Commit 041aa7a8 authored by Mark Rutland's avatar Mark Rutland Committed by Thomas Gleixner
Browse files

entry: Split preemption from irqentry_exit_to_kernel_mode() 

Some architecture-specific work needs to be performed between the state
management for exception entry/exit and the "real" work to handle the
exception. For example, arm64 needs to manipulate a number of exception
masking bits, with different exceptions requiring different masking.

Generally this can all be hidden in the architecture code, but for arm64
the current structure of irqentry_exit_to_kernel_mode() makes this
particularly difficult to handle in a way that is correct, maintainable,
and efficient.

The gory details are described in the thread surrounding:

  https://lore.kernel.org/lkml/acPAzdtjK5w-rNqC@J2N7QTR9R3/



The summary is:

* Currently, irqentry_exit_to_kernel_mode() handles both involuntary
  preemption AND state management necessary for exception return.

* When scheduling (including involuntary preemption), arm64 needs to
  have all arm64-specific exceptions unmasked, though regular interrupts
  must be masked.

* Prior to the state management for exception return, arm64 needs to
  mask a number of arm64-specific exceptions, and perform some work with
  these exceptions masked (with RCU watching, etc).

While in theory it is possible to handle this with a new arch_*() hook
called somewhere under irqentry_exit_to_kernel_mode(), this is fragile
and complicated, and doesn't match the flow used for exception return to
user mode, which has a separate 'prepare' step (where preemption can
occur) prior to the state management.

To solve this, refactor irqentry_exit_to_kernel_mode() to match the
style of {irqentry,syscall}_exit_to_user_mode(), moving preemption logic
into a new irqentry_exit_to_kernel_mode_preempt() function, and moving
state management in a new irqentry_exit_to_kernel_mode_after_preempt()
function. The existing irqentry_exit_to_kernel_mode() is left as a
caller of both of these, avoiding the need to modify existing callers.

There should be no functional change as a result of this change.

[ tglx: Updated kernel doc ]

Signed-off-by: default avatarMark Rutland <mark.rutland@arm.com>
Signed-off-by: default avatarThomas Gleixner <tglx@kernel.org>
Reviewed-by: default avatarJinjie Ruan <ruanjinjie@huawei.com>
Acked-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260407131650.3813777-6-mark.rutland@arm.com
parent c5538d01

include/linux/irq-entry-common.h

+59 −14
Original line number Diff line number Diff line
@@ -438,24 +438,46 @@ static __always_inline irqentry_state_t irqentry_enter_from_kernel_mode(struct p 
}



/**

 * irqentry_exit_to_kernel_mode - Run preempt checks and establish state after

 *				  invoking the interrupt handler

 * irqentry_exit_to_kernel_mode_preempt - Run preempt checks on return to kernel mode

 * @regs:	Pointer to current's pt_regs

 * @state:	Return value from matching call to irqentry_enter_from_kernel_mode()

 *

 * This is the counterpart of irqentry_enter_from_kernel_mode() and runs the

 * necessary preemption check if possible and required. It returns to the caller

 * with interrupts disabled and the correct state vs. tracing, lockdep and RCU

 * required to return to the interrupted context.

 * This is to be invoked before irqentry_exit_to_kernel_mode_after_preempt() to

 * allow kernel preemption on return from interrupt.

 *

 * Must be invoked with interrupts disabled and CPU state which allows kernel

 * preemption.

 *

 * It is the last action before returning to the low level ASM code which just

 * needs to return.

 * After returning from this function, the caller can modify CPU state before

 * invoking irqentry_exit_to_kernel_mode_after_preempt(), which is required to

 * re-establish the tracing, lockdep and RCU state for returning to the

 * interrupted context.

 */

static __always_inline void irqentry_exit_to_kernel_mode(struct pt_regs *regs,

static inline void irqentry_exit_to_kernel_mode_preempt(struct pt_regs *regs,

							irqentry_state_t state)

{

	lockdep_assert_irqs_disabled();

	if (regs_irqs_disabled(regs) || state.exit_rcu)

		return;



	if (IS_ENABLED(CONFIG_PREEMPTION))

		irqentry_exit_cond_resched();

}



/**

 * irqentry_exit_to_kernel_mode_after_preempt - Establish trace, lockdep and RCU state

 * @regs:	Pointer to current's pt_regs

 * @state:	Return value from matching call to irqentry_enter_from_kernel_mode()

 *

 * This is to be invoked after irqentry_exit_to_kernel_mode_preempt() and before

 * actually returning to the interrupted context.

 *

 * There are no requirements for the CPU state other than being able to complete

 * the tracing, lockdep and RCU state transitions. After this function returns

 * the caller must return directly to the interrupted context.

 */

static __always_inline void

irqentry_exit_to_kernel_mode_after_preempt(struct pt_regs *regs, irqentry_state_t state)

{

	if (!regs_irqs_disabled(regs)) {

		/*

		 * If RCU was not watching on entry this needs to be done
@@ -474,9 +496,6 @@ static __always_inline void irqentry_exit_to_kernel_mode(struct pt_regs *regs, 
		}



		instrumentation_begin();

		if (IS_ENABLED(CONFIG_PREEMPTION))

			irqentry_exit_cond_resched();



		/* Covers both tracing and lockdep */

		trace_hardirqs_on();

		instrumentation_end();
@@ -490,6 +509,32 @@ static __always_inline void irqentry_exit_to_kernel_mode(struct pt_regs *regs, 
	}

}



/**

 * irqentry_exit_to_kernel_mode - Run preempt checks and establish state after

 *				  invoking the interrupt handler

 * @regs:	Pointer to current's pt_regs

 * @state:	Return value from matching call to irqentry_enter_from_kernel_mode()

 *

 * This is the counterpart of irqentry_enter_from_kernel_mode() and combines

 * the calls to irqentry_exit_to_kernel_mode_preempt() and

 * irqentry_exit_to_kernel_mode_after_preempt().

 *

 * The requirement for the CPU state is that it can schedule. After the function

 * returns the tracing, lockdep and RCU state transitions are completed and the

 * caller must return directly to the interrupted context.

 */

static __always_inline void irqentry_exit_to_kernel_mode(struct pt_regs *regs,

							 irqentry_state_t state)

{

	lockdep_assert_irqs_disabled();



	instrumentation_begin();

	irqentry_exit_to_kernel_mode_preempt(regs, state);

	instrumentation_end();



	irqentry_exit_to_kernel_mode_after_preempt(regs, state);

}



/**

 * irqentry_enter - Handle state tracking on ordinary interrupt entries

 * @regs:	Pointer to pt_regs of interrupted context