On Fri, 2019-10-11 at 18:41 -0700, speck for Pawan Gupta wrote:
> On Thu, Oct 10, 2019 at 08:54:12AM +0200, speck for Greg KH wrote:
[...]
> > > +/* Take tsx_mutex lock and update tsx_ctrl_state when calling this function */
> > > +static void tsx_update_on_each_cpu(bool val)
> > > +{
> > > +	get_online_cpus();
> > > +	on_each_cpu(tsx_update_this_cpu, (void *)val, 1);
> > > +	put_online_cpus();
> > > +}
> > 
> > Why take the lock?  This is only called in one place.
> 
> So that TSX_CTRL MSR state stays consistent across all CPUs between
> multiple on_each_cpu() calls. Otherwise overlapping conflicting TSX_CTRL MSR
> writes could end up in some CPUs with TSX enabled and others with TSX
> disabled.
[...]

get_online_cpus() is a read lock, so it doesn't prevent concurrent
updates.

Ben.

-- 
Ben Hutchings
The obvious mathematical breakthrough [to break modern encryption]
would be development of an easy way to factor large prime numbers.
                                                           - Bill Gates