[PATCH printk v5 02/40] printk: Convert console_drivers list to hlist

From: John Ogness
Date: Wed Nov 16 2022 - 11:22:17 EST

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From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

Replace the open coded single linked list with a hlist so a conversion
to SRCU protected list walks can reuse the existing primitives.

Co-developed-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Reviewed-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>
---
fs/proc/consoles.c | 3 +-
include/linux/console.h | 8 ++--
kernel/printk/printk.c | 101 ++++++++++++++++++++++------------------
3 files changed, 62 insertions(+), 50 deletions(-)

diff --git a/fs/proc/consoles.c b/fs/proc/consoles.c
index dfe6ce3505ce..cf2e0788f9c7 100644
--- a/fs/proc/consoles.c
+++ b/fs/proc/consoles.c
@@ -74,8 +74,9 @@ static void *c_start(struct seq_file *m, loff_t *pos)
static void *c_next(struct seq_file *m, void *v, loff_t *pos)
{
struct console *con = v;
+
++*pos;
- return con->next;
+ return hlist_entry_safe(con->node.next, struct console, node);
}

static void c_stop(struct seq_file *m, void *v)
diff --git a/include/linux/console.h b/include/linux/console.h
index 8c1686e2c233..7b5f21f9e469 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -15,6 +15,7 @@
#define _LINUX_CONSOLE_H_ 1

#include <linux/atomic.h>
+#include <linux/list.h>
#include <linux/types.h>

struct vc_data;
@@ -154,14 +155,16 @@ struct console {
u64 seq;
unsigned long dropped;
void *data;
- struct console *next;
+ struct hlist_node node;
};

+extern struct hlist_head console_list;
+
/*
* for_each_console() allows you to iterate on each console
*/
#define for_each_console(con) \
- for (con = console_drivers; con != NULL; con = con->next)
+ hlist_for_each_entry(con, &console_list, node)

extern int console_set_on_cmdline;
extern struct console *early_console;
@@ -174,7 +177,6 @@ enum con_flush_mode {
extern int add_preferred_console(char *name, int idx, char *options);
extern void register_console(struct console *);
extern int unregister_console(struct console *);
-extern struct console *console_drivers;
extern void console_lock(void);
extern int console_trylock(void);
extern void console_unlock(void);
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index e4f1e7478b52..e6f0832e71f0 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -79,13 +79,12 @@ int oops_in_progress;
EXPORT_SYMBOL(oops_in_progress);

/*
- * console_sem protects the console_drivers list, and also
- * provides serialisation for access to the entire console
- * driver system.
+ * console_sem protects console_list and console->flags updates, and also
+ * provides serialization for access to the entire console driver system.
*/
static DEFINE_SEMAPHORE(console_sem);
-struct console *console_drivers;
-EXPORT_SYMBOL_GPL(console_drivers);
+HLIST_HEAD(console_list);
+EXPORT_SYMBOL_GPL(console_list);

/*
* System may need to suppress printk message under certain
@@ -2556,7 +2555,7 @@ static int console_cpu_notify(unsigned int cpu)
* console_lock - lock the console system for exclusive use.
*
* Acquires a lock which guarantees that the caller has
- * exclusive access to the console system and the console_drivers list.
+ * exclusive access to the console system and console_list.
*
* Can sleep, returns nothing.
*/
@@ -2576,7 +2575,7 @@ EXPORT_SYMBOL(console_lock);
* console_trylock - try to lock the console system for exclusive use.
*
* Try to acquire a lock which guarantees that the caller has exclusive
- * access to the console system and the console_drivers list.
+ * access to the console system and console_list.
*
* returns 1 on success, and 0 on failure to acquire the lock.
*/
@@ -2940,11 +2939,20 @@ void console_flush_on_panic(enum con_flush_mode mode)
console_may_schedule = 0;

if (mode == CONSOLE_REPLAY_ALL) {
+ struct hlist_node *tmp;
struct console *c;
u64 seq;

seq = prb_first_valid_seq(prb);
- for_each_console(c)
+ /*
+ * This cannot use for_each_console() because it's not established
+ * that the current context has console locked and neither there is
+ * a guarantee that there is no concurrency in that case.
+ *
+ * Open code it for documentation purposes and pretend that
+ * it works.
+ */
+ hlist_for_each_entry_safe(c, tmp, &console_list, node)
c->seq = seq;
}
console_unlock();
@@ -3081,6 +3089,9 @@ static void try_enable_default_console(struct console *newcon)
(con->flags & CON_BOOT) ? "boot" : "", \
con->name, con->index, ##__VA_ARGS__)

+#define console_first() \
+ hlist_entry(console_list.first, struct console, node)
+
/*
* The console driver calls this routine during kernel initialization
* to register the console printing procedure with printk() and to
@@ -3140,8 +3151,8 @@ void register_console(struct console *newcon)
* flag set and will be first in the list.
*/
if (preferred_console < 0) {
- if (!console_drivers || !console_drivers->device ||
- console_drivers->flags & CON_BOOT) {
+ if (hlist_empty(&console_list) || !console_first()->device ||
+ console_first()->flags & CON_BOOT) {
try_enable_default_console(newcon);
}
}
@@ -3169,20 +3180,22 @@ void register_console(struct console *newcon)
}

/*
- * Put this console in the list - keep the
- * preferred driver at the head of the list.
+ * Put this console in the list - keep the
+ * preferred driver at the head of the list.
*/
console_lock();
- if ((newcon->flags & CON_CONSDEV) || console_drivers == NULL) {
- newcon->next = console_drivers;
- console_drivers = newcon;
- if (newcon->next)
- newcon->next->flags &= ~CON_CONSDEV;
- /* Ensure this flag is always set for the head of the list */
+ if (hlist_empty(&console_list)) {
+ /* Ensure CON_CONSDEV is always set for the head. */
newcon->flags |= CON_CONSDEV;
+ hlist_add_head(&newcon->node, &console_list);
+
+ } else if (newcon->flags & CON_CONSDEV) {
+ /* Only the new head can have CON_CONSDEV set. */
+ console_first()->flags &= ~CON_CONSDEV;
+ hlist_add_head(&newcon->node, &console_list);
+
} else {
- newcon->next = console_drivers->next;
- console_drivers->next = newcon;
+ hlist_add_behind(&newcon->node, console_list.first);
}

newcon->dropped = 0;
@@ -3209,16 +3222,18 @@ void register_console(struct console *newcon)
if (bootcon_enabled &&
((newcon->flags & (CON_CONSDEV | CON_BOOT)) == CON_CONSDEV) &&
!keep_bootcon) {
- for_each_console(con)
+ struct hlist_node *tmp;
+
+ hlist_for_each_entry_safe(con, tmp, &console_list, node) {
if (con->flags & CON_BOOT)
unregister_console(con);
+ }
}
}
EXPORT_SYMBOL(register_console);

int unregister_console(struct console *console)
{
- struct console *con;
int res;

con_printk(KERN_INFO, console, "disabled\n");
@@ -3229,32 +3244,30 @@ int unregister_console(struct console *console)
if (res > 0)
return 0;

- res = -ENODEV;
console_lock();
- if (console_drivers == console) {
- console_drivers=console->next;
- res = 0;
- } else {
- for_each_console(con) {
- if (con->next == console) {
- con->next = console->next;
- res = 0;
- break;
- }
- }
+
+ /* Disable it unconditionally */
+ console->flags &= ~CON_ENABLED;
+
+ if (hlist_unhashed(&console->node)) {
+ console_unlock();
+ return -ENODEV;
}

- if (res)
- goto out_disable_unlock;
+ hlist_del_init(&console->node);

/*
+ * <HISTORICAL>
* If this isn't the last console and it has CON_CONSDEV set, we
* need to set it on the next preferred console.
+ * </HISTORICAL>
+ *
+ * The above makes no sense as there is no guarantee that the next
+ * console has any device attached. Oh well....
*/
- if (console_drivers != NULL && console->flags & CON_CONSDEV)
- console_drivers->flags |= CON_CONSDEV;
+ if (!hlist_empty(&console_list) && console->flags & CON_CONSDEV)
+ console_first()->flags |= CON_CONSDEV;

- console->flags &= ~CON_ENABLED;
console_unlock();
console_sysfs_notify();

@@ -3262,12 +3275,6 @@ int unregister_console(struct console *console)
res = console->exit(console);

return res;
-
-out_disable_unlock:
- console->flags &= ~CON_ENABLED;
- console_unlock();
-
- return res;
}
EXPORT_SYMBOL(unregister_console);

@@ -3317,10 +3324,11 @@ void __init console_init(void)
*/
static int __init printk_late_init(void)
{
+ struct hlist_node *tmp;
struct console *con;
int ret;

- for_each_console(con) {
+ hlist_for_each_entry_safe(con, tmp, &console_list, node) {
if (!(con->flags & CON_BOOT))
continue;

@@ -3340,6 +3348,7 @@ static int __init printk_late_init(void)
unregister_console(con);
}
}
+
ret = cpuhp_setup_state_nocalls(CPUHP_PRINTK_DEAD, "printk:dead", NULL,
console_cpu_notify);
WARN_ON(ret < 0);
--
2.30.2

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