Hi,
On Thu 02-11-17 13:06:05, Steven Rostedt wrote:
> + 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();
Hum, what prevents us from rescheduling here? And what if the process
stored in console_owner is scheduled out? Both seem to be possible with
CONFIG_PREEMPT kernel? Unless I'm missing something you will need to
disable preemption in some places...
Other than that I like the simplicity of your approach.
Honza
> +
> + 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;
> @@ -2141,6 +2196,7 @@ 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) {
> @@ -2215,6 +2271,20 @@ skip:
> goto skip;
> }
>
> + /*
> + * While actively printing out messages, if another printk()
> + * were to occur on another CPU, it may wait for this one to
> + * finish. This task can not be preempted if there is a
> + * waiter waiting to take over.
> + */
> +
> + /* The waiter may spin on us after this */
> + spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
> +
> + raw_spin_lock(&console_owner_lock);
> + console_owner = current;
> + raw_spin_unlock(&console_owner_lock);
> +
> len += msg_print_text(msg, false, text + len, sizeof(text) - len);
> if (nr_ext_console_drivers) {
> ext_len = msg_print_ext_header(ext_text,
> @@ -2232,11 +2302,48 @@ skip:
> 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 = 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_);
> +
> printk_safe_exit_irqrestore(flags);
>
> if (do_cond_resched)
> 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 */
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
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