Hosts the 0x221E linux distro kernel. https://git.0xinfinity.dev/distro/kernel.git/atom?h=linux-3.15.y 2013-09-27T10:58:48Z 2013-09-27T10:58:48Z Vineet Gupta 2013-09-25T11:23:32Z urn:sha1:6c00350b573c0bd3635436e43e8696951dd6e1b6 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> 2013-02-11T14:30:35Z Vineet Gupta 2013-01-18T09:42:18Z urn:sha1:6e35fa2d430538cd0609e499c6f789beea9e9798 Signed-off-by: Vineet Gupta <vgupta@synopsys.com> Acked-by: Arnd Bergmann <arnd@arndb.de>