On Thu, Oct 18, 2018 at 05:04:34PM +0200, Daniel Borkmann wrote:
> diff --git a/tools/include/linux/ring_buffer.h
> b/tools/include/linux/ring_buffer.h
> new file mode 100644
> index 0000000..48200e0
> --- /dev/null
> +++ b/tools/include/linux/ring_buffer.h
> @@ -0,0 +1,69 @@
> +#ifndef _TOOLS_LINUX_RING_BUFFER_H_
> +#define _TOOLS_LINUX_RING_BUFFER_H_
> +
> +#include <linux/compiler.h>
> +#include <asm/barrier.h>
> +
> +/*
> + * Below barriers pair as follows (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 makes sense not for all architectures since 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 (total store order),
> + * 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);
> +#else
> + u64 head = READ_ONCE(base->data_head);
> +
> + smp_rmb();
> + return head;
> +#endif
> +}
> +
> +static inline void ring_buffer_write_tail(struct perf_event_mmap_page *base,
> + u64 tail)
> +{
> + smp_store_release(&base->data_tail, tail);
> +}
> +
> +#endif /* _TOOLS_LINUX_RING_BUFFER_H_ */
(for the whole patch, but in particular the above)
Acked-by: Peter Zijlstra (Intel) <pet...@infradead.org>
