lib: Fix possible deadlock in flexible proportion code

When percpu counter function in fprop_new_period() is interrupted by an
interrupt while holding counter lock, it can cause deadlock when the
interrupt wants to take the lock as well. Fix the problem by disabling
interrupts when calling percpu counter functions.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>
lib/flex_proportions.c: fix corruption of denominator in flexible proportions

When racing with CPU hotplug, percpu_counter_sum() can return negative
values for the number of observed events.

This confuses fprop_new_period(), which uses unsigned type and as a
result number of events is set to big *positive* number.  From that
moment on, things go pear shaped and can result e.g.  in division by
zero as denominator is later truncated to 32-bits.

This bug causes a divide-by-zero oops in bdi_dirty_limit() in Borislav's
3.6.0-rc6 based kernel.

Fix the issue by using a signed type in fprop_new_period().  That makes
us bail out from the function without doing anything (mistakenly)
thinking there are no events to age.  That makes aging somewhat
inaccurate but getting accurate data would be rather hard.

Signed-off-by: Jan Kara <jack@suse.cz>
Reported-by: Borislav Petkov <bp@amd64.org>
Reported-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>