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Date:   Sat, 23 Jul 2022 20:09:07 +0300
From:   Alexey Dobriyan <adobriyan@gmail.com>
To:     akpm@linux-foundation.org
Cc:     linux-kernel@vger.kernel.org
Subject: [PATCH] proc: add some (hopefully) insightful comments
Message-ID: <YtwrM6sDC0OQ53YB@localhost.localdomain>
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* /proc/${pid}/net status
* removing PDE vs last close stuff (again!)
* random small stuff

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---

 fs/proc/array.c    |    4 ++++
 fs/proc/inode.c    |   17 ++++++++++++-----
 fs/proc/proc_net.c |    6 ++++++
 fs/proc/root.c     |    5 +++++
 4 files changed, 27 insertions(+), 5 deletions(-)

--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -99,6 +99,10 @@ void proc_task_name(struct seq_file *m, struct task_struct *p, bool escape)
 {
 	char tcomm[64];
 
+	/*
+	 * Test before PF_KTHREAD because all workqueue worker threads are
+	 * kernel threads.
+	 */
 	if (p->flags & PF_WQ_WORKER)
 		wq_worker_comm(tcomm, sizeof(tcomm), p);
 	else if (p->flags & PF_KTHREAD)
--- a/fs/proc/inode.c
+++ b/fs/proc/inode.c
@@ -212,7 +212,15 @@ static void unuse_pde(struct proc_dir_entry *pde)
 		complete(pde->pde_unload_completion);
 }
 
-/* pde is locked on entry, unlocked on exit */
+/*
+ * At most 2 contexts can enter this function: the one doing the last
+ * close on the descriptor and whoever is deleting PDE itself.
+ *
+ * First to enter calls ->proc_release hook and signals its completion
+ * to the second one which waits and then does nothing.
+ *
+ * PDE is locked on entry, unlocked on exit.
+ */
 static void close_pdeo(struct proc_dir_entry *pde, struct pde_opener *pdeo)
 	__releases(&pde->pde_unload_lock)
 {
@@ -222,9 +230,6 @@ static void close_pdeo(struct proc_dir_entry *pde, struct pde_opener *pdeo)
 	 *
 	 * rmmod (remove_proc_entry() et al) can't delete an entry and proceed:
 	 * "struct file" needs to be available at the right moment.
-	 *
-	 * Therefore, first process to enter this function does ->release() and
-	 * signals its completion to the other process which does nothing.
 	 */
 	if (pdeo->closing) {
 		/* somebody else is doing that, just wait */
@@ -238,10 +243,12 @@ static void close_pdeo(struct proc_dir_entry *pde, struct pde_opener *pdeo)
 
 		pdeo->closing = true;
 		spin_unlock(&pde->pde_unload_lock);
+
 		file = pdeo->file;
 		pde->proc_ops->proc_release(file_inode(file), file);
+
 		spin_lock(&pde->pde_unload_lock);
-		/* After ->release. */
+		/* Strictly after ->proc_release, see above. */
 		list_del(&pdeo->lh);
 		c = pdeo->c;
 		spin_unlock(&pde->pde_unload_lock);
--- a/fs/proc/proc_net.c
+++ b/fs/proc/proc_net.c
@@ -350,6 +350,12 @@ static __net_init int proc_net_ns_init(struct net *net)
 	kgid_t gid;
 	int err;
 
+	/*
+	 * This PDE acts only as an anchor for /proc/${pid}/net hierarchy.
+	 * Corresponding inode (PDE(inode) == net->proc_net) is never
+	 * instantiated therefore blanket zeroing is fine.
+	 * net->proc_net_stat inode is instantiated normally.
+	 */
 	err = -ENOMEM;
 	netd = kmem_cache_zalloc(proc_dir_entry_cache, GFP_KERNEL);
 	if (!netd)
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -302,6 +302,11 @@ void __init proc_root_init(void)
 	proc_mkdir("bus", NULL);
 	proc_sys_init();
 
+	/*
+	 * Last things last. It is not like userspace processes eager
+	 * to open /proc files exist at this point but register last
+	 * anyway.
+	 */
 	register_filesystem(&proc_fs_type);
 }