Re: ldcw inline assembler patch

[Helge Deller <deller@gmx.de>]

Hi Carlos,

Carlos O'Donell wrote:
> On Sat, Jun 14, 2008 at 11:36 AM, Helge Deller <deller@gmx.de> wrote:
>> I'm wondering if this patch might help people who are seeing locking
>> problems on SMP boxes ?
>> Helge
>>
>> diff --git a/include/asm-parisc/system.h b/include/asm-parisc/system.h
>> index ee80c92..4752684 100644
>> --- a/include/asm-parisc/system.h
>> +++ b/include/asm-parisc/system.h
>> @@ -168,8 +168,9 @@ static inline void set_eiem(unsigned long val)
>>  /* LDCW, the only atomic read-write operation PA-RISC has. *sigh*.  */
>>  #define __ldcw(a) ({                                           \
>>        unsigned __ret;                                         \
>> -       __asm__ __volatile__(__LDCW " 0(%1),%0"                 \
>> -               : "=r" (__ret) : "r" (a));                      \
>> +       __asm__ __volatile__(__LDCW " 0(%2),%0"                 \
>> +                       : "=r" (__ret), "=m" (*(a))             \
>> +                       : "r" (a), "m" (*(a))   );              \
>>        __ret;                                                  \
>>  })
> 
> You don't want to do that, the compiler might hold "=m" (*(a)) in a
> temporary memory location.
> 
> e.g.
> temp_mem = *a;
> reg2 = &temp_mem;
> ... operation ...
> *a = temp_mem;
> 
> You would be atomic with regards to the store to temp_mem, but not a.
> Infact this could always be the case.
> 
> I'm now of the opinion that we need a "memory" clobber in the original
> expression to prevent this from ever happening.

So, your proposal is (copy-and-pasted in here) the following ?

diff --git a/include/asm-parisc/system.h b/include/asm-parisc/system.h
index ee80c92..daeae39 100644
--- a/include/asm-parisc/system.h
+++ b/include/asm-parisc/system.h
@@ -169,7 +169,7 @@ static inline void set_eiem(unsigned long val)
  #define __ldcw(a) ({                                           \
         unsigned __ret;                                         \
         __asm__ __volatile__(__LDCW " 0(%1),%0"                 \
-               : "=r" (__ret) : "r" (a));                      \
+               : "=r" (__ret) : "r" (a) : "memory" );          \
         __ret;                                                  \
  })