On Monday 29 February 2016 11:32:21 Peter Zijlstra wrote:

On Sun, Feb 28, 2016 at 12:39:54AM +0000, Mathieu Desnoyers wrote:

quoted

/* This structure needs to be aligned cache line size. */
struct thread_local_abi {
  int32_t  cpu_id;
  uint32_t rseq_seqnum;
  uint64_t rseq_post_commit_ip;
  /* Add new fields at the end. */ 
} __attribute__((packed));

I would really not use packed; that can lead to horrible layout.

Suppose someone would add:

	uint32_t foo;
	uint64_t bar;

With packed, you get an unaligned uint64_t in there, which is horrible.
Without packed, you get a hole, which you can later fill.

What's making things worse is that on some architectures, adding
__packed will force access by bytes rather than just reading
a 32-bit or 64-bit numbers directly, so it's slow and non-atomic.

	Arnd