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Subject[PATCH 3/4] ARC: Workaround spinlock livelock in SMP SystemC simulation
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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>
---
arch/arc/include/asm/spinlock.h | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/arc/include/asm/spinlock.h b/arch/arc/include/asm/spinlock.h
index f158197..b6a8c2d 100644
--- a/arch/arc/include/asm/spinlock.h
+++ b/arch/arc/include/asm/spinlock.h
@@ -45,7 +45,14 @@ static inline int arch_spin_trylock(arch_spinlock_t *lock)

static inline void arch_spin_unlock(arch_spinlock_t *lock)
{
- lock->slock = __ARCH_SPIN_LOCK_UNLOCKED__;
+ unsigned int tmp = __ARCH_SPIN_LOCK_UNLOCKED__;
+
+ __asm__ __volatile__(
+ " ex %0, [%1] \n"
+ : "+r" (tmp)
+ : "r"(&(lock->slock))
+ : "memory");
+
smp_mb();
}

--
1.8.1.2


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