musl - 64-bit atomic ops on 32-bit ARM

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Message-ID: <52F2C1AF.8020903@nicta.com.au>
Date: Thu, 6 Feb 2014 09:56:47 +1100
From: Matthew Fernandez <matthew.fernandez@...ta.com.au>
To: <musl@...ts.openwall.com>
Subject: 64-bit atomic ops on 32-bit ARM

Hi all,

The header atomic.h contains a couple of functions for operations on 64-bit types. Despite their
name, these do not appear to be atomic to me. For example:

  static inline void a_and_64(volatile uint64_t *p, uint64_t v)
  {
    union { uint64_t v; uint32_t r[2]; } u = { v };
    a_and((int *)p, u.r[0]);
    a_and((int *)p+1, u.r[1]);
  }

Have I misunderstood something or is there some other reason behind their implementation? AIUI on
certain ARM platforms there is actually no way to implement atomic 64-bit ops without kernel support.

If replying, please CC me directly as I'm not on the list.

Thanks,
Matt

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