Messages in this thread | | | Subject | Re: [PATCH 3/3] bitops.h: set_mask_bits() to return old value | From | Vineet Gupta <> | Date | Fri, 11 Jan 2019 12:58:22 -0800 |
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On 1/11/19 1:24 AM, Peter Zijlstra wrote: > diff --git a/include/linux/bitops.h b/include/linux/bitops.h > index 705f7c442691..2060d26a35f5 100644 > --- a/include/linux/bitops.h > +++ b/include/linux/bitops.h > @@ -241,10 +241,10 @@ static __always_inline void __assign_bit(long nr, volatile unsigned long *addr, > const typeof(*(ptr)) mask__ = (mask), bits__ = (bits); \ > typeof(*(ptr)) old__, new__; \ > \ > + old__ = READ_ONCE(*(ptr)); \ > do { \ > - old__ = READ_ONCE(*(ptr)); \ > new__ = (old__ & ~mask__) | bits__; \ > - } while (cmpxchg(ptr, old__, new__) != old__); \ > + } while (!try_cmpxchg(ptr, &old__, new__)); \ > \ > new__; \ > }) > > > While there you probably want something like the above...
As a separate change perhaps so that a revert (unlikely as it might be) could be done with less pain.
> although, > looking at it now, we seem to have 'forgotten' to add try_cmpxchg to the > generic code :/
So it _has_ to be a separate change ;-)
But can we even provide a sane generic try_cmpxchg. The asm-generic cmpxchg relies on local irq save etc so it is clearly only to prevent a new arch from failing to compile. atomic*_cmpxchg() is different story since atomics have to be provided by arch.
Anyhow what is more interesting is the try_cmpxchg API itself. So commit a9ebf306f52c756 introduced/use of try_cmpxchg(), which indeed makes the looping "nicer" to read and obvious code gen improvements.
So, for (;;) { new = val $op $imm; old = cmpxchg(ptr, val, new); if (old == val) break; val = old; }
becomes
do { } while (!try_cmpxchg(ptr, &val, val $op $imm));
But on pure LL/SC retry based arches, we still end up with generated code having 2 loops. We discussed something similar a while back: see [1]
First loop is inside inline asm to retry LL/SC and the outer one due to code above. Explicit return of try_cmpxchg() means setting up a register with a boolean status of cmpxchg (AFAIKR ARMv7 already does that but ARC e.g. uses a CPU flag thus requires an additional insn or two). We could arguably remove the inline asm loop and retry LL/SC from the outer loop, but it seems cleaner to keep the retry where it belongs.
Also under the hood, try_cmpxchg() would end up re-reading it for the issue fixed by commit 44fe84459faf1a.
Heck, it would all be simpler if we could express this w/o use of cmpxchg.
try_some_op(ptr, &val, val $op $imm);
P.S. the horrible API name is for indicative purposes only
This would remove the outer loop completely, also avoid any re-reads due to the semantics of cmpxchg etc.
[1] https://www.spinics.net/lists/kernel/msg2029217.html
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