Re: [PATCH 3/3] bitops.h: set_mask_bits() to return old value
On Thu, Jan 10, 2019 at 04:26:27PM -0800, Vineet Gupta wrote: > | > Also, set_mask_bits is used in fs quite a bit and we can possibly come up > | > with a generic llsc based implementation (w/o the cmpxchg loop) > | > | May I also suggest changing the return value of set_mask_bits() to old. > | > | You can compute the new value given old, but you cannot compute the old > | value given new, therefore old is the better return value. Also, no > | current user seems to use the return value, so changing it is without > | risk. > > Link: > http://lkml.kernel.org/g/20150807110955.gh16...@twins.programming.kicks-ass.net > Suggested-by: Peter Zijlstra > Cc: Miklos Szeredi > Cc: Ingo Molnar > Cc: Jani Nikula > Cc: Chris Wilson > Cc: Andrew Morton > Cc: Will Deacon > Signed-off-by: Vineet Gupta > --- > include/linux/bitops.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/include/linux/bitops.h b/include/linux/bitops.h > index 705f7c442691..602af23b98c7 100644 > --- a/include/linux/bitops.h > +++ b/include/linux/bitops.h > @@ -246,7 +246,7 @@ static __always_inline void __assign_bit(long nr, > volatile unsigned long *addr, > new__ = (old__ & ~mask__) | bits__; \ > } while (cmpxchg(ptr, old__, new__) != old__); \ > \ > - new__; \ > + old__; \ > }) > #endif Acked-by: Will Deacon May also explain why no in-tree users appear to use the return value! Will ___ linux-snps-arc mailing list linux-snps-arc@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-snps-arc
Re: [PATCH 3/3] bitops.h: set_mask_bits() to return old value
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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Re: [PATCH 3/3] bitops.h: set_mask_bits() to return old value
On Thu, Jan 10, 2019 at 04:26:27PM -0800, Vineet Gupta wrote:
> @@ -246,7 +246,7 @@ static __always_inline void __assign_bit(long nr,
> volatile unsigned long *addr,
> new__ = (old__ & ~mask__) | bits__; \
> } while (cmpxchg(ptr, old__, new__) != old__); \
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... although,
looking at it now, we seem to have 'forgotten' to add try_cmpxchg to the
generic code :/
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Re: [PATCH 3/3] bitops.h: set_mask_bits() to return old value
On 1/10/19 4:26 PM, Vineet Gupta wrote: > | > Also, set_mask_bits is used in fs quite a bit and we can possibly come up > | > with a generic llsc based implementation (w/o the cmpxchg loop) > | > | May I also suggest changing the return value of set_mask_bits() to old. > | > | You can compute the new value given old, but you cannot compute the old > | value given new, therefore old is the better return value. Also, no > | current user seems to use the return value, so changing it is without > | risk. > > Link: > http://lkml.kernel.org/g/20150807110955.gh16...@twins.programming.kicks-ass.net > Suggested-by: Peter Zijlstra > Cc: Miklos Szeredi > Cc: Ingo Molnar > Cc: Jani Nikula > Cc: Chris Wilson > Cc: Andrew Morton > Cc: Will Deacon > Signed-off-by: Vineet Gupta > Reviewed-by: Anthony Yznaga ___ linux-snps-arc mailing list linux-snps-arc@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-snps-arc
