Hello Paul,
"Paul E. McKenney" wrote:
>
> +void init_srcu_struct(struct srcu_struct *sp)
> +{
> + int cpu;
> +
> + sp->completed = 0;
> + sp->per_cpu_ref = (struct srcu_struct_array *)
> + kmalloc(NR_CPUS * sizeof(*sp->per_cpu_ref),
> + GFP_KERNEL);
> + for_each_cpu(cpu) {
> + sp->per_cpu_ref[cpu].c[0] = 0;
> + sp->per_cpu_ref[cpu].c[1] = 0;
> + }
Isn't it simpler to just do:
sp->per_cpu_ref = kzmalloc(NR_CPUS * sizeof(*sp->per_cpu_ref),
GFP_KERNEL);
and drop 'for_each_cpu(cpu)' initialization ?
> +int srcu_read_lock(struct srcu_struct *sp)
> +{
> + int idx;
> +
> + preempt_disable();
> + idx = sp->completed & 0x1;
> + barrier();
> + sp->per_cpu_ref[smp_processor_id()].c[idx]++;
> + preempt_enable();
> + return idx;
> +}
Could you explain this 'barrier()' ?
> +void synchronize_srcu(struct srcu_struct *sp)
> +{
> + int cpu;
> + int idx;
> + int sum;
> +
> + might_sleep();
> +
> + mutex_lock(&sp->mutex);
> +
> + smp_mb(); /* Prevent operations from leaking in. */
Why smp_wmb() is not enough? We are doing synchronize_sched() below
before reading ->per_cpu_ref, and ->completed is protected by ->mutex.
> + idx = sp->completed & 0x1;
> + sp->completed++;
But srcu_read_lock()'s path and rcu_dereference() doesn't have rmb(),
and the reader can block, so I can't understand how this all works.
Suppose ->completed == 0,
WRITER: READER:
old = global_ptr;
rcu_assign_pointer(global_ptr, new);
synchronize_srcu:
locks mutex, does mb,
->completed++;
srcu_read_lock();
// reads ->completed == 1
// does .c[1]++
ptr = rcu_dereference(global_ptr)
// reads the *OLD* value,
// because we don't have rmb()
block_on_something();
synchronize_sched();
// ... still blocked ...
checks sum_of(.c[0]) == 0, yes
synchronize_sched();
// ... still blocked ...
kfree(old);
// wake up
do_something(ptr);
Also, I can't understand the purpose of 2-nd synchronize_sched() in
synchronize_srcu().
Please help!
Oleg.
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