[boost] [interprocess] problem in boost/interprocess/detail/atomic.hpp

From: Glenn Schrader (gschrad_at_[hidden])
Date: 2006-12-15 16:56:59

I ran across this bug while running on a 2.6.9 Linux kernel. Since that 
kernel version doesn't have native interprocess mutexes and condition 
variables the interprocess library uses some emulation code. The x86 
verion of the emulated semaphore uses atomic_dec32() from atomic.hpp but 
that function wasn't behaving properly. The following change in 
oost/interprocess/detail/atomic.hpp made things work properly:

BEFORE:

inline boost::uint32_t atomic_dec32(volatile boost::uint32_t *mem)
{
   unsigned char prev;

   asm volatile ("lock; decl %1;\n\t"
               "setnz %%al"
               : "=a" (prev)
               : "m" (*(mem))
               : "memory", "cc");
   return prev;
}

AFTER:

inline boost::uint32_t atomic_dec32(volatile boost::uint32_t *mem)
{
   boost::uint32_t prev;

   // acts like an atomic 'return (*mem)--;'
   asm volatile ("movl $-1, %0;\n\t"
                 "lock; xaddl %0, %1"
                 : "=r" (prev)
                 : "m" (*(mem))
                 : "memory", "cc" );
   return prev;
}

-glenn