On Fri, Aug 04, 2023 at 05:11:27AM +0000, Joel Fernandes wrote: > On Thu, Aug 03, 2023 at 11:52:06AM -0700, Paul E. McKenney wrote: > > On Thu, Aug 03, 2023 at 03:24:07AM +0000, Joel Fernandes (Google) wrote: > > > The compiler has the ability to cause misordering by destroying > > > address-dependency barriers if comparison operations are used. Add a > > > note about this to memory-barriers.txt and point to rcu-dereference.rst > > > for more information. > > > > > > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > > > --- > > > Documentation/memory-barriers.txt | 5 +++++ > > > 1 file changed, 5 insertions(+) > > > > > > diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt > > > index 06e14efd8662..acc8ec5ce563 100644 > > > --- a/Documentation/memory-barriers.txt > > > +++ b/Documentation/memory-barriers.txt > > > @@ -435,6 +435,11 @@ Memory barriers come in four basic varieties: > > > variables such as READ_ONCE() and rcu_dereference() provide implicit > > > address-dependency barriers. > > > > > > + [!] Note that address dependency barriers can be destroyed by comparison > > > + of a pointer obtained by a marked accessor such as READ_ONCE() or > > > + rcu_dereference() with some value. For an example of this, see > > > + rcu_dereference.rst (part where the comparison of pointers is discussed). > > > > Hmmm... > > > > Given that this is in a section marked "historical" (for the old > > smp_read_barrier_depends() API), why not instead add a pointer to > > Documentation/RCU/rcu_dereference.rst to the beginning of the section, > > noted as the updated material? > > Sounds good. There's also another section in the same file on Address > dependency barriers (also marked historical). So something like the > following? Given a Signed-off-by and so forth, I would be happy to take this one. Thanx, Paul > ---8<----------------------- > > diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt > index acc8ec5ce563..ba50220716ca 100644 > --- a/Documentation/memory-barriers.txt > +++ b/Documentation/memory-barriers.txt > @@ -396,6 +396,10 @@ Memory barriers come in four basic varieties: > > > (2) Address-dependency barriers (historical). > + [!] This section is marked as HISTORICAL: For more up-to-date > + information, including how compiler transformations related to pointer > + comparisons can sometimes cause problems, see > + Documentation/RCU/rcu_dereference.rst. > > An address-dependency barrier is a weaker form of read barrier. In the > case where two loads are performed such that the second depends on the > @@ -561,6 +565,9 @@ There are certain things that the Linux kernel memory barriers do not guarantee: > > ADDRESS-DEPENDENCY BARRIERS (HISTORICAL) > ---------------------------------------- > +[!] This section is marked as HISTORICAL: For more up-to-date information, > +including how compiler transformations related to pointer comparisons can > +sometimes cause problems, see Documentation/RCU/rcu_dereference.rst. > > As of v4.15 of the Linux kernel, an smp_mb() was added to READ_ONCE() for > DEC Alpha, which means that about the only people who need to pay attention
