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SubjectRe: [PATCH v3 1/4] cpumask: Fix invalid uniprocessor mask assumption
On Sun, Jun 05, 2022 at 08:22:38AM +0200, Sander Vanheule wrote:
> On uniprocessor builds, any CPU mask is assumed to contain exactly one
> CPU (cpu0). This assumption ignores the existence of empty masks,
> resulting in incorrect behaviour.
> cpumask_first_zero(), cpumask_next_zero(), and for_each_cpu_not() don't
> provide behaviour matching the assumption that a UP mask is always "1",
> and instead provide behaviour matching the empty mask.
>
> Drop the incorrectly optimised code and use the generic implementations
> in all cases.
>
> Signed-off-by: Sander Vanheule <sander@svanheule.net>
> ---
> Changes since v1:
> - Drop UP implementations instead of trying to fix them
> ---
> include/linux/cpumask.h | 80 -----------------------------------------
> lib/Makefile | 3 +-
> 2 files changed, 1 insertion(+), 82 deletions(-)
>
> diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
> index fe29ac7cc469..d6add0e29ef4 100644
> --- a/include/linux/cpumask.h
> +++ b/include/linux/cpumask.h
> @@ -116,85 +116,6 @@ static __always_inline unsigned int cpumask_check(unsigned int cpu)
> return cpu;
> }
>
> -#if NR_CPUS == 1
> -/* Uniprocessor. Assume all masks are "1". */
> -static inline unsigned int cpumask_first(const struct cpumask *srcp)
> -{
> - return 0;
> -}
> -
> -static inline unsigned int cpumask_first_zero(const struct cpumask *srcp)
> -{
> - return 0;
> -}
> -
> -static inline unsigned int cpumask_first_and(const struct cpumask *srcp1,
> - const struct cpumask *srcp2)
> -{
> - return 0;
> -}
> -
> -static inline unsigned int cpumask_last(const struct cpumask *srcp)
> -{
> - return 0;
> -}
> -
> -/* Valid inputs for n are -1 and 0. */
> -static inline unsigned int cpumask_next(int n, const struct cpumask *srcp)
> -{
> - return n+1;
> -}
> -
> -static inline unsigned int cpumask_next_zero(int n, const struct cpumask *srcp)
> -{
> - return n+1;
> -}
> -
> -static inline unsigned int cpumask_next_and(int n,
> - const struct cpumask *srcp,
> - const struct cpumask *andp)
> -{
> - return n+1;
> -}
> -
> -static inline unsigned int cpumask_next_wrap(int n, const struct cpumask *mask,
> - int start, bool wrap)
> -{
> - /* cpu0 unless stop condition, wrap and at cpu0, then nr_cpumask_bits */
> - return (wrap && n == 0);
> -}
> -
> -/* cpu must be a valid cpu, ie 0, so there's no other choice. */
> -static inline unsigned int cpumask_any_but(const struct cpumask *mask,
> - unsigned int cpu)
> -{
> - return 1;
> -}
> -
> -static inline unsigned int cpumask_local_spread(unsigned int i, int node)
> -{
> - return 0;
> -}
> -
> -static inline int cpumask_any_and_distribute(const struct cpumask *src1p,
> - const struct cpumask *src2p) {
> - return cpumask_first_and(src1p, src2p);
> -}
> -
> -static inline int cpumask_any_distribute(const struct cpumask *srcp)
> -{
> - return cpumask_first(srcp);
> -}

It looks like cpumask_local_spread, cpumask_any_and_distribute and
cpumask_any_distribute were correct and better optimized in UP case.
cpumask_local_spread - for sure. I think it's worth keeping them
optimized.

Thanks,
Yury

> -#define for_each_cpu(cpu, mask) \
> - for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask)
> -#define for_each_cpu_not(cpu, mask) \
> - for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask)
> -#define for_each_cpu_wrap(cpu, mask, start) \
> - for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask, (void)(start))
> -#define for_each_cpu_and(cpu, mask1, mask2) \
> - for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask1, (void)mask2)
> -#else
> /**
> * cpumask_first - get the first cpu in a cpumask
> * @srcp: the cpumask pointer
> @@ -324,7 +245,6 @@ extern int cpumask_next_wrap(int n, const struct cpumask *mask, int start, bool
> for ((cpu) = -1; \
> (cpu) = cpumask_next_and((cpu), (mask1), (mask2)), \
> (cpu) < nr_cpu_ids;)
> -#endif /* SMP */
>
> #define CPU_BITS_NONE \
> { \
> diff --git a/lib/Makefile b/lib/Makefile
> index 89fcae891361..6f26a429115b 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> @@ -34,10 +34,9 @@ lib-y := ctype.o string.o vsprintf.o cmdline.o \
> is_single_threaded.o plist.o decompress.o kobject_uevent.o \
> earlycpio.o seq_buf.o siphash.o dec_and_lock.o \
> nmi_backtrace.o nodemask.o win_minmax.o memcat_p.o \
> - buildid.o
> + buildid.o cpumask.o
>
> lib-$(CONFIG_PRINTK) += dump_stack.o
> -lib-$(CONFIG_SMP) += cpumask.o
>
> lib-y += kobject.o klist.o
> obj-y += lockref.o
> --
> 2.36.1

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