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    Subject[PATCH printk-rework 11/12] printk: remove logbuf_lock
    Date
    Since the ringbuffer is lockless, there is no need for it to be
    protected by @logbuf_lock. Remove @logbuf_lock.

    This means that printk_nmi_direct and printk_safe_flush_on_panic()
    no longer need to acquire any lock to run.

    @console_seq, @exclusive_console_stop_seq, @console_dropped are
    protected by @console_lock.

    Signed-off-by: John Ogness <john.ogness@linutronix.de>
    ---
    kernel/printk/internal.h | 4 +-
    kernel/printk/printk.c | 134 +++++++++++-------------------------
    kernel/printk/printk_safe.c | 29 +++-----
    3 files changed, 50 insertions(+), 117 deletions(-)

    diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
    index 3a8fd491758c..e7acc2888c8e 100644
    --- a/kernel/printk/internal.h
    +++ b/kernel/printk/internal.h
    @@ -12,8 +12,6 @@

    #define PRINTK_NMI_CONTEXT_OFFSET 0x010000000

    -extern raw_spinlock_t logbuf_lock;
    -
    __printf(4, 0)
    int vprintk_store(int facility, int level,
    const struct dev_printk_info *dev_info,
    @@ -59,7 +57,7 @@ void defer_console_output(void);
    __printf(1, 0) int vprintk_func(const char *fmt, va_list args) { return 0; }

    /*
    - * In !PRINTK builds we still export logbuf_lock spin_lock, console_sem
    + * In !PRINTK builds we still export console_sem
    * semaphore and some of console functions (console_unlock()/etc.), so
    * printk-safe must preserve the existing local IRQ guarantees.
    */
    diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
    index d14a4afc5b72..b57dba7f077d 100644
    --- a/kernel/printk/printk.c
    +++ b/kernel/printk/printk.c
    @@ -355,41 +355,6 @@ enum log_flags {
    LOG_CONT = 8, /* text is a fragment of a continuation line */
    };

    -/*
    - * The logbuf_lock protects kmsg buffer, indices, counters. This can be taken
    - * within the scheduler's rq lock. It must be released before calling
    - * console_unlock() or anything else that might wake up a process.
    - */
    -DEFINE_RAW_SPINLOCK(logbuf_lock);
    -
    -/*
    - * Helper macros to lock/unlock logbuf_lock and switch between
    - * printk-safe/unsafe modes.
    - */
    -#define logbuf_lock_irq() \
    - do { \
    - printk_safe_enter_irq(); \
    - raw_spin_lock(&logbuf_lock); \
    - } while (0)
    -
    -#define logbuf_unlock_irq() \
    - do { \
    - raw_spin_unlock(&logbuf_lock); \
    - printk_safe_exit_irq(); \
    - } while (0)
    -
    -#define logbuf_lock_irqsave(flags) \
    - do { \
    - printk_safe_enter_irqsave(flags); \
    - raw_spin_lock(&logbuf_lock); \
    - } while (0)
    -
    -#define logbuf_unlock_irqrestore(flags) \
    - do { \
    - raw_spin_unlock(&logbuf_lock); \
    - printk_safe_exit_irqrestore(flags); \
    - } while (0)
    -
    /* syslog_lock protects syslog_* variables and write access to clear_seq. */
    static DEFINE_RAW_SPINLOCK(syslog_lock);

    @@ -401,6 +366,7 @@ static u64 syslog_seq;
    static size_t syslog_partial;
    static bool syslog_time;

    +/* All 3 protected by @console_sem. */
    /* the next printk record to write to the console */
    static u64 console_seq;
    static u64 exclusive_console_stop_seq;
    @@ -762,27 +728,27 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
    if (ret)
    return ret;

    - logbuf_lock_irq();
    + printk_safe_enter_irq();
    if (!prb_read_valid(prb, atomic64_read(&user->seq), r)) {
    if (file->f_flags & O_NONBLOCK) {
    ret = -EAGAIN;
    - logbuf_unlock_irq();
    + printk_safe_exit_irq();
    goto out;
    }

    - logbuf_unlock_irq();
    + printk_safe_exit_irq();
    ret = wait_event_interruptible(log_wait,
    prb_read_valid(prb, atomic64_read(&user->seq), r));
    if (ret)
    goto out;
    - logbuf_lock_irq();
    + printk_safe_enter_irq();
    }

    if (atomic64_read(&user->seq) < prb_first_valid_seq(prb)) {
    /* our last seen message is gone, return error and reset */
    atomic64_set(&user->seq, prb_first_valid_seq(prb));
    ret = -EPIPE;
    - logbuf_unlock_irq();
    + printk_safe_exit_irq();
    goto out;
    }

    @@ -792,7 +758,7 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
    &r->info->dev_info);

    atomic64_set(&user->seq, r->info->seq + 1);
    - logbuf_unlock_irq();
    + printk_safe_exit_irq();

    if (len > count) {
    ret = -EINVAL;
    @@ -827,7 +793,7 @@ static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence)
    if (offset)
    return -ESPIPE;

    - logbuf_lock_irq();
    + printk_safe_enter_irq();
    switch (whence) {
    case SEEK_SET:
    /* the first record */
    @@ -848,7 +814,7 @@ static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence)
    default:
    ret = -EINVAL;
    }
    - logbuf_unlock_irq();
    + printk_safe_exit_irq();
    return ret;
    }

    @@ -862,7 +828,7 @@ static __poll_t devkmsg_poll(struct file *file, poll_table *wait)

    poll_wait(file, &log_wait, wait);

    - logbuf_lock_irq();
    + printk_safe_enter_irq();
    if (prb_read_valid(prb, atomic64_read(&user->seq), NULL)) {
    /* return error when data has vanished underneath us */
    if (atomic64_read(&user->seq) < prb_first_valid_seq(prb))
    @@ -870,7 +836,7 @@ static __poll_t devkmsg_poll(struct file *file, poll_table *wait)
    else
    ret = EPOLLIN|EPOLLRDNORM;
    }
    - logbuf_unlock_irq();
    + printk_safe_exit_irq();

    return ret;
    }
    @@ -903,9 +869,9 @@ static int devkmsg_open(struct inode *inode, struct file *file)
    prb_rec_init_rd(&user->record, &user->info,
    &user->text_buf[0], sizeof(user->text_buf));

    - logbuf_lock_irq();
    + printk_safe_enter_irq();
    atomic64_set(&user->seq, prb_first_valid_seq(prb));
    - logbuf_unlock_irq();
    + printk_safe_exit_irq();

    file->private_data = user;
    return 0;
    @@ -1518,11 +1484,11 @@ static int syslog_print(char __user *buf, int size)
    size_t n;
    size_t skip;

    - logbuf_lock_irq();
    + printk_safe_enter_irq();
    raw_spin_lock(&syslog_lock);
    if (!prb_read_valid(prb, syslog_seq, &r)) {
    raw_spin_unlock(&syslog_lock);
    - logbuf_unlock_irq();
    + printk_safe_exit_irq();
    break;
    }
    if (r.info->seq != syslog_seq) {
    @@ -1552,7 +1518,7 @@ static int syslog_print(char __user *buf, int size)
    } else
    n = 0;
    raw_spin_unlock(&syslog_lock);
    - logbuf_unlock_irq();
    + printk_safe_exit_irq();

    if (!n)
    break;
    @@ -1586,7 +1552,7 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
    return -ENOMEM;

    time = printk_time;
    - logbuf_lock_irq();
    + printk_safe_enter_irq();
    /*
    * Find first record that fits, including all following records,
    * into the user-provided buffer for this dump.
    @@ -1607,12 +1573,12 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
    break;
    }

    - logbuf_unlock_irq();
    + printk_safe_exit_irq();
    if (copy_to_user(buf + len, text, textlen))
    len = -EFAULT;
    else
    len += textlen;
    - logbuf_lock_irq();
    + printk_safe_enter_irq();

    if (len < 0)
    break;
    @@ -1623,7 +1589,7 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
    latched_seq_write(&clear_seq, seq);
    raw_spin_unlock(&syslog_lock);
    }
    - logbuf_unlock_irq();
    + printk_safe_exit_irq();

    kfree(text);
    return len;
    @@ -1631,11 +1597,11 @@ static int syslog_print_all(char __user *buf, int size, bool clear)

    static void syslog_clear(void)
    {
    - logbuf_lock_irq();
    + printk_safe_enter_irq();
    raw_spin_lock(&syslog_lock);
    latched_seq_write(&clear_seq, prb_next_seq(prb));
    raw_spin_unlock(&syslog_lock);
    - logbuf_unlock_irq();
    + printk_safe_exit_irq();
    }

    int do_syslog(int type, char __user *buf, int len, int source)
    @@ -1716,7 +1682,7 @@ int do_syslog(int type, char __user *buf, int len, int source)
    break;
    /* Number of chars in the log buffer */
    case SYSLOG_ACTION_SIZE_UNREAD:
    - logbuf_lock_irq();
    + printk_safe_enter_irq();
    raw_spin_lock(&syslog_lock);
    if (syslog_seq < prb_first_valid_seq(prb)) {
    /* messages are gone, move to first one */
    @@ -1745,7 +1711,7 @@ int do_syslog(int type, char __user *buf, int len, int source)
    error -= syslog_partial;
    }
    raw_spin_unlock(&syslog_lock);
    - logbuf_unlock_irq();
    + printk_safe_exit_irq();
    break;
    /* Size of the log buffer */
    case SYSLOG_ACTION_SIZE_BUFFER:
    @@ -2593,7 +2559,6 @@ void console_unlock(void)
    size_t len;

    printk_safe_enter_irqsave(flags);
    - raw_spin_lock(&logbuf_lock);
    skip:
    if (!prb_read_valid(prb, console_seq, &r))
    break;
    @@ -2637,7 +2602,6 @@ void console_unlock(void)
    console_msg_format & MSG_FORMAT_SYSLOG,
    printk_time);
    console_seq++;
    - raw_spin_unlock(&logbuf_lock);

    /*
    * While actively printing out messages, if another printk()
    @@ -2664,8 +2628,6 @@ void console_unlock(void)

    console_locked = 0;

    - raw_spin_unlock(&logbuf_lock);
    -
    up_console_sem();

    /*
    @@ -2674,9 +2636,7 @@ void console_unlock(void)
    * there's a new owner and the console_unlock() from them will do the
    * flush, no worries.
    */
    - raw_spin_lock(&logbuf_lock);
    retry = prb_read_valid(prb, console_seq, NULL);
    - raw_spin_unlock(&logbuf_lock);
    printk_safe_exit_irqrestore(flags);

    if (retry && console_trylock())
    @@ -2743,9 +2703,9 @@ void console_flush_on_panic(enum con_flush_mode mode)
    if (mode == CONSOLE_REPLAY_ALL) {
    unsigned long flags;

    - logbuf_lock_irqsave(flags);
    + printk_safe_enter_irqsave(flags);
    console_seq = prb_first_valid_seq(prb);
    - logbuf_unlock_irqrestore(flags);
    + printk_safe_exit_irqrestore(flags);
    }
    console_unlock();
    }
    @@ -2973,9 +2933,7 @@ void register_console(struct console *newcon)
    /*
    * console_unlock(); will print out the buffered messages
    * for us.
    - */
    - logbuf_lock_irqsave(flags);
    - /*
    + *
    * We're about to replay the log buffer. Only do this to the
    * just-registered console to avoid excessive message spam to
    * the already-registered consoles.
    @@ -2988,11 +2946,9 @@ void register_console(struct console *newcon)
    exclusive_console_stop_seq = console_seq;

    /* Get a consistent copy of @syslog_seq. */
    - raw_spin_lock(&syslog_lock);
    + raw_spin_lock_irqsave(&syslog_lock, flags);
    console_seq = syslog_seq;
    - raw_spin_unlock(&syslog_lock);
    -
    - logbuf_unlock_irqrestore(flags);
    + raw_spin_unlock_irqrestore(&syslog_lock, flags);
    }
    console_unlock();
    console_sysfs_notify();
    @@ -3359,7 +3315,6 @@ void kmsg_dump(enum kmsg_dump_reason reason)
    {
    struct kmsg_dumper_iter iter;
    struct kmsg_dumper *dumper;
    - unsigned long flags;

    rcu_read_lock();
    list_for_each_entry_rcu(dumper, &dump_list, list) {
    @@ -3378,10 +3333,7 @@ void kmsg_dump(enum kmsg_dump_reason reason)

    /* initialize iterator with data about the stored records */
    iter.active = true;
    - logbuf_lock_irqsave(flags);
    - iter.cur_seq = latched_seq_read_nolock(&clear_seq);
    - iter.next_seq = prb_next_seq(prb);
    - logbuf_unlock_irqrestore(flags);
    + kmsg_dump_rewind_nolock(&iter);

    /* invoke dumper which will iterate over records */
    dumper->dump(dumper, reason, &iter);
    @@ -3414,9 +3366,11 @@ bool kmsg_dump_get_line_nolock(struct kmsg_dumper_iter *iter, bool syslog,
    struct printk_info info;
    unsigned int line_count;
    struct printk_record r;
    + unsigned long flags;
    size_t l = 0;
    bool ret = false;

    + printk_safe_enter_irqsave(flags);
    prb_rec_init_rd(&r, &info, line, size);

    if (!iter->active)
    @@ -3440,6 +3394,7 @@ bool kmsg_dump_get_line_nolock(struct kmsg_dumper_iter *iter, bool syslog,
    iter->cur_seq = r.info->seq + 1;
    ret = true;
    out:
    + printk_safe_exit_irqrestore(flags);
    if (len)
    *len = l;
    return ret;
    @@ -3465,14 +3420,7 @@ bool kmsg_dump_get_line_nolock(struct kmsg_dumper_iter *iter, bool syslog,
    bool kmsg_dump_get_line(struct kmsg_dumper_iter *iter, bool syslog,
    char *line, size_t size, size_t *len)
    {
    - unsigned long flags;
    - bool ret;
    -
    - logbuf_lock_irqsave(flags);
    - ret = kmsg_dump_get_line_nolock(iter, syslog, line, size, len);
    - logbuf_unlock_irqrestore(flags);
    -
    - return ret;
    + return kmsg_dump_get_line_nolock(iter, syslog, line, size, len);
    }
    EXPORT_SYMBOL_GPL(kmsg_dump_get_line);

    @@ -3510,7 +3458,7 @@ bool kmsg_dump_get_buffer(struct kmsg_dumper_iter *iter, bool syslog,
    if (!iter->active || !buf || !size)
    goto out;

    - logbuf_lock_irqsave(flags);
    + printk_safe_enter_irqsave(flags);
    if (iter->cur_seq < prb_first_valid_seq(prb)) {
    /* messages are gone, move to first available one */
    iter->cur_seq = prb_first_valid_seq(prb);
    @@ -3518,7 +3466,7 @@ bool kmsg_dump_get_buffer(struct kmsg_dumper_iter *iter, bool syslog,

    /* last entry */
    if (iter->cur_seq >= iter->next_seq) {
    - logbuf_unlock_irqrestore(flags);
    + printk_safe_exit_irqrestore(flags);
    goto out;
    }

    @@ -3549,7 +3497,7 @@ bool kmsg_dump_get_buffer(struct kmsg_dumper_iter *iter, bool syslog,

    iter->next_seq = next_seq;
    ret = true;
    - logbuf_unlock_irqrestore(flags);
    + printk_safe_exit_irqrestore(flags);
    out:
    if (len_out)
    *len_out = len;
    @@ -3569,8 +3517,12 @@ EXPORT_SYMBOL_GPL(kmsg_dump_get_buffer);
    */
    void kmsg_dump_rewind_nolock(struct kmsg_dumper_iter *iter)
    {
    + unsigned long flags;
    +
    + printk_safe_enter_irqsave(flags);
    iter->cur_seq = latched_seq_read_nolock(&clear_seq);
    iter->next_seq = prb_next_seq(prb);
    + printk_safe_exit_irqrestore(flags);
    }

    /**
    @@ -3583,11 +3535,7 @@ void kmsg_dump_rewind_nolock(struct kmsg_dumper_iter *iter)
    */
    void kmsg_dump_rewind(struct kmsg_dumper_iter *iter)
    {
    - unsigned long flags;
    -
    - logbuf_lock_irqsave(flags);
    kmsg_dump_rewind_nolock(iter);
    - logbuf_unlock_irqrestore(flags);
    }
    EXPORT_SYMBOL_GPL(kmsg_dump_rewind);

    diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
    index a0e6f746de6c..a9a3137bd972 100644
    --- a/kernel/printk/printk_safe.c
    +++ b/kernel/printk/printk_safe.c
    @@ -16,7 +16,7 @@
    #include "internal.h"

    /*
    - * printk() could not take logbuf_lock in NMI context. Instead,
    + * In NMI and safe mode, printk() avoids taking locks. Instead,
    * it uses an alternative implementation that temporary stores
    * the strings into a per-CPU buffer. The content of the buffer
    * is later flushed into the main ring buffer via IRQ work.
    @@ -266,18 +266,6 @@ void printk_safe_flush(void)
    */
    void printk_safe_flush_on_panic(void)
    {
    - /*
    - * Make sure that we could access the main ring buffer.
    - * Do not risk a double release when more CPUs are up.
    - */
    - if (raw_spin_is_locked(&logbuf_lock)) {
    - if (num_online_cpus() > 1)
    - return;
    -
    - debug_locks_off();
    - raw_spin_lock_init(&logbuf_lock);
    - }
    -
    printk_safe_flush();
    }

    @@ -311,9 +299,7 @@ void noinstr printk_nmi_exit(void)
    * reordering.
    *
    * It has effect only when called in NMI context. Then printk()
    - * will try to store the messages into the main logbuf directly
    - * and use the per-CPU buffers only as a fallback when the lock
    - * is not available.
    + * will store the messages into the main logbuf directly.
    */
    void printk_nmi_direct_enter(void)
    {
    @@ -368,20 +354,21 @@ __printf(1, 0) int vprintk_func(const char *fmt, va_list args)
    #endif

    /*
    - * Try to use the main logbuf even in NMI. But avoid calling console
    + * Use the main logbuf even in NMI. But avoid calling console
    * drivers that might have their own locks.
    */
    - if ((this_cpu_read(printk_context) & PRINTK_NMI_DIRECT_CONTEXT_MASK) &&
    - raw_spin_trylock(&logbuf_lock)) {
    + if ((this_cpu_read(printk_context) & PRINTK_NMI_DIRECT_CONTEXT_MASK)) {
    + unsigned long flags;
    int len;

    + printk_safe_enter_irqsave(flags);
    len = vprintk_store(0, LOGLEVEL_DEFAULT, NULL, fmt, args);
    - raw_spin_unlock(&logbuf_lock);
    + printk_safe_exit_irqrestore(flags);
    defer_console_output();
    return len;
    }

    - /* Use extra buffer in NMI when logbuf_lock is taken or in safe mode. */
    + /* Use extra buffer in NMI. */
    if (this_cpu_read(printk_context) & PRINTK_NMI_CONTEXT_MASK)
    return vprintk_nmi(fmt, args);

    --
    2.20.1
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