Re: [PATCH v3 1/4] fuse: Make the fuse unique value a per-cpu counter

[Miklos Szeredi <miklos@szeredi.hu>]

On Thu, 3 Apr 2025 at 22:23, Bernd Schubert <bschubert@ddn.com> wrote:

> +/**
> + * Get the next unique ID for a request
> + */
> +static inline u64 fuse_get_unique(struct fuse_iqueue *fiq)
> +{
> +       int step = FUSE_REQ_ID_STEP * (task_cpu(current));
> +       u64 cntr = this_cpu_inc_return(*fiq->reqctr);
> +
> +       return cntr * FUSE_REQ_ID_STEP * NR_CPUS + step;

Thinking a bit... this looks wrong.

The reason is that the task could be migrated to a different CPU
between the task_cpu() and the this_cpu_inc_return(), resulting in a
possibly duplicated value.

This could be fixed with a preempt_disable()/preempt_enable() pair,
but I think it would be cleaner to go with my original idea and
initialize the percpu counters to  CPUID and increment by NR_CPU *
FUSE_REQ_ID_STEP when fetching a new value.

Thanks,
Miklos