This is a note to let you know that I've just added the patch titled
ARC: Workaround spinlock livelock in SMP SystemC simulation
to the 3.11-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
arc-workaround-spinlock-livelock-in-smp-systemc-simulation.patch
and it can be found in the queue-3.11 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 6c00350b573c0bd3635436e43e8696951dd6e1b6 Mon Sep 17 00:00:00 2001
From: Vineet Gupta <vgupta@xxxxxxxxxxxx>
Date: Wed, 25 Sep 2013 16:53:32 +0530
Subject: ARC: Workaround spinlock livelock in SMP SystemC simulation
From: Vineet Gupta <vgupta@xxxxxxxxxxxx>
commit 6c00350b573c0bd3635436e43e8696951dd6e1b6 upstream.
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@xxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
arch/arc/include/asm/spinlock.h | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)
--- 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
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();
}
Patches currently in stable-queue which might be from vgupta@xxxxxxxxxxxx are
queue-3.11/arc-fix-32-bit-wrap-around-in-access_ok.patch
queue-3.11/arc-fix-signal-frame-management-for-sa_siginfo.patch
queue-3.11/arc-ignore-ptrace-setregset-request-for-synthetic-register-stop_pc.patch
queue-3.11/arc-handle-zero-overhead-loop-in-unaligned-access-handler.patch
queue-3.11/arc-fix-__udelay-calculation.patch
queue-3.11/arc-workaround-spinlock-livelock-in-smp-systemc-simulation.patch
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