Re: [Qemu-devel] [PATCH 16/16] cpus-common: lock-free fast path for cpu_exec_start/end

[Paolo Bonzini]

On 26/09/2016 09:28, Alex Bennée wrote:
> > cpu->running is only read under the mutex, but can be written _by the
> > owner thread only_ outside the mutex.  So writes outside the mutex must
> > be atomic, but writes under the mutex don't because:
> >
> > - no other thread ever writes to cpu->running
> >
> > - no other thread can be reading cpu->running
>
> Should we add some comments to cpu.h's definitions to make the rules clear?

I don't know... It's awfully easy for such documentation to get out of 
date.  Currently it says:

 * @running: #true if CPU is currently running (lockless).
 * @has_waiter: #true if a CPU is currently waiting for the cpu_exec_end;
 * valid under cpu_list_lock.

I think it's a good middle ground; it's kind of obvious that only the
CPU itself writes cpu->running.  I'm changing anyway the cpu->running
assignment to atomic_set as suggested by Richard, and that makes it valid
to read cpu->running outside the lock.

Paolo