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Commit 421dd6fa authored by Chris Metcalf's avatar Chris Metcalf Committed by Will Deacon
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arm64: factor work_pending state machine to C 



Currently ret_fast_syscall, work_pending, and ret_to_user form an ad-hoc
state machine that can be difficult to reason about due to duplicated
code and a large number of branch targets.

This patch factors the common logic out into the existing
do_notify_resume function, converting the code to C in the process,
making the code more legible.

This patch tries to closely mirror the existing behaviour while using
the usual C control flow primitives. As local_irq_{disable,enable} may
be instrumented, we balance exception entry (where we will almost most
likely enable IRQs) with a call to trace_hardirqs_on just before the
return to userspace.

Signed-off-by: default avatarChris Metcalf <cmetcalf@mellanox.com>
Signed-off-by: default avatarWill Deacon <will.deacon@arm.com>
parent 0a7d87a7

arch/arm64/kernel/entry.S

+4 −8
Original line number Diff line number Diff line
@@ -707,18 +707,13 @@ ret_fast_syscall_trace: 
 * Ok, we need to do extra processing, enter the slow path.

 */

work_pending:

	tbnz	x1, #TIF_NEED_RESCHED, work_resched

	/* TIF_SIGPENDING, TIF_NOTIFY_RESUME or TIF_FOREIGN_FPSTATE case */

	mov	x0, sp				// 'regs'

	enable_irq				// enable interrupts for do_notify_resume()

	bl	do_notify_resume

	b	ret_to_user

work_resched:

#ifdef CONFIG_TRACE_IRQFLAGS

	bl	trace_hardirqs_off		// the IRQs are off here, inform the tracing code

	bl	trace_hardirqs_on		// enabled while in userspace

#endif

	bl	schedule



	ldr	x1, [tsk, #TI_FLAGS]		// re-check for single-step

	b	finish_ret_to_user

/*

 * "slow" syscall return path.

 */
@@ -727,6 +722,7 @@ ret_to_user: 
	ldr	x1, [tsk, #TI_FLAGS]

	and	x2, x1, #_TIF_WORK_MASK

	cbnz	x2, work_pending

finish_ret_to_user:

	enable_step_tsk x1, x2

	kernel_exit 0

ENDPROC(ret_to_user)

arch/arm64/kernel/signal.c

+26 −10
Original line number Diff line number Diff line
@@ -402,6 +402,18 @@ static void do_signal(struct pt_regs *regs) 
asmlinkage void do_notify_resume(struct pt_regs *regs,

				 unsigned int thread_flags)

{

	/*

	 * The assembly code enters us with IRQs off, but it hasn't

	 * informed the tracing code of that for efficiency reasons.

	 * Update the trace code with the current status.

	 */

	trace_hardirqs_off();

	do {

		if (thread_flags & _TIF_NEED_RESCHED) {

			schedule();

		} else {

			local_irq_enable();



			if (thread_flags & _TIF_SIGPENDING)

				do_signal(regs);
@@ -412,5 +424,9 @@ asmlinkage void do_notify_resume(struct pt_regs *regs, 


			if (thread_flags & _TIF_FOREIGN_FPSTATE)

				fpsimd_restore_current_state();

		}



		local_irq_disable();

		thread_flags = READ_ONCE(current_thread_info()->flags);

	} while (thread_flags & _TIF_WORK_MASK);

}