Re: One question about API cpumask_setall()

[Yury Norov <ynorov@nvidia.com>]

On Wed, Jul 29, 2026 at 10:13:56AM +0800, Bibo Mao wrote:
> 
> 
> On 2026/7/29 上午9:55, Yury Norov wrote:
> > On Tue, Jul 28, 2026 at 04:16:27PM +0800, Bibo Mao wrote:
> > > Hi,
> > > 
> > > I am preparing to use API cpumask_setall() for TLB flush on LoongArch KVM
> > > side. There are cpumask_test_and_clear_cpu/cpumask_setall() APIs, it seems
> > > that API cpumask_setall() does not support atomic operation.
> > > 
> > > Do you think it is reasonable to add new API similar with cpumask_setall()
> > > which supports atomic operation?
> > 
> > Maybe reasonable, but it's impossible. cpumask is a bitmap, which in
> > turn is an array of unsigned longs. There's no atomic operations on
> > arrays.
> It should be workable with atomic operation on unsigned long, rather than
> the array of unsigned longs, the API cpumask_test_and_clear_cpu is atomic
> operation on unsigned long also.

No, cpumask_test_and_clear_cpu is atomic() operates on cpumask_t*,
which is a pointer to unsigned long.
 
> Is it ok to use smp_store_mb() to fill the bitmap with the similar API
> cpumask_setall()? where xchg() is used in smp_store_mb() wrapper instead.

If your bitmap is longer than 1 word, you'll have to issue 2 or more
smp_store_mb()'s. Then, a series of atomic operations is not atomic
itself.