Date:	Sat, 24 Mar 2012 00:58:33 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Lucas De Marchi <lucas.demarchi@...fusion.mobi>
Cc:	Linux Kernel <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [REVIEW][PATCH] Making poll generally useful for sysctls

Lucas De Marchi <lucas.demarchi@...fusion.mobi> writes:

>>  /* A sysctl table is an array of struct ctl_table: */
>>  struct ctl_table
>>  {
>>        const char *procname;           /* Text ID for /proc/sys, or zero */
>>        void *data;
>> +       atomic_t event;
>>        int maxlen;
>>        umode_t mode;
>>        struct ctl_table *child;        /* Deprecated */
>>        proc_handler *proc_handler;     /* Callback for text formatting */
>> -       struct ctl_table_poll *poll;
>>        void *extra1;
>>        void *extra2;
>>  };
>> @@ -1042,6 +1025,7 @@ struct ctl_table_header
>>                };
>>                struct rcu_head rcu;
>>        };
>> +       wait_queue_head_t wait;
>
> If you have a waitqueue per table instead of per entry you will get
> spurious notifications when other entries change. The easiest way to
> test this is to poll /proc/sys/kernel/hostname and change your
> domainname.

You will get spurious wakeups but not spurious notifications to
userspace since event is still per entry.

For my money that seemed a nice compromise of code simplicity, and
generality.  We could of course do something much closer to what
sysfs does and allocate and refcount something similar to your
ctl_table_poll when we have a ctl_table opened.  But that just looked
like a pain.

Of course we already have spurious notifications for hostname and
domainname when multiple uts namespaces are involved, but that
is a different problem.

> I couldn't apply this patch to any tree (linus/master + my previous
> patch, your master, 3.3 + my previous patch), so I couldn't test. On
> top of your tree:

How odd.  It should have applied cleanly to my tree and it applies
with just a two line offset top of Linus's latest with my tree merged
in.  Those two lines of offset coming from the two extra includes
that came in through the merge.

patch -p1 --dry-run <  ~/tmp/sysctl-poll-test.patch 
patching file fs/proc/proc_sysctl.c
Hunk #1 succeeded at 18 (offset 2 lines).
Hunk #2 succeeded at 173 (offset 2 lines).
Hunk #3 succeeded at 245 (offset 2 lines).
Hunk #4 succeeded at 512 (offset 2 lines).
Hunk #5 succeeded at 542 (offset 2 lines).
Hunk #6 succeeded at 561 (offset 2 lines).
patching file include/linux/sysctl.h
patching file kernel/utsname_sysctl.c

> [lucas@...er kernel]$ git am /tmp/a.patch
> Applying: Making poll generally useful for sysctls
> error: patch failed: fs/proc/proc_sysctl.c:16
> error: fs/proc/proc_sysctl.c: patch does not apply
> error: patch failed: include/linux/sysctl.h:992
> error: include/linux/sysctl.h: patch does not apply
> Patch failed at 0001 Making poll generally useful for sysctls

Here is rebased version of the patch just in case that helps.

---

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index 21d836f..739615c 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -18,13 +18,15 @@ static const struct inode_operations proc_sys_inode_operations;
 static const struct file_operations proc_sys_dir_file_operations;
 static const struct inode_operations proc_sys_dir_operations;
 
-void proc_sys_poll_notify(struct ctl_table_poll *poll)
+static inline void *proc_sys_poll_event(struct ctl_table *table)
 {
-	if (!poll)
-		return;
+	return (void *)(unsigned long)atomic_read(&table->event);
+}
 
-	atomic_inc(&poll->event);
-	wake_up_interruptible(&poll->wait);
+void proc_sys_poll_notify(struct ctl_table_header *head, struct ctl_table *table)
+{
+	atomic_inc(&table->event);
+	wake_up_interruptible(&head->wait);
 }
 
 static struct ctl_table root_table[] = {
@@ -171,6 +173,7 @@ static void init_header(struct ctl_table_header *head,
 		for (entry = table; entry->procname; entry++, node++) {
 			rb_init_node(&node->node);
 			node->header = head;
+			atomic_set(&entry->event, 1);
 		}
 	}
 }
@@ -242,6 +245,8 @@ static void start_unregistering(struct ctl_table_header *p)
 		/* anything non-NULL; we'll never dereference it */
 		p->unregistering = ERR_PTR(-EINVAL);
 	}
+	/* Don't let poll sleep forever on deleted entries */
+	wake_up_interruptible(&p->wait);
 	/*
 	 * do not remove from the list until nobody holds it; walking the
 	 * list in do_sysctl() relies on that.
@@ -507,6 +512,9 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *buf,
 	error = table->proc_handler(table, write, buf, &res, ppos);
 	if (!error)
 		error = res;
+
+	if (write)
+		proc_sys_poll_notify(head, table);
 out:
 	sysctl_head_finish(head);
 
@@ -534,8 +542,7 @@ static int proc_sys_open(struct inode *inode, struct file *filp)
 	if (IS_ERR(head))
 		return PTR_ERR(head);
 
-	if (table->poll)
-		filp->private_data = proc_sys_poll_event(table->poll);
+	filp->private_data = proc_sys_poll_event(table);
 
 	sysctl_head_finish(head);
 
@@ -554,21 +561,14 @@ static unsigned int proc_sys_poll(struct file *filp, poll_table *wait)
 	if (IS_ERR(head))
 		return POLLERR | POLLHUP;
 
-	if (!table->proc_handler)
-		goto out;
-
-	if (!table->poll)
-		goto out;
-
 	event = (unsigned long)filp->private_data;
-	poll_wait(filp, &table->poll->wait, wait);
+	poll_wait(filp, &head->wait, wait);
 
-	if (event != atomic_read(&table->poll->event)) {
-		filp->private_data = proc_sys_poll_event(table->poll);
+	if (event != atomic_read(&table->event)) {
+		filp->private_data = proc_sys_poll_event(table);
 		ret = POLLIN | POLLRDNORM | POLLERR | POLLPRI;
 	}
 
-out:
 	sysctl_head_finish(head);
 
 	return ret;
diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index c34b4c8..8647ee0 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -992,34 +992,17 @@ extern int proc_do_large_bitmap(struct ctl_table *, int,
  * cover common cases.
  */
 
-/* Support for userspace poll() to watch for changes */
-struct ctl_table_poll {
-	atomic_t event;
-	wait_queue_head_t wait;
-};
-
-static inline void *proc_sys_poll_event(struct ctl_table_poll *poll)
-{
-	return (void *)(unsigned long)atomic_read(&poll->event);
-}
-
-#define __CTL_TABLE_POLL_INITIALIZER(name) {				\
-	.event = ATOMIC_INIT(0),					\
-	.wait = __WAIT_QUEUE_HEAD_INITIALIZER(name.wait) }
-
-#define DEFINE_CTL_TABLE_POLL(name)					\
-	struct ctl_table_poll name = __CTL_TABLE_POLL_INITIALIZER(name)
 
 /* A sysctl table is an array of struct ctl_table: */
 struct ctl_table 
 {
 	const char *procname;		/* Text ID for /proc/sys, or zero */
 	void *data;
+	atomic_t event;
 	int maxlen;
 	umode_t mode;
 	struct ctl_table *child;	/* Deprecated */
 	proc_handler *proc_handler;	/* Callback for text formatting */
-	struct ctl_table_poll *poll;
 	void *extra1;
 	void *extra2;
 };
@@ -1042,6 +1025,7 @@ struct ctl_table_header
 		};
 		struct rcu_head rcu;
 	};
+	wait_queue_head_t wait;
 	struct completion *unregistering;
 	struct ctl_table *ctl_table_arg;
 	struct ctl_table_root *root;
@@ -1076,7 +1060,7 @@ struct ctl_path {
 
 #ifdef CONFIG_SYSCTL
 
-void proc_sys_poll_notify(struct ctl_table_poll *poll);
+void proc_sys_poll_notify(struct ctl_table_header *head, struct ctl_table *table);
 
 extern void setup_sysctl_set(struct ctl_table_set *p,
 	struct ctl_table_root *root,
diff --git a/kernel/utsname_sysctl.c b/kernel/utsname_sysctl.c
index 63da38c..3e91a50 100644
--- a/kernel/utsname_sysctl.c
+++ b/kernel/utsname_sysctl.c
@@ -53,18 +53,12 @@ static int proc_do_uts_string(ctl_table *table, int write,
 	r = proc_dostring(&uts_table,write,buffer,lenp, ppos);
 	put_uts(table, write, uts_table.data);
 
-	if (write)
-		proc_sys_poll_notify(table->poll);
-
 	return r;
 }
 #else
 #define proc_do_uts_string NULL
 #endif
 
-static DEFINE_CTL_TABLE_POLL(hostname_poll);
-static DEFINE_CTL_TABLE_POLL(domainname_poll);
-
 static struct ctl_table uts_kern_table[] = {
 	{
 		.procname	= "ostype",
@@ -93,7 +87,6 @@ static struct ctl_table uts_kern_table[] = {
 		.maxlen		= sizeof(init_uts_ns.name.nodename),
 		.mode		= 0644,
 		.proc_handler	= proc_do_uts_string,
-		.poll		= &hostname_poll,
 	},
 	{
 		.procname	= "domainname",
@@ -101,7 +94,6 @@ static struct ctl_table uts_kern_table[] = {
 		.maxlen		= sizeof(init_uts_ns.name.domainname),
 		.mode		= 0644,
 		.proc_handler	= proc_do_uts_string,
-		.poll		= &domainname_poll,
 	},
 	{}
 };
@@ -115,6 +107,8 @@ static struct ctl_table uts_root_table[] = {
 	{}
 };
 
+static struct ctl_table_header *uts_header;
+
 #ifdef CONFIG_PROC_SYSCTL
 /*
  * Notify userspace about a change in a certain entry of uts_kern_table,
@@ -124,13 +118,13 @@ void uts_proc_notify(enum uts_proc proc)
 {
 	struct ctl_table *table = &uts_kern_table[proc];
 
-	proc_sys_poll_notify(table->poll);
+	proc_sys_poll_notify(uts_header, table);
 }
 #endif
 
 static int __init utsname_sysctl_init(void)
 {
-	register_sysctl_table(uts_root_table);
+	uts_header = register_sysctl_table(uts_root_table);
 	return 0;
 }
 
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