Re: [PATCH v5 2/2] printk: Hide console waiter logic into helpers

Steven Rostedt
2018-01-12T16:36:33Z

On Fri, 12 Jan 2018 17:08:37 +0100
Petr Mladek <pmladek@suse.com> wrote:

> On Fri 2018-01-12 10:37:54, Steven Rostedt wrote:
> > On Thu, 11 Jan 2018 13:03:41 +0100
> > Petr Mladek <pmladek@suse.com> wrote:
> > > All the other changes look good to me. I will use them in the next version.
> >
> > Great.
>
> Please, find below the updated version. If I get Ack at least from
> Steven and no nack's, I will put it into linux-next next week.
>

Typos below.

>
> >From f67f70d910d9cf310a7bc73e97bf14097d31b059 Mon Sep 17 00:00:00 2001
> From: Petr Mladek <pmladek@suse.com>
> Date: Fri, 22 Dec 2017 18:58:46 +0100
> Subject: [PATCH v6 2/4] printk: Hide console waiter logic into helpers
>
> The commit ("printk: Add console owner and waiter logic to load balance
> console writes") made vprintk_emit() and console_unlock() even more
> complicated.
>
> This patch extracts the new code into 3 helper functions. They should
> help to keep it rather self-contained. It will be easier to use and
> maintain.
>
> This patch just shuffles the existing code. It does not change
> the functionality.
>
> Signed-off-by: Petr Mladek <pmladek@suse.com>
> ---
> Changes against v5:
>
> + updated some comments (Steven)
> + do console_trylock() in console_trylock_spinning() (Steven)
>
> kernel/printk/printk.c | 245 +++++++++++++++++++++++++++++--------------------
> 1 file changed, 148 insertions(+), 97 deletions(-)
>
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 7e6459abba43..3057dbc69b4f 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -86,15 +86,8 @@ EXPORT_SYMBOL_GPL(console_drivers);
> static struct lockdep_map console_lock_dep_map = {
> .name = "console_lock"
> };
> -static struct lockdep_map console_owner_dep_map = {
> - .name = "console_owner"
> -};
> #endif
>
> -static DEFINE_RAW_SPINLOCK(console_owner_lock);
> -static struct task_struct *console_owner;
> -static bool console_waiter;
> -
> enum devkmsg_log_bits {
> __DEVKMSG_LOG_BIT_ON = 0,
> __DEVKMSG_LOG_BIT_OFF,
> @@ -1551,6 +1544,146 @@ SYSCALL_DEFINE3(syslog, int, type, char __user *, buf, int, len)
> }
>
> /*
> + * Special console_lock variants that help to reduce the risk of soft-lockups.
> + * They allow to pass console_lock to another printk() call using a busy wait.
> + */
> +
> +#ifdef CONFIG_LOCKDEP
> +static struct lockdep_map console_owner_dep_map = {
> + .name = "console_owner"
> +};
> +#endif
> +
> +static DEFINE_RAW_SPINLOCK(console_owner_lock);
> +static struct task_struct *console_owner;
> +static bool console_waiter;
> +
> +/**
> + * console_lock_spinning_enable - mark beginning of code where another
> + * thread might safely busy wait
> + *
> + * This basically converts console_lock into a spinlock. This marks
> + * the section where the console_lock owner can not sleep, because
> + * there may be a waiter spinning (like a spinlock). Also it must be
> + * ready to hand over the lock at the end of the section.
> + */
> +static void console_lock_spinning_enable(void)
> +{
> + raw_spin_lock(&console_owner_lock);
> + console_owner = current;
> + raw_spin_unlock(&console_owner_lock);
> +
> + /* The waiter may spin on us after setting console_owner */
> + spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
> +}
> +
> +/**
> + * console_lock_spinning_disable_and_check - mark end of code where another
> + * thread was able to busy wait and check if there is a waiter
> + *
> + * This is called at the end of the section where spinning is allowed.
> + * It has two functions. First, it is a signal that it is not longer

"it is no longer safe"

> + * safe to start busy waiting for the lock. Second, it checks if
> + * there is a busy waiter and passes the lock rights to her.
> + *
> + * Important: Callers lose the lock if there was the busy waiter.

"if there was a busy waiter"

> + * They must not touch items synchronized by console_lock
> + * in this case.
> + *
> + * Return: 1 if the lock rights were passed, 0 otherwise.
> + */
> +static int console_lock_spinning_disable_and_check(void)
> +{
> + int waiter;
> +
> + raw_spin_lock(&console_owner_lock);
> + waiter = READ_ONCE(console_waiter);
> + console_owner = NULL;
> + raw_spin_unlock(&console_owner_lock);
> +
> + if (!waiter) {
> + spin_release(&console_owner_dep_map, 1, _THIS_IP_);
> + return 0;
> + }
> +
> + /* The waiter is now free to continue */
> + WRITE_ONCE(console_waiter, false);
> +
> + spin_release(&console_owner_dep_map, 1, _THIS_IP_);
> +
> + /*
> + * Hand off console_lock to waiter. The waiter will perform
> + * the up(). After this, the waiter is the console_lock owner.
> + */
> + mutex_release(&console_lock_dep_map, 1, _THIS_IP_);
> + return 1;
> +}
> +
> +/**
> + * console_trylock_spinning - try to get console_lock by busy waiting
> + *
> + * This allows to busy wait for the console_lock when the current
> + * owner is running in a special marked sections. It means that

"running in specially marked sections."

> + * the current owner is running and cannot reschedule until it
> + * is ready to loose the lock.

"ready to lose the lock."

> + *
> + * Return: 1 if we got the lock, 0 othrewise
> + */
> +static int console_trylock_spinning(void)
> +{
> + struct task_struct *owner = NULL;
> + bool waiter;
> + bool spin = false;
> + unsigned long flags;
> +
> + if (console_trylock())
> + return 1;
> +
> + printk_safe_enter_irqsave(flags);
> +
> + raw_spin_lock(&console_owner_lock);
> + owner = READ_ONCE(console_owner);
> + waiter = READ_ONCE(console_waiter);
> + if (!waiter && owner && owner != current) {
> + WRITE_ONCE(console_waiter, true);
> + spin = true;
> + }
> + raw_spin_unlock(&console_owner_lock);
> +
> + /*
> + * If there is an active printk() writing to the
> + * consoles, instead of having it write our data too,
> + * see if we can offload that load from the active
> + * printer, and do some printing ourselves.
> + * Go into a spin only if there isn't already a waiter
> + * spinning, and there is an active printer, and
> + * that active printer isn't us (recursive printk?).
> + */
> + if (!spin) {
> + printk_safe_exit_irqrestore(flags);
> + return 0;
> + }
> +
> + /* We spin waiting for the owner to release us */
> + spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
> + /* Owner will clear console_waiter on hand off */
> + while (READ_ONCE(console_waiter))
> + cpu_relax();
> + spin_release(&console_owner_dep_map, 1, _THIS_IP_);
> +
> + printk_safe_exit_irqrestore(flags);
> + /*
> + * The owner passed the console lock to us.
> + * Since we did not spin on console lock, annotate
> + * this as a trylock. Otherwise lockdep will
> + * complain.
> + */
> + mutex_acquire(&console_lock_dep_map, 0, 1, _THIS_IP_);
> +
> + return 1;
> +}
> +
> +/*
> * Call the console drivers, asking them to write out
> * log_buf[start] to log_buf[end - 1].
> * The console_lock must be held.
> @@ -1760,56 +1893,8 @@ asmlinkage int vprintk_emit(int facility, int level,
> * semaphore. The release will print out buffers and wake up
> * /dev/kmsg and syslog() users.
> */
> - if (console_trylock()) {
> + if (console_trylock_spinning())
> console_unlock();
> - } else {
> - struct task_struct *owner = NULL;
> - bool waiter;
> - bool spin = false;
> -
> - printk_safe_enter_irqsave(flags);
> -
> - raw_spin_lock(&console_owner_lock);
> - owner = READ_ONCE(console_owner);
> - waiter = READ_ONCE(console_waiter);
> - if (!waiter && owner && owner != current) {
> - WRITE_ONCE(console_waiter, true);
> - spin = true;
> - }
> - raw_spin_unlock(&console_owner_lock);
> -
> - /*
> - * If there is an active printk() writing to the
> - * consoles, instead of having it write our data too,
> - * see if we can offload that load from the active
> - * printer, and do some printing ourselves.
> - * Go into a spin only if there isn't already a waiter
> - * spinning, and there is an active printer, and
> - * that active printer isn't us (recursive printk?).
> - */
> - if (spin) {
> - /* We spin waiting for the owner to release us */
> - spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
> - /* Owner will clear console_waiter on hand off */
> - while (READ_ONCE(console_waiter))
> - cpu_relax();
> -
> - spin_release(&console_owner_dep_map, 1, _THIS_IP_);
> - printk_safe_exit_irqrestore(flags);
> -
> - /*
> - * The owner passed the console lock to us.
> - * Since we did not spin on console lock, annotate
> - * this as a trylock. Otherwise lockdep will
> - * complain.
> - */
> - mutex_acquire(&console_lock_dep_map, 0, 1, _THIS_IP_);
> - console_unlock();
> - printk_safe_enter_irqsave(flags);
> - }
> - printk_safe_exit_irqrestore(flags);
> -
> - }
> }
>
> return printed_len;
> @@ -1910,6 +1995,8 @@ static ssize_t msg_print_ext_header(char *buf, size_t size,
> static ssize_t msg_print_ext_body(char *buf, size_t size,
> char *dict, size_t dict_len,
> char *text, size_t text_len) { return 0; }
> +static void console_lock_spinning_enable(void) { }
> +static int console_lock_spinning_disable_and_check(void) { return 0; }
> static void call_console_drivers(const char *ext_text, size_t ext_len,
> const char *text, size_t len) {}
> static size_t msg_print_text(const struct printk_log *msg,
> @@ -2196,7 +2283,6 @@ void console_unlock(void)
> static u64 seen_seq;
> unsigned long flags;
> bool wake_klogd = false;
> - bool waiter = false;
> bool do_cond_resched, retry;
>
> if (console_suspended) {
> @@ -2291,31 +2377,16 @@ void console_unlock(void)
> * finish. This task can not be preempted if there is a
> * waiter waiting to take over.
> */
> - raw_spin_lock(&console_owner_lock);
> - console_owner = current;
> - raw_spin_unlock(&console_owner_lock);
> -
> - /* The waiter may spin on us after setting console_owner */
> - spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
> + console_lock_spinning_enable();
>
> stop_critical_timings(); /* don't trace print latency */
> call_console_drivers(ext_text, ext_len, text, len);
> start_critical_timings();
>
> - raw_spin_lock(&console_owner_lock);
> - waiter = READ_ONCE(console_waiter);
> - console_owner = NULL;
> - raw_spin_unlock(&console_owner_lock);
> -
> - /*
> - * If there is a waiter waiting for us, then pass the
> - * rest of the work load over to that waiter.
> - */
> - if (waiter)
> - break;
> -
> - /* There was no waiter, and nothing will spin on us here */
> - spin_release(&console_owner_dep_map, 1, _THIS_IP_);
> + if (console_lock_spinning_disable_and_check()) {
> + printk_safe_exit_irqrestore(flags);
> + return;
> + }
>
> printk_safe_exit_irqrestore(flags);
>
> @@ -2323,26 +2394,6 @@ void console_unlock(void)
> cond_resched();
> }
>
> - /*
> - * If there is an active waiter waiting on the console_lock.
> - * Pass off the printing to the waiter, and the waiter
> - * will continue printing on its CPU, and when all writing
> - * has finished, the last printer will wake up klogd.
> - */
> - if (waiter) {
> - WRITE_ONCE(console_waiter, false);
> - /* The waiter is now free to continue */
> - spin_release(&console_owner_dep_map, 1, _THIS_IP_);
> - /*
> - * Hand off console_lock to waiter. The waiter will perform
> - * the up(). After this, the waiter is the console_lock owner.
> - */
> - mutex_release(&console_lock_dep_map, 1, _THIS_IP_);
> - printk_safe_exit_irqrestore(flags);
> - /* Note, if waiter is set, logbuf_lock is not held */
> - return;
> - }
> -
> console_locked = 0;
>
> /* Release the exclusive_console once it is used */

Besides the typos (which should be fixed)...

Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

-- Steve