Re: [PATCH 1/2] fix cmpxchg instruction

[Richard Henderson]

On 3/19/22 09:06, Wei Li wrote:
> We need a branch to determine when the instruction can touch the
> accumulator. But there is a branch provided by movcond.

There is no branch in movcond -- this expands to cmov.

> -                /* store value = (old == cmp ? new : old);  */
> -                tcg_gen_movcond_tl(TCG_COND_EQ, newv, oldv, cmpv, newv, oldv);
> +                tcg_gen_brcond_tl(TCG_COND_EQ, oldv, cmpv, label1);
...
>                       /* Perform an unconditional store cycle like physical cpu;
>                          must be before changing accumulator to ensure
>                          idempotency if the store faults and the instruction
>                          is restarted */

Your branch invalidates the comment -- the store becomes conditional, and we no longer get 
a write fault on read-only pages when the comparison fails.  OTOH, we're already getting 
the incorrect SIGSEGV behaviour, since we get a read fault on an unmapped page instead of 
a write fault.

The faulting behaviour could be addressed with a write_probe prior to the original load. 
Alternately, we can use the tcg_gen_atomic_cmpxchg_tl path whenever mod != 3.  While an 
unlocked cmpxchg need not be atomic, it is not required to be non-atomic either, and it 
would reduce code duplication.


r~