lkml.org 
[lkml]   [2023]   [Aug]   [4]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [PATCH 1/2] docs: rcu: Add cautionary note on plain-accesses to requirements
[...]
> > >> However, the kernel consider the volatile access to be atomic, right?
> > >
> > > The compiler must therefore act as if a volatile access to an aligned
> > > machine-word size location is atomic. To see this, consider accesses
> > > to memory that is shared by a device driver and that device's firmware,
> > > both of which are written in either C or C++.
> >
> > Btw it appears TSAN complaints bitterly on even volatile 4 byte data races.
> > Hence we have to explicitly use atomic API for data race accesses in Chrome.
>
> That might have been a conscious and deliberate choice on the part of
> the TSAN guys. Volatile does not imply any ordering in the standard
> (other than the compiler avoiding emitting volatile operations out of
> order), but some compilers did emit memory-barrier instructions for
> volatile accesses. Which resulted in a lot of problems when such code
> found compilers that did not cause the CPU to order volatile operations.
>
> So a lot of people decided to thrown the volatile baby out with the
> unordered bathwather. ;-)

Thanks for the input, I think TSAN was indeed worried about
memory-ordering even if relaxed ordering was intended. I think there
is a way to tell TSAN to shut-up in such situations but in my last
Chrome sprint, I just used the atomic API with relaxed ordering and
called it a day. :-)

- Joel

\
 
 \ /
  Last update: 2023-08-04 18:18    [W:0.046 / U:0.508 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site