Local variables reordering and 'asm volatile("" ::: "memory");'

[Alexander Monakov]

On Sun, 8 Sep 2019, Ayrat Gaskarov wrote:

> mutex_lock()
> function_to_lock()
> some_really_slow_not_observable_function()
> mutex_unlock()
> 
> How could we avoid this? For example if
> some_really_slow_not_observable_function is long running loop using
> only local variables.

One possibility is by tying control flow and data flow in the inline asm.
Suppose your some_really_slow_not_observable_function returns an int, then

  int ret = some_really_slow_not_observable_function();
  asm volatile ("" :: "g"(ret), "X" (&mut) : "memory");
  
  mutex_lock(&mut);
  func_under_lock();
  mutex_unlock(&mut);

  <use ret>

enforces order by creating a dependency between the value returned from
the first call and mutex state used in the following call. A similar barrier
can be placed between the unlock and the use of 'ret':

  asm volatile ("" : "=g"(ret) : "X" (&mut) : "memory");

(in general empty volatile asms with non-empty constraints is a nice tool for
limiting what the compiler may do)

Alexander