* [PATCH 3/3] bitops.h: set_mask_bits() to return old value
@ 2019-01-11 0:26 ` Vineet Gupta
0 siblings, 0 replies; 29+ messages in thread
From: Vineet Gupta @ 2019-01-11 0:26 UTC (permalink / raw)
To: linux-snps-arc
| > 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.GH16853 at twins.programming.kicks-ass.net
Suggested-by: Peter Zijlstra <peterz at infradead.org>
Cc: Miklos Szeredi <mszeredi at redhat.com>
Cc: Ingo Molnar <mingo at kernel.org>
Cc: Jani Nikula <jani.nikula at intel.com>
Cc: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Andrew Morton <akpm at linux-foundation.org>
Cc: Will Deacon <will.deacon at arm.com>
Signed-off-by: Vineet Gupta <vgupta at synopsys.com>
---
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
--
2.7.4
^ permalink raw reply related [flat|nested] 29+ messages in thread
* [PATCH 3/3] bitops.h: set_mask_bits() to return old value
@ 2019-01-11 0:26 ` Vineet Gupta
0 siblings, 0 replies; 29+ messages in thread
From: Vineet Gupta @ 2019-01-11 0:26 UTC (permalink / raw)
To: linux-kernel
Cc: linux-snps-arc, linux-mm, peterz, Vineet Gupta, Miklos Szeredi,
Ingo Molnar, Jani Nikula, Chris Wilson, Andrew Morton,
Will Deacon
| > 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.GH16853@twins.programming.kicks-ass.net
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Cc: Miklos Szeredi <mszeredi@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
---
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
--
2.7.4
^ permalink raw reply related [flat|nested] 29+ messages in thread
* [PATCH 3/3] bitops.h: set_mask_bits() to return old value
@ 2019-01-11 0:26 ` Vineet Gupta
0 siblings, 0 replies; 29+ messages in thread
From: Vineet Gupta @ 2019-01-11 0:26 UTC (permalink / raw)
To: linux-kernel
Cc: linux-snps-arc, linux-mm, peterz, Vineet Gupta, Miklos Szeredi,
Ingo Molnar, Jani Nikula, Chris Wilson, Andrew Morton,
Will Deacon
| > 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.GH16853@twins.programming.kicks-ass.net
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Cc: Miklos Szeredi <mszeredi@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
---
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
--
2.7.4
^ permalink raw reply related [flat|nested] 29+ messages in thread
* Re: [PATCH 3/3] bitops.h: set_mask_bits() to return old value
2019-01-11 0:26 ` Vineet Gupta
@ 2019-01-11 4:25 ` Anthony Yznaga
-1 siblings, 0 replies; 29+ messages in thread
From: Anthony Yznaga @ 2019-01-11 4:25 UTC (permalink / raw)
To: Vineet Gupta, linux-kernel
Cc: linux-snps-arc, linux-mm, peterz, Miklos Szeredi, Ingo Molnar,
Jani Nikula, Chris Wilson, Andrew Morton, Will Deacon
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.GH16853@twins.programming.kicks-ass.net
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Cc: Miklos Szeredi <mszeredi@redhat.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Jani Nikula <jani.nikula@intel.com>
> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Will Deacon <will.deacon@arm.com>
> Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
>
Reviewed-by: Anthony Yznaga <anthony.yznaga@oracle.com>
^ permalink raw reply [flat|nested] 29+ messages in thread
* [PATCH 3/3] bitops.h: set_mask_bits() to return old value
@ 2019-01-11 4:25 ` Anthony Yznaga
0 siblings, 0 replies; 29+ messages in thread
From: Anthony Yznaga @ 2019-01-11 4:25 UTC (permalink / raw)
To: linux-snps-arc
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.GH16853 at twins.programming.kicks-ass.net
> Suggested-by: Peter Zijlstra <peterz at infradead.org>
> Cc: Miklos Szeredi <mszeredi at redhat.com>
> Cc: Ingo Molnar <mingo at kernel.org>
> Cc: Jani Nikula <jani.nikula at intel.com>
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Andrew Morton <akpm at linux-foundation.org>
> Cc: Will Deacon <will.deacon at arm.com>
> Signed-off-by: Vineet Gupta <vgupta at synopsys.com>
>
Reviewed-by: Anthony Yznaga <anthony.yznaga at oracle.com>
^ permalink raw reply [flat|nested] 29+ messages in thread
* Re: [PATCH 3/3] bitops.h: set_mask_bits() to return old value
2019-01-11 0:26 ` Vineet Gupta
@ 2019-01-11 9:24 ` Peter Zijlstra
-1 siblings, 0 replies; 29+ messages in thread
From: Peter Zijlstra @ 2019-01-11 9:24 UTC (permalink / raw)
To: Vineet Gupta
Cc: linux-kernel, linux-snps-arc, linux-mm, Miklos Szeredi,
Ingo Molnar, Jani Nikula, Chris Wilson, Andrew Morton,
Will Deacon, Mark Rutland
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 :/
^ permalink raw reply related [flat|nested] 29+ messages in thread
* [PATCH 3/3] bitops.h: set_mask_bits() to return old value
@ 2019-01-11 9:24 ` Peter Zijlstra
0 siblings, 0 replies; 29+ messages in thread
From: Peter Zijlstra @ 2019-01-11 9:24 UTC (permalink / raw)
To: linux-snps-arc
On Thu, Jan 10, 2019@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 :/
^ permalink raw reply related [flat|nested] 29+ messages in thread
* Re: [PATCH 3/3] bitops.h: set_mask_bits() to return old value
2019-01-11 9:24 ` Peter Zijlstra
(?)
@ 2019-01-11 20:58 ` Vineet Gupta
-1 siblings, 0 replies; 29+ messages in thread
From: Vineet Gupta @ 2019-01-11 20:58 UTC (permalink / raw)
To: Peter Zijlstra
Cc: Mark Rutland, Miklos Szeredi, Jani Nikula, Will Deacon,
linux-kernel, Chris Wilson, linux-mm, Andrew Morton,
linux-snps-arc, Ingo Molnar
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
^ permalink raw reply [flat|nested] 29+ messages in thread
* [PATCH 3/3] bitops.h: set_mask_bits() to return old value
@ 2019-01-11 20:58 ` Vineet Gupta
0 siblings, 0 replies; 29+ messages in thread
From: Vineet Gupta @ 2019-01-11 20:58 UTC (permalink / raw)
To: linux-snps-arc
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
^ permalink raw reply [flat|nested] 29+ messages in thread
* Re: [PATCH 3/3] bitops.h: set_mask_bits() to return old value
@ 2019-01-11 20:58 ` Vineet Gupta
0 siblings, 0 replies; 29+ messages in thread
From: Vineet Gupta @ 2019-01-11 20:58 UTC (permalink / raw)
To: Peter Zijlstra
Cc: Mark Rutland, Miklos Szeredi, Jani Nikula, Will Deacon,
linux-kernel, Chris Wilson, linux-mm, Andrew Morton,
linux-snps-arc, Ingo Molnar
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
^ permalink raw reply [flat|nested] 29+ messages in thread
* Re: [PATCH 3/3] bitops.h: set_mask_bits() to return old value
2019-01-11 0:26 ` Vineet Gupta
@ 2019-01-14 19:00 ` Will Deacon
-1 siblings, 0 replies; 29+ messages in thread
From: Will Deacon @ 2019-01-14 19:00 UTC (permalink / raw)
To: Vineet Gupta
Cc: linux-kernel, linux-snps-arc, linux-mm, peterz, Miklos Szeredi,
Ingo Molnar, Jani Nikula, Chris Wilson, Andrew Morton
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.GH16853@twins.programming.kicks-ass.net
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Cc: Miklos Szeredi <mszeredi@redhat.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Jani Nikula <jani.nikula@intel.com>
> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Will Deacon <will.deacon@arm.com>
> Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
> ---
> 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 <will.deacon@arm.com>
May also explain why no in-tree users appear to use the return value!
Will
^ permalink raw reply [flat|nested] 29+ messages in thread
* [PATCH 3/3] bitops.h: set_mask_bits() to return old value
@ 2019-01-14 19:00 ` Will Deacon
0 siblings, 0 replies; 29+ messages in thread
From: Will Deacon @ 2019-01-14 19:00 UTC (permalink / raw)
To: linux-snps-arc
On Thu, Jan 10, 2019@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.GH16853 at twins.programming.kicks-ass.net
> Suggested-by: Peter Zijlstra <peterz at infradead.org>
> Cc: Miklos Szeredi <mszeredi at redhat.com>
> Cc: Ingo Molnar <mingo at kernel.org>
> Cc: Jani Nikula <jani.nikula at intel.com>
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Andrew Morton <akpm at linux-foundation.org>
> Cc: Will Deacon <will.deacon at arm.com>
> Signed-off-by: Vineet Gupta <vgupta at synopsys.com>
> ---
> 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 <will.deacon at arm.com>
May also explain why no in-tree users appear to use the return value!
Will
^ permalink raw reply [flat|nested] 29+ messages in thread