[PATCH 1/2] perf mmap: Use the head selected by read_init

[Paul Moses <p@1g4.org>]

perf_mmap__push() reads the producer head before calling
perf_mmap__read_init(), which reads it again to select the range to copy.

If the producer advances between those reads, push() copies through the
newer head but stores the earlier head in prev. The next drain starts at
that earlier position and can write the same records to perf.data again.

Use the head selected by read_init(): end for forward buffers and start
for overwrite buffers. Save it before the copy loop advances start so
prev matches the range that was copied.

Fixes: 8872481bd048 ("perf mmap: Introduce perf_mmap__read_init()")
Signed-off-by: Paul Moses <p@1g4.org>
---
 tools/perf/util/mmap.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/perf/util/mmap.c b/tools/perf/util/mmap.c
index 358e70c4f3edd077c9752115329f29040f1c2a76..6dcc4a9304b926c4a8d4f401e40bfd79d18dd199 100644
--- a/tools/perf/util/mmap.c
+++ b/tools/perf/util/mmap.c
@@ -330,7 +330,7 @@ int mmap__mmap(struct mmap *map, struct mmap_params *mp, int fd, struct perf_cpu
 int perf_mmap__push(struct mmap *md, void *to,
 		    int push(struct mmap *map, void *to, void *buf, size_t size))
 {
-	u64 head = perf_mmap__read_head(&md->core);
+	u64 head;
 	unsigned char *data = md->core.base + page_size;
 	unsigned long size;
 	void *buf;
@@ -340,6 +340,7 @@ int perf_mmap__push(struct mmap *md, void *to,
 	if (rc < 0)
 		return (rc == -EAGAIN) ? 1 : -1;
 
+	head = md->core.overwrite ? md->core.start : md->core.end;
 	size = md->core.end - md->core.start;
 
 	if ((md->core.start & md->core.mask) + size != (md->core.end & md->core.mask)) {
-- 
2.56.GIT