Re: [PATCH v1 2/2] Documentation: RCU: Refer to ptr_eq()

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2024-09-29 17:51, Paul E. McKenney wrote:
> On Sun, Sep 29, 2024 at 07:16:08AM -0400, Mathieu Desnoyers wrote:
>> Refer to ptr_eq() in the rcu_dereference() documentation.
>>
>> ptr_eq() is a mechanism that preserves address dependencies when
>> comparing pointers, and should be favored when comparing a pointer
>> obtained from rcu_dereference() against another pointer.
>>
>> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
>> Cc: "Paul E. McKenney" <paulmck@kernel.org>
>> Cc: Will Deacon <will@kernel.org>
>> Cc: Peter Zijlstra <peterz@infradead.org>
>> Cc: Boqun Feng <boqun.feng@gmail.com>
>> Cc: Alan Stern <stern@rowland.harvard.edu>
>> Cc: John Stultz <jstultz@google.com>
>> Cc: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
>> Cc: Linus Torvalds <torvalds@linux-foundation.org>
>> Cc: Boqun Feng <boqun.feng@gmail.com>
>> Cc: Frederic Weisbecker <frederic@kernel.org>
>> Cc: Joel Fernandes <joel@joelfernandes.org>
>> Cc: Josh Triplett <josh@joshtriplett.org>
>> Cc: Uladzislau Rezki <urezki@gmail.com>
>> Cc: Steven Rostedt <rostedt@goodmis.org>
>> Cc: Lai Jiangshan <jiangshanlai@gmail.com>
>> Cc: Zqiang <qiang.zhang1211@gmail.com>
>> Cc: Ingo Molnar <mingo@redhat.com>
>> Cc: Waiman Long <longman@redhat.com>
>> Cc: Mark Rutland <mark.rutland@arm.com>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Cc: Vlastimil Babka <vbabka@suse.cz>
>> Cc: maged.michael@gmail.com
>> Cc: Mateusz Guzik <mjguzik@gmail.com>
>> Cc: Gary Guo <gary@garyguo.net>
>> Cc: Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
>> Cc: rcu@vger.kernel.org
>> Cc: linux-mm@kvack.org
>> Cc: lkmm@lists.linux.dev
>> Cc: Nikita Popov <github@npopov.com>
>> Cc: llvm@lists.linux.dev
>> ---
>> Changes since v0:
>> - Include feedback from Alan Stern.
>> ---
>>   Documentation/RCU/rcu_dereference.rst | 32 ++++++++++++++++++++++-----
>>   1 file changed, 27 insertions(+), 5 deletions(-)
>>
>> diff --git a/Documentation/RCU/rcu_dereference.rst b/Documentation/RCU/rcu_dereference.rst
>> index 2524dcdadde2..9ef97b7ca74d 100644
>> --- a/Documentation/RCU/rcu_dereference.rst
>> +++ b/Documentation/RCU/rcu_dereference.rst
>> @@ -104,11 +104,12 @@ readers working properly:
>>   	after such branches, but can speculate loads, which can again
>>   	result in misordering bugs.
>>   
>> --	Be very careful about comparing pointers obtained from
>> -	rcu_dereference() against non-NULL values.  As Linus Torvalds
>> -	explained, if the two pointers are equal, the compiler could
>> -	substitute the pointer you are comparing against for the pointer
>> -	obtained from rcu_dereference().  For example::
>> +-	Use operations that preserve address dependencies (such as
>> +	"ptr_eq()") to compare pointers obtained from rcu_dereference()
>> +	against non-NULL pointers. As Linus Torvalds explained, if the
>> +	two pointers are equal, the compiler could substitute the
>> +	pointer you are comparing against for the pointer obtained from
>> +	rcu_dereference().  For example::
>>   
>>   		p = rcu_dereference(gp);
>>   		if (p == &default_struct)
>> @@ -125,6 +126,23 @@ readers working properly:
>>   	On ARM and Power hardware, the load from "default_struct.a"
>>   	can now be speculated, such that it might happen before the
>>   	rcu_dereference().  This could result in bugs due to misordering.
>> +	Performing the comparison with "ptr_eq()" ensures the compiler
>> +	does not perform such transformation.
>> +
>> +	If the comparison is against another pointer, the compiler is
>> +	allowed to use either pointer for the following accesses, which
>> +	loses the address dependency and allows weakly-ordered
>> +	architectures such as ARM and PowerPC to speculate the
>> +	address-dependent load before rcu_dereference().  For example::
>> +
>> +		p1 = READ_ONCE(gp);
>> +		p2 = rcu_dereference(gp);
>> +		if (p1 == p2)
>> +			do_default(p2->a);
>> +
>> +	The compiler can use p1->a rather than p2->a, destroying the
>> +	address dependency.  Performing the comparison with "ptr_eq()"
>> +	ensures the compiler preserves the address dependencies.
> 
> Bitter experience leads me to suggest a "// BUGGY" comment on the "if"
> statement in the above example, and a corrected code snippet right here.  :-/

Changing for the following:

+               p1 = READ_ONCE(gp);
+               p2 = rcu_dereference(gp);
+               if (p1 == p2)  /* BUGGY!!! */
+                       do_default(p2->a);
+
+       The compiler can use p1->a rather than p2->a, destroying the
+       address dependency.  Performing the comparison with "ptr_eq()"
+       ensures the compiler preserves the address dependencies.
+       Corrected code::
+
+               p1 = READ_ONCE(gp);
+               p2 = rcu_dereference(gp);
+               if (ptr_eq(p1, p2))
+                       do_default(p2->a);

> 
> Other than that, loks good!

Let me know if I should add an acked-by from you on this
documentation patch as well.

Thanks,

Mathieu

> 
> 							Thanx, Paul
> 
>>   	However, comparisons are OK in the following cases:
>>   
>> @@ -204,6 +222,10 @@ readers working properly:
>>   		comparison will provide exactly the information that the
>>   		compiler needs to deduce the value of the pointer.
>>   
>> +	When in doubt, use operations that preserve address dependencies
>> +	(such as "ptr_eq()") to compare pointers obtained from
>> +	rcu_dereference() against non-NULL pointers.
>> +
>>   -	Disable any value-speculation optimizations that your compiler
>>   	might provide, especially if you are making use of feedback-based
>>   	optimizations that take data collected from prior runs.  Such
>> -- 
>> 2.39.2
>>

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com