blob: 6c02617377c29b4f9fb758593dd5f2566518cfd9 [file] [log] [blame]
#include <asm/barrier.h>
#include <linux/perf_event.h>
* Contract with kernel for walking the perf ring buffer from
* user space requires the following barrier pairing (quote
* from kernel/events/ring_buffer.c):
* Since the mmap() consumer (userspace) can run on a
* different CPU:
* kernel user
* if (LOAD ->data_tail) { LOAD ->data_head
* (A) smp_rmb() (C)
* STORE $data LOAD $data
* smp_wmb() (B) smp_mb() (D)
* STORE ->data_head STORE ->data_tail
* }
* Where A pairs with D, and B pairs with C.
* In our case A is a control dependency that separates the
* load of the ->data_tail and the stores of $data. In case
* ->data_tail indicates there is no room in the buffer to
* store $data we do not.
* D needs to be a full barrier since it separates the data
* READ from the tail WRITE.
* For B a WMB is sufficient since it separates two WRITEs,
* and for C an RMB is sufficient since it separates two READs.
* Note, instead of B, C, D we could also use smp_store_release()
* in B and D as well as smp_load_acquire() in C.
* However, this optimization does not make sense for all kernel
* supported architectures since for a fair number it would
* resolve into READ_ONCE() + smp_mb() pair for smp_load_acquire(),
* and smp_mb() + WRITE_ONCE() pair for smp_store_release().
* Thus for those smp_wmb() in B and smp_rmb() in C would still
* be less expensive. For the case of D this has either the same
* cost or is less expensive, for example, due to TSO x86 can
* avoid the CPU barrier entirely.
static inline u64 ring_buffer_read_head(struct perf_event_mmap_page *base)
* Architectures where smp_load_acquire() does not fallback to
* READ_ONCE() + smp_mb() pair.
#if defined(__x86_64__) || defined(__aarch64__) || defined(__powerpc64__) || \
defined(__ia64__) || defined(__sparc__) && defined(__arch64__)
return smp_load_acquire(&base->data_head);
u64 head = READ_ONCE(base->data_head);
return head;
static inline void ring_buffer_write_tail(struct perf_event_mmap_page *base,
u64 tail)
smp_store_release(&base->data_tail, tail);