *proc_dentry;
+        struct dentry *dentry, *leader, *dir;
+        char buf[30];
+        struct qstr name;
+
+        name.name = buf;
+        name.len = snprintf(buf, sizeof(buf), "%d", task->pid);
+        dentry = d_hash_and_lookup(proc_mnt->mnt_root, &name);
+        if (dentry) {
+                shrink_dcache_parent(dentry);
+                d_drop(dentry);
+                dput(dentry);
+        }
 
-        proc_dentry = p->proc_dentry;
-        if (proc_dentry != NULL) {
+        if (thread_group_leader(task))
+                goto out;
 
-                spin_lock(&dcache_lock);
-                spin_lock(&proc_dentry->d_lock);
-                if (!d_unhashed(proc_dentry)) {
-                        dget_locked(proc_dentry);
-                        __d_drop(proc_dentry);
-                        spin_unlock(&proc_dentry->d_lock);
-                } else {
-                        spin_unlock(&proc_dentry->d_lock);
-                        proc_dentry = NULL;
-                }
-                spin_unlock(&dcache_lock);
-        }
-        return proc_dentry;
-}
+        name.name = buf;
+        name.len = snprintf(buf, sizeof(buf), "%d", task->tgid);
+        leader = d_hash_and_lookup(proc_mnt->mnt_root, &name);
+        if (!leader)
+                goto out;
 
-/**
- * proc_pid_flush - recover memory used by stale /proc/@pid/x entries
- * @proc_dentry: directoy to prune.
- *
- * Shrink the /proc directory that was used by the just killed thread.
- */
-        
-void proc_pid_flush(struct dentry *proc_dentry)
-{
-        might_sleep();
-        if(proc_dentry != NULL) {
-                shrink_dcache_parent(proc_dentry);
-                dput(proc_dentry);
+        name.name = "task";
+        name.len = strlen(name.name);
+        dir = d_hash_and_lookup(leader, &name);
+        if (!dir)
+                goto out_put_leader;
+
+        name.name = buf;
+        name.len = snprintf(buf, sizeof(buf), "%d", task->pid);
+        dentry = d_hash_and_lookup(dir, &name);
+        if (dentry) {
+                shrink_dcache_parent(dentry);
+                d_drop(dentry);
+                dput(dentry);
         }
+
+        dput(dir);
+out_put_leader:
+        dput(leader);
+out:
+        return;
 }
 
 /* SMP-safe */
@@ -1919,7 +1915,6 @@ struct dentry *proc_pid_lookup(struct inode *dir, struct dentry * dentry, struct
         struct inode *inode;
         struct proc_inode *ei;
         unsigned tgid;
-        int died;
 
         if (dentry->d_name.len == 4 && !memcmp(dentry->d_name.name,"self",4)) {
                 inode = new_inode(dir->i_sb);
@@ -1965,23 +1960,16 @@ struct dentry *proc_pid_lookup(struct inode *dir, struct dentry * dentry, struct
         inode->i_nlink = 4;
 #endif
 
-        dentry->d_op = &pid_base_dentry_operations;
+        dentry->d_op = &pid_dentry_operations;
 
-        died = 0;
         d_add(dentry, inode);
-        spin_lock(&task->proc_lock);
-        task->proc_dentry = dentry;
         if (!pid_alive(task)) {
-                dentry = proc_pid_unhash(task);
-                died = 1;
+                d_drop(dentry);
+                shrink_dcache_parent(dentry);
+                goto out;
         }
-        spin_unlock(&task->proc_lock);
 
         put_task_struct(task);
-        if (died) {
-                proc_pid_flush(dentry);
-                goto out;
-        }
         return NULL;
 out:
         return ERR_PTR(-ENOENT);
@@ -2024,7 +2012,7 @@ static struct dentry *proc_task_lookup(struct inode *dir, struct dentry * dentry
         inode->i_nlink = 3;
 #endif
 
-        dentry->d_op = &pid_base_dentry_operations;
+        dentry->d_op = &pid_dentry_operations;
 
         d_add(dentry, inode);