perf_counter: Fix lockup with interrupting counters

Commit 8e3747c1 ("perf_counter: Change data head from u32 to u64")
changed the type of 'head' in struct perf_mmap_data from atomic_t
to atomic_long_t, but missed converting one use of atomic_read on
it to atomic_long_read.  The effect of using atomic_read rather than
atomic_long_read on powerpc (and other big-endian architectures) is
that we get the high half of the 64-bit quantity, resulting in the
cmpxchg retry loop in perf_output_begin spinning forever as soon as
data->head becomes non-zero.  On little-endian architectures such as
x86 we would get the low half, resulting in a lockup once data->head
becomes greater than 4G.

This fixes it by using atomic_long_read rather than atomic_read.

[ Impact: fix perfcounter lockup on PowerPC / big-endian systems ]
Signed-off-by: NPaul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <18984.33964.21541.743096@cargo.ozlabs.ibm.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/perf_counter.c

@@ -2234,7 +2234,7 @@ static int perf_output_begin(struct perf_output_handle *handle,
 	perf_output_lock(handle);
 
 	do {
-		offset = head = atomic_read(&data->head);
+		offset = head = atomic_long_read(&data->head);
 		head += size;
 	} while (atomic_long_cmpxchg(&data->head, offset, head) != offset);