On Fri, Mar 18, 2005 at 11:03:39AM +0100, Ingo Molnar wrote:
>
> there's a problem in #5's rcu_read_lock():
>
> void
> rcu_read_lock(void)
> {
> preempt_disable();
> if (current->rcu_read_lock_nesting++ == 0) {
> current->rcu_read_lock_ptr =
> &__get_cpu_var(rcu_data).lock;
> read_lock(current->rcu_read_lock_ptr);
> }
> preempt_enable();
> }
>
> not only are read_lock()-ed sections preemptible, read_lock() itself may
> block, so it cannot be called from within preempt_disable(). How about
> something like:
>
> void
> rcu_read_lock(void)
> {
> preempt_disable();
> if (current->rcu_read_lock_nesting++ == 0) {
> current->rcu_read_lock_ptr =
> &__get_cpu_var(rcu_data).lock;
> preempt_enable();
> read_lock(current->rcu_read_lock_ptr);
> } else
> preempt_enable();
> }
>
> this would still make it 'statistically scalable' - but is it correct?
Good catch!
Also good question...
Strictly speaking, it is not necessary to block callback invocation until
just after rcu_read_lock() returns.
It is correct as long as there is no sort of "upcall" or "callback" that
can masquerade as this task. I know of no such thing in the Linux kernel.
In fact such a thing would break a lot of code, right?
Any tool that relied on the ->rcu_read_lock_nesting counter to deduce
RCU state would be confused by this change, but there might be other
ways of handling this. Also, we are currently making do without such
a tool.
It should be possible to move the preempt_enable() further forward
ahead of the assignment to ->rcu_read_lock_ptr, since the assignment
to ->rcu_read_lock_ptr is strictly local. Not sure that this is
worthwhile, thoughts?
void
rcu_read_lock(void)
{
preempt_disable();
if (current->rcu_read_lock_nesting++ == 0) {
preempt_enable();
current->rcu_read_lock_ptr =
&__get_cpu_var(rcu_data).lock;
read_lock(current->rcu_read_lock_ptr);
} else
preempt_enable();
}
The other question is whether preempt_disable() is needed in the first
place. The two task-structure fields are not accessed except by the
task itself. I bet that the following is just fine:
void
rcu_read_lock(void)
{
if (current->rcu_read_lock_nesting++ == 0) {
current->rcu_read_lock_ptr =
&__get_cpu_var(rcu_data).lock;
read_lock(current->rcu_read_lock_ptr);
}
}
Thoughts?
Thanx, Paul
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