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On the Alpha, interprocessor interrupts come in at the same priority
as normal device interrupts.  Because of this, we won't get IPIs while
servicing such interrupts.  This can lead to the following deadlock
scenario as reported by Bill Sommerfeld:
- Process runs on cpu1, but has FP state on cpu0.
- Process executes FP-using insn, causing an FP trap, which causes
  the kernel lock to be acquired.
- At roughly the same time, cpu0 receives a device interrupt, and attempts
  to acquire the kernel lock, which blocks since cpu1 already has it.
- cpu1 sends cpu0 a SYNCH FPU IPI, and waits for cpu0 to release its
  FP state.
- Since cpu0 cannot notice the IPI until it has processed the device
  interrupt, which it cannot do because it cannot acquire the kernel
  lock, we have deadlock.

Solve the problem by adding a spinlock interlock release hook which
checks for pending IPIs and processes them.

Idea from Bill Sommerfeld.

/* $NetBSD: lock.h,v 1.11 2000/11/20 20:17:13 thorpej Exp $ */

/*-
 * Copyright (c) 1998, 1999, 2000 The NetBSD Foundation, Inc.
 * All rights reserved.
 *
 * This code is derived from software contributed to The NetBSD Foundation
 * by Jason R. Thorpe of the Numerical Aerospace Simulation Facility,
 * NASA Ames Research Center.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *	This product includes software developed by the NetBSD
 *	Foundation, Inc. and its contributors.
 * 4. Neither the name of The NetBSD Foundation nor the names of its
 *    contributors may be used to endorse or promote products derived
 *    from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

/*
 * Machine-dependent spin lock operations.
 */

#ifndef _ALPHA_LOCK_H_
#define	_ALPHA_LOCK_H_

typedef	__volatile int		__cpu_simple_lock_t;

#define	__SIMPLELOCK_LOCKED	1
#define	__SIMPLELOCK_UNLOCKED	0

static __inline void
__cpu_simple_lock_init(__cpu_simple_lock_t *alp)
{

	__asm __volatile(
		"# BEGIN __cpu_simple_lock_init\n"
		"	stl	$31, %0		\n"
		"	mb			\n"
		"	# END __cpu_simple_lock_init"
		: "=m" (*alp));
}

static __inline void
__cpu_simple_lock(__cpu_simple_lock_t *alp)
{
	unsigned long t0;

	/*
	 * Note, if we detect that the lock is held when
	 * we do the initial load-locked, we spin using
	 * a non-locked load to save the coherency logic
	 * some work.
	 */

	__asm __volatile(
		"# BEGIN __cpu_simple_lock\n"
		"1:	ldl_l	%0, %3		\n"
		"	bne	%0, 2f		\n"
		"	bis	$31, %2, %0	\n"
		"	stl_c	%0, %1		\n"
		"	beq	%0, 3f		\n"
		"	mb			\n"
		"	br	4f		\n"
		"2:	ldl	%0, %3		\n"
		"	beq	%0, 1b		\n"
		"	br	2b		\n"
		"3:	br	1b		\n"
		"4:				\n"
		"	# END __cpu_simple_lock\n"
		: "=r" (t0), "=m" (*alp)
		: "i" (__SIMPLELOCK_LOCKED), "1" (*alp));
}

static __inline int
__cpu_simple_lock_try(__cpu_simple_lock_t *alp)
{
	unsigned long t0, v0;

	__asm __volatile(
		"# BEGIN __cpu_simple_lock_try\n"
		"1:	ldl_l	%0, %4		\n"
		"	bne	%0, 2f		\n"
		"	bis	$31, %3, %0	\n"
		"	stl_c	%0, %2		\n"
		"	beq	%0, 3f		\n"
		"	mb			\n"
		"	bis	$31, 1, %1	\n"
		"	br	4f		\n"
		"2:	bis	$31, $31, %1	\n"
		"	br	4f		\n"
		"3:	br	1b		\n"
		"4:				\n"
		"	# END __cpu_simple_lock_try"
		: "=r" (t0), "=r" (v0), "=m" (*alp)
		: "i" (__SIMPLELOCK_LOCKED), "2" (*alp));

	return (v0 != 0);
}

static __inline void
__cpu_simple_unlock(__cpu_simple_lock_t *alp)
{

	__asm __volatile(
		"# BEGIN __cpu_simple_unlock\n"
		"	mb			\n"
		"	stl	$31, %0		\n"
		"	# END __cpu_simple_unlock"
		: "=m" (*alp));
}

/*
 * On the Alpha, interprocessor interrupts come in at device priority
 * level.  This can cause some problems while waiting for r/w spinlocks
 * from a high'ish priority level: IPIs that come in will not be processed.
 * This can lead to deadlock.
 *
 * This hook allows IPIs to be processed when a spinlock's interlock
 * is released.
 */
#define	SPINLOCK_INTERLOCK_RELEASE_HOOK					\
do {									\
	struct cpu_info *__ci = curcpu();				\
									\
	if (__ci->ci_ipis != 0)						\
		alpha_ipi_process(__ci);				\
} while (0)

#endif /* _ALPHA_LOCK_H_ */