Parmenides <mobile.parmenides@xxxxxxxxx> writes:
> static inline int test_and_set_bit(int nr, volatile unsigned long *
> addr)
> {
> int oldbit;
>
> __asm__ __volatile__( LOCK_PREFIX
> "btsl %2,%1\n\tsbbl %0,%0"
> :"=r" (oldbit),"+m" (ADDR)
> :"Ir" (nr) : "memory");
> return oldbit;
> }
>
> 1. There are two instructions in the inline assemably, namely btsl and
> sbbl. But, can the only one LOCK_PREFIX ensure that the operation is
> atomic?
The sbbl instruction here is a register-to-register operation anyhow.
There would be no point to using a lock prefix on it. It's just there
to move the result of the atomic btsl operation from the carry flag to a
register.
> 2. The clobber list of the inline assembly contains a string of
> "memory". What is the meaning of this declaration and why does the
> operation need it ? After all, some other operations such as
> clear_bit() (see the following code) dose not need "memory" indeed.
> The declaration of "memory" appears in source code of kernel
> here and there, and bothers me for a long time. So any details about
> it will be appreciated.
>
> static inline void clear_bit(int nr, volatile unsigned long * addr)
> {
> __asm__ __volatile__( LOCK_PREFIX
> "btrl %1,%0"
> :"+m" (ADDR)
> :"Ir" (nr));
> }
This is documented here
http://gcc.gnu.org/onlinedocs/gcc-4.6.0/gcc/Extended-Asm.html
A "memory" clobber means that the instruction modifies memory in a way
which is difficult to express. As far as I can see, it is not needed in
the first example above, as the '+' constraint on the memory location
ADDR tells gcc about the modification to memory.
Ian
