Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [patch 03/50] sched: Prepare for RT sleeping spin/rwlocks | Date | Thu, 15 Jul 2021 00:20:28 +0100 |
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Hi,
On 13/07/21 17:10, Thomas Gleixner wrote: > From: Thomas Gleixner <tglx@linutronix.de> > > Waiting for spinlocks and rwlocks on non RT enabled kernels is task::state > preserving. Any wakeup which matches the state is valid. > > RT enabled kernels substitutes them with 'sleeping' spinlocks. This creates > an issue vs. task::state. > > In order to block on the lock the task has to overwrite task::state and a > consecutive wakeup issued by the unlocker sets the state back to > TASK_RUNNING. As a consequence the task loses the state which was set > before the lock acquire and also any regular wakeup targeted at the task > while it is blocked on the lock. >
I'm not sure I get this for spinlocks - p->__state != TASK_RUNNING means task is stopped (or about to be), IMO that doesn't go with spinning. I was thinking perhaps ptrace could be an issue, but I don't have a clear picture on that either. What am I missing?
> @@ -213,6 +234,47 @@ struct task_group; > raw_spin_unlock_irqrestore(¤t->pi_lock, flags); \ > } while (0) > > +/* > + * PREEMPT_RT specific variants for "sleeping" spin/rwlocks > + * > + * RT's spin/rwlock substitutions are state preserving. The state of the > + * task when blocking on the lock is saved in task_struct::saved_state and > + * restored after the lock has been acquired. These operations are > + * serialized by task_struct::pi_lock against try_to_wake_up(). Any non RT > + * lock related wakeups while the task is blocked on the lock are > + * redirected to operate on task_struct::saved_state to ensure that these > + * are not dropped. On restore task_struct::saved_state is set to > + * TASK_RUNNING so any wakeup attempt redirected to saved_state will fail. > + * > + * The lock operation looks like this: > + * > + * current_save_and_set_rtlock_wait_state(); > + * for (;;) { > + * if (try_lock()) > + * break; > + * raw_spin_unlock_irq(&lock->wait_lock); > + * schedule_rtlock(); > + * raw_spin_lock_irq(&lock->wait_lock); > + * set_current_state(TASK_RTLOCK_WAIT); > + * } > + * current_restore_rtlock_saved_state(); > + */ > +#define current_save_and_set_rtlock_wait_state() \ > + do { \ > + raw_spin_lock(¤t->pi_lock); \ > + current->saved_state = current->state; \ ^^^^^ That one somehow survived the s/state/__state/ renaming.
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