Commit 6dc5f2a4 authored by Paul Mackerras's avatar Paul Mackerras Committed by Ingo Molnar
Browse files

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: default avatarPaul Mackerras <>
Cc: Peter Zijlstra <>
LKML-Reference: <>
Signed-off-by: default avatarIngo Molnar <>
parent 76a0f40f
......@@ -2234,7 +2234,7 @@ static int perf_output_begin(struct perf_output_handle *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);
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