Bump On 19/09/2021 20.36, Avi Kivity wrote:
perf_event_open(2) instructs the user to issue an rmb() after reading data_head to ensure that user-space sees all writes to the memory it reads. rmb() is a kernel-internal term that might not mean much to the reader; and further it is too strict. It's enough to require the weaker load-acquire fence. This is an industry standard term that does not require the user to understand kernel terminology. In addition, require a store-release fence before writing data_tail. This prevents the user's reads from being reordered with the kernel's writes to the just-freed space. The documentation in <linux/perf_event.h> also suggests doing this. Signed-off-by: Avi Kivity <avi@xxxxxxxxxxxx> --- man2/perf_event_open.2 | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/man2/perf_event_open.2 b/man2/perf_event_open.2 index 81c1b10f2..db5ce746b 100644 --- a/man2/perf_event_open.2 +++ b/man2/perf_event_open.2 @@ -1837,18 +1837,19 @@ The value needs to be manually wrapped by the size of the mmap buffer before accessing the samples. .IP On SMP-capable platforms, after reading the .I data_head value, -user space should issue an rmb(). +user space should issue a load-acquire fence. .TP .I data_tail When the mapping is .BR PROT_WRITE , the .I data_tail value should be written by user space to reflect the last read data. +Before writing, issue a store-release fence. In this case, the kernel will not overwrite unread data. .TP .IR data_offset " (since Linux 4.1)" .\" commit e8c6deac69629c0cb97c3d3272f8631ef17f8f0f Contains the offset of the location in the mmap buffer
