atomic operations in user space

[Liu Dave-r63238]

> [snip]
> > I surely know all the theories you mentioned clearly.  But 
> please do 
> > look at the case I gave.  Correct me if I missed anything.  Thanks
> > 
> > All the lwarx and stwcx operate on the same address.
> > 
> > > Task A		Task B
> > > lwarx				
> > // Get RESERVATION
> > > 			......
> > > 			lwarx
> > > 			stwcx
> > 
> > // RESERVATION cleared
> > >
> > > 			.....
> > > 			.....
> > > 			lwarx
> > 
> > // Get RESERVATION again
> > > stwcx				
> > 
> > //Note here: RESERVATION is valid, address is the same.
> > So the result is commited, no retry for task A
> > 
> > > .....
> > > 			stwcx
> > //RESERVATION is cleared, retry atomic op for task B
> > 
> > Please be noted that reservation is only identified by 
> reservation bit 
> > and address operated on.  So different lwarx's on the same address, 
> > may be considered as the same reservation.
> 
> Is this your reason we cannot do atomic operation in user space?
> 
> How about the kernel space? You can image it.
> The context switching as above also happen in kernel space, 
> Why we can do atomic operation in kernel space, not do in user space?
> 
> You are assuming the context switching cause the reservation broken.
> but we can do atomic operation in kernel space.  The context 
> switching really is the execption of processor, If we can 
> clear the wrong RESERVATION before exception return, I think 
> we can solve this problem. We can dummy stwcx. before 
> exception return or the processor automaticly clear the 
> reservation in exception. 
> 
> Are you missing these important things?
> 
> -DAve

I got it. I noticed that all of execption return in kernel did stwcx.
to clear the wrong reserved bit. See the source code.

        .globl  ret_from_except_full
ret_from_except_full:
        REST_NVGPRS(r1)
        /* fall through */

        .globl  ret_from_except
ret_from_except:
           ......

restore:
        lwz     r0,GPR0(r1)
        lwz     r2,GPR2(r1)
        REST_4GPRS(3, r1)
        REST_2GPRS(7, r1)

        lwz     r10,_XER(r1)
        lwz     r11,_CTR(r1)
        mtspr   SPRN_XER,r10
        mtctr   r11

        PPC405_ERR77(0,r1)
        stwcx.  r0,0,r1                 /* to clear the reservation */

          ......

         rfi

-DAve