Re: [Qemu-devel] [RFC v1 03/11] tcg: comment on which functions have to be called with tb_lock held

[Paolo Bonzini <pbonzini@redhat.com>]

On 18/03/2016 17:18, Alex Bennée wrote:
> +
> +    /* Protected by tb_lock.  */

Only writes are protected by tb_lock.  Read happen outside the lock.

Reads are not quite thread safe yet, because of tb_flush.  In order to
fix that, there's either the async_safe_run() mechanism from Fred or
preferrably the code generation buffer could be moved under RCU.

Because tb_flush is really rare, my suggestion is simply to allocate two
code generation buffers and do something like

static int which_buffer_is_in_use_bit_mask = 1;
...

   /* in tb_flush */
   assert (which_buffer_is_in_use_bit_mask != 3);
   if (which_buffer_is_in_use_bit_mask == 1) {
       which_buffer_is_in_use_bit_mask |= 2;
       call_rcu(function doing which_buffer_is_in_use_bit_mask &= ~1);
       point TCG to second buffer
    } else if (which_buffer_is_in_use_bit_mask == 2) {
       which_buffer_is_in_use_bit_mask |= 1;
       call_rcu(function doing which_buffer_is_in_use_bit_mask &= ~2);
       point TCG to first buffer
    }

Basically, we just assert that call_rcu makes at least one pass between
two tb_flushes.

All this is also a prerequisite for patch 1.

Paolo

>      struct TranslationBlock *tb_jmp_cache[TB_JMP_CACHE_SIZE];
> +