On Fri, Oct 23, 2020 at 11:50:11PM +0200, Thomas Gleixner wrote:
On Thu, Oct 22 2020 at 15:26, ira weiny wrote:
From: Thomas Gleixner tglx@linutronix.de
Lockdep state handling on NMI enter and exit is nothing specific to X86. It's not any different on other architectures. Also the extra state type is not necessary, irqentry_state_t can carry the necessary information as well.
Move it to common code and extend irqentry_state_t to carry lockdep state.
This lacks something like:
[ Ira: Made the states a union as they are mutually exclusive and added the missing kernel doc ]
Fair enough. done.
Hrm.
#ifndef irqentry_state typedef struct irqentry_state {
- bool exit_rcu;
- union {
bool exit_rcu;bool lockdep;- };
} irqentry_state_t; #endif
-E_NO_KERNELDOC
Adding: Paul McKenney
I'm happy to write something but I'm very unfamiliar with this code. So I'm getting confused what exactly exit_rcu is flagging.
I can see that exit_rcu is a bad name for the state used in irqentry_nmi_[enter|exit](). Furthermore, I see why 'lockdep' is a better name. But similar lockdep handling is used in irqentry_exit() if exit_rcu is true...
Given my limited knowledge; here is my proposed text:
/** * struct irqentry_state - Opaque object for exception state storage * @exit_rcu: Used exclusively in the irqentry_*() calls; tracks if the * exception hit the idle task which requires special handling, * including calling rcu_irq_exit(), when the exception exits. * @lockdep: Used exclusively in the irqentry_nmi_*() calls; ensures lockdep * tracking is maintained if hardirqs were already enabled * * This opaque object is filled in by the irqentry_*_enter() functions and * should be passed back into the corresponding irqentry_*_exit() functions * when the exception is complete. * * Callers of irqentry_*_[enter|exit]() should consider this structure opaque * and all members private. Descriptions of the members are provided to aid in * the maintenance of the irqentry_*() functions. */
Perhaps Paul can enlighten me on how exit_rcu is used beyond just flagging a call to rcu_irq_exit()?
Why do we call lockdep_hardirqs_off() only when in the idle task? That implies that regs_irqs_disabled() can only be false if we were in the idle task to match up the lockdep on/off calls. This does not make sense to me because why do we need the extra check for exit_rcu? I'm still trying to understand when regs_irqs_disabled() is false.
} else if (!regs_irqs_disabled(regs)) { ... } else { /* * IRQ flags state is correct already. Just tell RCU if it * was not watching on entry. */ if (state.exit_rcu) rcu_irq_exit(); }
Also, the comment in irqentry_enter() refers to irq_enter_from_user_mode() which does not seem to exist anymore. So I'm not sure what careful sequence it is referring to.
/* * If RCU is not watching then the same careful * sequence vs. lockdep and tracing is required * as in irq_enter_from_user_mode(). */
?
Ira