Change API

* rcu_read_lock

> A patch below to allow nested rcu_read_lock() while keeping to the Linux
> kernel API, just FYI.  One can argue that the overhead of accessing the
> extra per-thread variables is offset by the fact that there no longer
> needs to be a return value from rcu_read_lock() nor an argument to
> rcu_read_unlock(), but hard to say.
>

I ran your modified version within my benchmarks :

with return value : 14.164 cycles per read
without return value : 16.4017 cycles per read

So we have a 14% performance decrease due to this. We also pollute the
branch prediction buffer and we add a cache access due to the added
variables in the TLS. Returning the value has the clear advantage of
letting the compiler keep it around in registers or on the stack, which
clearly costs less.

So I think the speed factor outweights the visual considerations. Maybe
we could switch to something like :

unsigned int qparity;

urcu_read_lock(&qparity);
...
urcu_read_unlock(&qparity);

That would be a bit like local_irq_save() in the kernel, except that we
could do it in a static inline because we pass the address. I
personnally dislike the local_irq_save() way of hiding the fact that it
writes to the variable in a "clever" macro. I'd really prefer to leave
the " & ".

* rcu_write_lock

Removed.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>