> On Aug 3, 2023, at 8:09 AM, Alan Huang <mmpgouride@xxxxxxxxx> wrote: > > >> 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. Yes, a bad outcome is perfectly legal amidst data race. Who said it is not legal? > > But the example here is different, That is intentional. Wills discussion partially triggered this. Though I am wondering if we should document that as well. > 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? Maybe you are trying to rationalize too much that the sequence mentioned cannot result in a counter intuitive outcome like I did? The point AFAIU is not just about line 10 but that the compiler can replace any of the lines after the plain access with the cached value. Thanks. > >> +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 >> >
