Messages in this thread | | | Date | Mon, 16 Sep 2013 10:20:13 +0200 | Subject | Re: Why does test_bit() take a volatile addr? | From | Geert Uytterhoeven <> |
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On Mon, Sep 16, 2013 at 6:08 AM, Rusty Russell <rusty@rustcorp.com.au> wrote: > Predates git, does anyone remember the rationale? > > ie: > int test_bit(int nr, const volatile unsigned long *addr)
That's why we have full-history-linux ;-)
Unfortunately it doesn't show the rationale, as this change also predates bitkeeper.
Since 2.1.30, volatile is used unconditionally:
commit 84d59b7dda1092a22663f4e2da77a6ce581b539e Author: linus1 <torvalds@linuxfoundation.org> Date: Mon Mar 10 12:00:00 1997 -0600
Import 2.1.30
#ifdef __SMP__ #define LOCK_PREFIX "lock ; " -#define SMPVOL volatile #else #define LOCK_PREFIX "" -#define SMPVOL #endif
/* * This routine doesn't need to be atomic. */ -extern __inline__ int test_bit(int nr, const SMPVOL void * addr) +extern __inline__ int __constant_test_bit(int nr, const volatile void * addr) { - return ((1UL << (nr & 31)) & (((const unsigned int *) addr)[nr >> 5])) ! + return ((1UL << (nr & 31)) & (((const volatile unsigned int *) addr)[nr }
+extern __inline__ int __test_bit(int nr, volatile void * addr) +{ + int oldbit; + + __asm__ __volatile__( + "btl %2,%1\n\tsbbl %0,%0" + :"=r" (oldbit) + :"m" (ADDR),"ir" (nr)); + return oldbit; +} + +#define test_bit(nr,addr) \ +(__builtin_constant_p(nr) ? \ + __constant_test_bit((nr),(addr)) : \ + __test_bit((nr),(addr))) + /*
Between 1.3.75 and 2.1.30, volatile was used on SMP only:
commit 08c5b9d698e6ca2233aec0f7d7f0fdd6eb3ad235 Author: linus1 <torvalds@linuxfoundation.org> Date: Thu Mar 7 12:00:00 1996 -0600
Import 1.3.75
#ifdef __SMP__ #define LOCK_PREFIX "lock ; " +#define SMPVOL volatile #else #define LOCK_PREFIX "" +#define SMPVOL #endif
/* * This routine doesn't need to be atomic. */ -extern __inline__ int test_bit(int nr, const void * addr) +extern __inline__ int test_bit(int nr, const SMPVOL void * addr) { return 1UL & (((const unsigned int *) addr)[nr >> 5] >> (nr & 31)); }
Gr{oetje,eeting}s,
Geert
-- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org
In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds
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