Messages in this thread | | | From | Arnd Bergmann <> | Subject | Re: [PATCH v4 1/5] getcpu_cache system call: cache CPU number of running thread | Date | Mon, 29 Feb 2016 11:39:34 +0100 |
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On Monday 29 February 2016 11:32:21 Peter Zijlstra wrote: > On Sun, Feb 28, 2016 at 12:39:54AM +0000, Mathieu Desnoyers wrote: > > > /* 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
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