Re: Arm ldaxr / stxr loop question

Lawrence D'Oliveiro <ldo@nz.invalid> writes:

On Mon, 28 Oct 2024 15:13:03 -0400, jseigh wrote:
>

So if were to implement a spinlock using the above instructions
something along the lines of
>
.L0
   ldaxr    -- load lockword exclusive w/ acquire membar
   cmp      -- compare to zero
   bne  .LO -- loop if currently locked
          stxr     -- store 1
          cbnz .LO -- retry if stxr failed

>
The closest I could find to this was on page 8367
of DDI0487G_a_armv8_arm.pdf from infocenter.arm.com:

 DDI0487K_a is the most recent.
 

>
    Loop
        LDAXR W5, [X1]       ; read lock with acquire
        CBNZ W5, Loop        ; check if 0
        STXR W5, W0, [X1]    ; attempt to store new value
        CBNZ W5, Loop        ; test if store succeeded and retry if not

 A real world example from the linux kernel:
 static __always_inline s64
__ll_sc_atomic64_dec_if_positive(atomic64_t *v)
{
         s64 result;
         unsigned long tmp;
          asm volatile("// atomic64_dec_if_positive\n"
         "       prfm    pstl1strm, %2\n"
         "1:     ldxr    %0, %2\n"
         "       subs    %0, %0, #1\n"
         "       b.lt    2f\n"
         "       stlxr   %w1, %0, %2\n"
         "       cbnz    %w1, 1b\n"
         "       dmb     ish\n"

         "2:"
         : "=&r" (result), "=&r" (tmp), "+Q" (v->counter)
         :
         : "cc", "memory");
          return result;
}