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    Subject[PATCH printk v2 02/38] printk: Convert console_drivers list to hlist
    Date
    Replace the open coded single linked list with a hlist so a conversion
    to SRCU protected list walks can reuse the existing primitives.

    Signed-off-by: John Ogness <john.ogness@linutronix.de>
    ---
    fs/proc/consoles.c | 3 +-
    include/linux/console.h | 8 ++--
    kernel/printk/printk.c | 99 +++++++++++++++++++++++------------------
    3 files changed, 63 insertions(+), 47 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..867becc40021 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)) {
    + res = -ENODEV;
    + goto out_unlock;
    }

    - 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();

    @@ -3263,10 +3276,8 @@ int unregister_console(struct console *console)

    return res;

    -out_disable_unlock:
    - console->flags &= ~CON_ENABLED;
    +out_unlock:
    console_unlock();
    -
    return res;
    }
    EXPORT_SYMBOL(unregister_console);
    @@ -3317,10 +3328,11 @@ 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 +3352,7 @@ 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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