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Some ARC SMP systems lack native atomic R-M-W (LLOCK/SCOND) insns and
can only use atomic EX insn (reg with mem) to build higher level R-M-W
primitives. This includes a SystemC based SMP simulation model.
So rwlocks need to use a protecting spinlock for atomic cmp-n-exchange
operation to update reader(s)/writer count.
The spinlock operation itself looks as follows:
mov reg, 1 ; 1=locked, 0=unlocked
retry:
EX reg, [lock] ; load existing, store 1, atomically
BREQ reg, 1, rety ; if already locked, retry
In single-threaded simulation, SystemC alternates between the 2 cores
with "N" insn each based scheduling. Additionally for insn with global
side effect, such as EX writing to shared mem, a core switch is
enforced too.
Given that, 2 cores doing a repeated EX on same location, Linux often
got into a livelock e.g. when both cores were fiddling with tasklist
lock (gdbserver / hackbench) for read/write respectively as the
sequence diagram below shows:
core1 core2
-------- --------
1. spin lock [EX r=0, w=1] - LOCKED
2. rwlock(Read) - LOCKED
3. spin unlock [ST 0] - UNLOCKED
spin lock [EX r=0,w=1] - LOCKED
-- resched core 1----
5. spin lock [EX r=1] - ALREADY-LOCKED
-- resched core 2----
6. rwlock(Write) - READER-LOCKED
7. spin unlock [ST 0]
8. rwlock failed, retry again
9. spin lock [EX r=0, w=1]
-- resched core 1----
10 spinlock locked in #9, retry #5
11. spin lock [EX gets 1]
-- resched core 2----
...
...
The fix was to unlock using the EX insn too (step 7), to trigger another
SystemC scheduling pass which would let core1 proceed, eliding the
livelock.
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
152 lines
3.5 KiB
C
152 lines
3.5 KiB
C
/*
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* Copyright (C) 2004, 2007-2010, 2011-2012 Synopsys, Inc. (www.synopsys.com)
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License version 2 as
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* published by the Free Software Foundation.
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*/
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#ifndef __ASM_SPINLOCK_H
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#define __ASM_SPINLOCK_H
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#include <asm/spinlock_types.h>
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#include <asm/processor.h>
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#include <asm/barrier.h>
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#define arch_spin_is_locked(x) ((x)->slock != __ARCH_SPIN_LOCK_UNLOCKED__)
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#define arch_spin_lock_flags(lock, flags) arch_spin_lock(lock)
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#define arch_spin_unlock_wait(x) \
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do { while (arch_spin_is_locked(x)) cpu_relax(); } while (0)
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static inline void arch_spin_lock(arch_spinlock_t *lock)
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{
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unsigned int tmp = __ARCH_SPIN_LOCK_LOCKED__;
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__asm__ __volatile__(
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"1: ex %0, [%1] \n"
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" breq %0, %2, 1b \n"
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: "+&r" (tmp)
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: "r"(&(lock->slock)), "ir"(__ARCH_SPIN_LOCK_LOCKED__)
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: "memory");
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}
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static inline int arch_spin_trylock(arch_spinlock_t *lock)
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{
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unsigned int tmp = __ARCH_SPIN_LOCK_LOCKED__;
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__asm__ __volatile__(
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"1: ex %0, [%1] \n"
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: "+r" (tmp)
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: "r"(&(lock->slock))
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: "memory");
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return (tmp == __ARCH_SPIN_LOCK_UNLOCKED__);
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}
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static inline void arch_spin_unlock(arch_spinlock_t *lock)
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{
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unsigned int tmp = __ARCH_SPIN_LOCK_UNLOCKED__;
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__asm__ __volatile__(
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" ex %0, [%1] \n"
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: "+r" (tmp)
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: "r"(&(lock->slock))
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: "memory");
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smp_mb();
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}
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/*
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* Read-write spinlocks, allowing multiple readers but only one writer.
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*
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* The spinlock itself is contained in @counter and access to it is
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* serialized with @lock_mutex.
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*
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* Unfair locking as Writers could be starved indefinitely by Reader(s)
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*/
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/* Would read_trylock() succeed? */
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#define arch_read_can_lock(x) ((x)->counter > 0)
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/* Would write_trylock() succeed? */
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#define arch_write_can_lock(x) ((x)->counter == __ARCH_RW_LOCK_UNLOCKED__)
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/* 1 - lock taken successfully */
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static inline int arch_read_trylock(arch_rwlock_t *rw)
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{
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int ret = 0;
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arch_spin_lock(&(rw->lock_mutex));
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/*
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* zero means writer holds the lock exclusively, deny Reader.
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* Otherwise grant lock to first/subseq reader
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*/
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if (rw->counter > 0) {
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rw->counter--;
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ret = 1;
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}
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arch_spin_unlock(&(rw->lock_mutex));
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smp_mb();
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return ret;
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}
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/* 1 - lock taken successfully */
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static inline int arch_write_trylock(arch_rwlock_t *rw)
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{
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int ret = 0;
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arch_spin_lock(&(rw->lock_mutex));
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/*
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* If reader(s) hold lock (lock < __ARCH_RW_LOCK_UNLOCKED__),
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* deny writer. Otherwise if unlocked grant to writer
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* Hence the claim that Linux rwlocks are unfair to writers.
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* (can be starved for an indefinite time by readers).
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*/
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if (rw->counter == __ARCH_RW_LOCK_UNLOCKED__) {
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rw->counter = 0;
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ret = 1;
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}
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arch_spin_unlock(&(rw->lock_mutex));
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return ret;
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}
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static inline void arch_read_lock(arch_rwlock_t *rw)
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{
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while (!arch_read_trylock(rw))
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cpu_relax();
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}
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static inline void arch_write_lock(arch_rwlock_t *rw)
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{
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while (!arch_write_trylock(rw))
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cpu_relax();
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}
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static inline void arch_read_unlock(arch_rwlock_t *rw)
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{
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arch_spin_lock(&(rw->lock_mutex));
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rw->counter++;
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arch_spin_unlock(&(rw->lock_mutex));
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}
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static inline void arch_write_unlock(arch_rwlock_t *rw)
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{
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arch_spin_lock(&(rw->lock_mutex));
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rw->counter = __ARCH_RW_LOCK_UNLOCKED__;
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arch_spin_unlock(&(rw->lock_mutex));
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}
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#define arch_read_lock_flags(lock, flags) arch_read_lock(lock)
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#define arch_write_lock_flags(lock, flags) arch_write_lock(lock)
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#define arch_spin_relax(lock) cpu_relax()
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#define arch_read_relax(lock) cpu_relax()
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#define arch_write_relax(lock) cpu_relax()
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#endif /* __ASM_SPINLOCK_H */
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