mirror of https://github.com/torvalds/linux.git
6c00350b57
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>
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| .. | ||
| Kbuild | ||
| arcregs.h | ||
| asm-offsets.h | ||
| atomic.h | ||
| barrier.h | ||
| bitops.h | ||
| bug.h | ||
| cache.h | ||
| cacheflush.h | ||
| checksum.h | ||
| clk.h | ||
| cmpxchg.h | ||
| current.h | ||
| delay.h | ||
| disasm.h | ||
| dma-mapping.h | ||
| dma.h | ||
| elf.h | ||
| entry.h | ||
| exec.h | ||
| futex.h | ||
| io.h | ||
| irq.h | ||
| irqflags.h | ||
| kdebug.h | ||
| kgdb.h | ||
| kprobes.h | ||
| linkage.h | ||
| mach_desc.h | ||
| mmu.h | ||
| mmu_context.h | ||
| module.h | ||
| mutex.h | ||
| page.h | ||
| perf_event.h | ||
| pgalloc.h | ||
| pgtable.h | ||
| processor.h | ||
| prom.h | ||
| ptrace.h | ||
| sections.h | ||
| segment.h | ||
| serial.h | ||
| setup.h | ||
| shmparam.h | ||
| smp.h | ||
| spinlock.h | ||
| spinlock_types.h | ||
| string.h | ||
| switch_to.h | ||
| syscall.h | ||
| syscalls.h | ||
| thread_info.h | ||
| timex.h | ||
| tlb-mmu1.h | ||
| tlb.h | ||
| tlbflush.h | ||
| uaccess.h | ||
| unaligned.h | ||
| unwind.h | ||