On Thu, Aug 30, 2018 at 08:31:59PM +0000, Vineet Gupta wrote: > On 08/30/2018 07:29 AM, Peter Zijlstra wrote: > > On Thu, Aug 30, 2018 at 03:23:55PM +0100, Will Deacon wrote: > > > >> Yes, that would be worth trying. However, I also just noticed that the > >> fetch-ops (which are now used to implement test_and_set_bit_lock()) seem > >> to be missing the backwards branch in the LL/SC case. Yet another diff > >> below. > >> > >> Will > >> > >> --->8 > >> > >> diff --git a/arch/arc/include/asm/atomic.h b/arch/arc/include/asm/atomic.h > >> index 4e0072730241..f06c5ed672b3 100644 > >> --- a/arch/arc/include/asm/atomic.h > >> +++ b/arch/arc/include/asm/atomic.h > >> @@ -84,7 +84,7 @@ static inline int atomic_fetch_##op(int i, atomic_t *v) \ > >> "1: llock %[orig], [%[ctr]] \n" \ > >> " " #asm_op " %[val], %[orig], %[i] \n" \ > >> " scond %[val], [%[ctr]] \n" \ > >> - " \n" \ > >> + " bnz 1b \n" \ > >> : [val] "=&r" (val), \ > >> [orig] "=&r" (orig) \ > >> : [ctr] "r" (&v->counter), \ > > ACK!! sorry about that, no idea how I messed that up. > > > > Also, once it all works, they should look at switching to _relaxed > > atomics for LL/SC. > > Indeed this is the mother of all issues, I tried and system is clearly hosed with > and works after. > What's amazing is the commit 4aef66c8ae9 which introduced it is from 2016 ;-) > Back then we had a retry branch with backoff stuff which I'd reverted for new > cores and the merge conflict somehow missed it. > > @PeterZ I'll create a patch with you as author ? do I need any formal sign offs, > acks etc ? Well, Will spotted it, give authorship to him, you have my ack per the above. > So after this there are 2 other things to be addresses / looked at still while we > are still here. > > 1. After 84c6591103db __clear_bit_lock() implementation will be broken (or atleast > not consistent with what we had after), do we need to reinstate it. > 2. Will's proposed change to remove the underlying issue, but the issue in #1 > remains ? No, like explained, for spinlock based atomics the issue _should_ not exist, and if you look at your atomic_set() implementation for that variant, you'll see it does the right thing by taking the lock. Basically atomic_set() for spinlock based atomics ends up being (void)atomic_xchg(). FWIW, also ACK on Will's patch to switch you over to asm-generic bitops entirely.
