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

Steven Rostedt
2018-01-10T17:52:24Z

On Wed, 10 Jan 2018 14:24:18 +0100
Petr Mladek <pmladek@suse.com> wrote:

> 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>
> ---
> kernel/printk/printk.c | 242 +++++++++++++++++++++++++++++--------------------
> 1 file changed, 145 insertions(+), 97 deletions(-)
>
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 7e6459abba43..6217c280e6c1 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 +1544,143 @@ 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 might be called in sections where the current console_lock owner

"might be"? It has to be called in sections where the current
console_lock owner can not sleep. It's basically saying "console lock is
now acting like a spinlock".

> + * cannot sleep. It is a signal that another thread might start busy
> + * waiting for console_lock.
> + */
> +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 section when spinning was enabled by
> + * console_lock_spinning_enable(). It has two functions. First, it

"This is called at the end of the section where spinning is allowed."

> + * is a signal that it is not longer safe to start busy waiting

"it is no longer safe"

> + * 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.
> + * They must not longer touch items synchornized by console_lock

"They must not touch items ..."

> + * in this case.
> + *
> + * Return: 1 if the lock rights were passed, 0 othrewise.

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

Can we add here:

if (console_trylock())
return 1;

And then we can simplify the below from:

if (console_trylock() || console_trylock_spinning())

to just

if (console_trylock_spinning())

-- Steve

> +
> + 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 +1890,8 @@ 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() || console_trylock_spinning())
> console_unlock();
>