generic-ipi: remove kmalloc()

generic-ipi: remove kmalloc()

Remove the use of kmalloc() from the smp_call_function_*()
calls.

Steven's generic-ipi patch (d7240b98: generic-ipi: use per cpu
data for single cpu ipi calls) started the discussion on the use
of kmalloc() in this code and fixed the
smp_call_function_single(.wait=0) fallback case.

In this patch we complete this by also providing means for the
_many() call, which fully removes the need for kmalloc() in this
code.

The problem with the _many() call is that other cpus might still
be observing our entry when we're done with it. It solved this
by dynamically allocating data elements and RCU-freeing it.

We solve it by using a single per-cpu entry which provides
static storage and solves one half of the problem (avoiding
referencing freed data).

The other half, ensuring the queue iteration it still possible,
is done by placing re-used entries at the head of the list. This
means that if someone was still iterating that entry when it got
moved, he will now re-visit the entries on the list he had
already seen, but avoids skipping over entries like would have
happened had we placed the new entry at the end.

Furthermore, visiting entries twice is not a problem, since we
remove our cpu from the entry's cpumask once its called.

Many thanks to Oleg for his suggestions and him poking holes in
my earlier attempts.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nick Piggin <npiggin@suse.de>
Cc: Jens Axboe <jens.axboe@oracle.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Ingo Molnar <mingo@elte.hu>