> 2023年8月3日 11:24，Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> 写道： > > Add a detailed note to explain the potential side effects of > plain-accessing the gp pointer using a plain load, without using the > rcu_dereference() macros; which might trip neighboring code that does > use rcu_dereference(). > > I haven't verified this with a compiler, but this is what I gather from > the below link using Will's experience with READ_ONCE(). > > Link: https://lore.kernel.org/all/20230728124412.GA21303@willie-the-truck/ > Cc: Will Deacon <will@xxxxxxxxxx> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > --- > .../RCU/Design/Requirements/Requirements.rst | 32 +++++++++++++++++++ > 1 file changed, 32 insertions(+) > > diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst > index f3b605285a87..e0b896d3fb9b 100644 > --- a/Documentation/RCU/Design/Requirements/Requirements.rst > +++ b/Documentation/RCU/Design/Requirements/Requirements.rst > @@ -376,6 +376,38 @@ mechanism, most commonly locking or reference counting > .. |high-quality implementation of C11 memory_order_consume [PDF]| replace:: high-quality implementation of C11 ``memory_order_consume`` [PDF] > .. _high-quality implementation of C11 memory_order_consume [PDF]: http://www.rdrop.com/users/paulmck/RCU/consume.2015.07.13a.pdf > > +Note that, there can be strange side effects (due to compiler optimizations) if > +``gp`` is ever accessed using a plain load (i.e. without ``READ_ONCE()`` or > +``rcu_dereference()``) potentially hurting any succeeding > +``rcu_dereference()``. For example, consider the code: > + > + :: > + > + 1 bool do_something_gp(void) > + 2 { > + 3 void *tmp; > + 4 rcu_read_lock(); > + 5 tmp = gp; // Plain-load of GP. > + 6 printk("Point gp = %p\n", tmp); > + 7 > + 8 p = rcu_dereference(gp); > + 9 if (p) { > + 10 do_something(p->a, p->b); > + 11 rcu_read_unlock(); > + 12 return true; > + 13 } > + 14 rcu_read_unlock(); > + 15 return false; > + 16 } > + > +The behavior of plain accesses involved in a data race is non-deterministic in > +the face of compiler optimizations. Since accesses to the ``gp`` pointer is > +by-design a data race, the compiler could trip this code by caching the value > +of ``gp`` into a register in line 5, and then using the value of the register > +to satisfy the load in line 10. Thus it is important to never mix Will’s example is: // Assume *ptr is initially 0 and somebody else writes it to 1 // concurrently foo = *ptr; bar = READ_ONCE(*ptr); baz = *ptr; Then the compiler is within its right to reorder it to: foo = *ptr; baz = *ptr; bar = READ_ONCE(*ptr); So, the result foo == baz == 0 but bar == 1 is perfectly legal. But the example here is different, the compiler can not use the value loaded from line 5 unless the compiler can deduce that the tmp is equals to p in which case the address dependency doesn’t exist anymore. What am I missing here? > +plain accesses of a memory location with rcu_dereference() of the same memory > +location, in code involved in a data race. > + > In short, updaters use rcu_assign_pointer() and readers use > rcu_dereference(), and these two RCU API elements work together to > ensure that readers have a consistent view of newly added data elements. > -- > 2.41.0.585.gd2178a4bd4-goog >
