md/raid10: remove use-after-free bug.

We always need to be careful when calling generic_make_request, as it
can start a chain of events which might free something that we are
using.

Here is one place I wasn't careful enough.  If the wbio2 is not in
use, then it might get freed at the first generic_make_request call.
So perform all necessary tests first.

This bug was introduced in 3.3-rc3 (24afd80d99) and can cause an
oops, so fix is suitable for any -stable since then.

Cc: stable@vger.kernel.org (3.3+)
Signed-off-by: NeilBrown <neilb@suse.de>
md: don't call md_allow_write in get_bitmap_file.

There is no really need as GFP_NOIO is very likely sufficient,
and failure is not catastrophic.

Calling md_allow_write here will convert a read-auto array to
read/write which could be confusing when you are just performing
a read operation.

Signed-off-by: NeilBrown <neilb@suse.de>
raid5: create multiple threads to handle stripes

This is a new tempt to make raid5 handle stripes in multiple threads, as
suggested by Neil to have maxium flexibility and better numa binding. It
basically is a combination of my first and second generation patches. By
default, no multiple thread is enabled (all stripes are handled by raid5d).

An example to enable multiple threads:
#echo 3 > /sys/block/md0/md/auxthread_number
This will create 3 auxiliary threads to handle stripes. The threads can run
on any cpus and handle stripes produced by any cpus.

#echo 1-3 > /sys/block/md0/md/auxth0/cpulist
This will bind auxiliary thread 0 to cpu 1-3, and this thread will only handle
stripes produced by cpu 1-3. User tool can further change the thread's
affinity, but the thread can only handle stripes produced by cpu 1-3 till the
sysfs entry is changed again.

If stripes produced by a CPU aren't handled by any auxiliary thread, such
stripes will be handled by raid5d. Otherwise, raid5d doesn't handle any
stripes.

Signed-off-by: Shaohua Li <shli@fusionio.com>
Signed-off-by: NeilBrown <neilb@suse.de>
md/raid5: make release_stripe lockless

release_stripe still has big lock contention. We just add the stripe to a llist
without taking device_lock. We let the raid5d thread to do the real stripe
release, which must hold device_lock anyway. In this way, release_stripe
doesn't hold any locks.

The side effect is the released stripes order is changed. But sounds not a big
deal, stripes are never handled in order. And I thought block layer can already
do nice request merge, which means order isn't that important.

I kept the unplug release batch, which is unnecessary with this patch from lock
contention avoid point of view, and actually if we delete it, the stripe_head
release_list and lru can share storage. But the unplug release batch is also
helpful for request merge. We probably can delay wakeup raid5d till unplug, but
I'm still afraid of the case which raid5d is running.

Signed-off-by: Shaohua Li <shli@fusionio.com>
Signed-off-by: NeilBrown <neilb@suse.de>