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

[Paul E. McKenney]

On Sun, Sep 29, 2024 at 12:09:54PM -0400, Mathieu Desnoyers wrote:
> 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@xxxxxxxxxxxx>
> > > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > > Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> > > Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> > > Cc: Will Deacon <will@xxxxxxxxxx>
> > > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > > Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> > > Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> > > Cc: John Stultz <jstultz@xxxxxxxxxx>
> > > Cc: Neeraj Upadhyay <Neeraj.Upadhyay@xxxxxxx>
> > > Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> > > Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> > > Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
> > > Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> > > Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> > > Cc: Uladzislau Rezki <urezki@xxxxxxxxx>
> > > Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > > Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
> > > Cc: Zqiang <qiang.zhang1211@xxxxxxxxx>
> > > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > > Cc: Waiman Long <longman@xxxxxxxxxx>
> > > Cc: Mark Rutland <mark.rutland@xxxxxxx>
> > > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > > Cc: Vlastimil Babka <vbabka@xxxxxxx>
> > > Cc: maged.michael@xxxxxxxxx
> > > Cc: Mateusz Guzik <mjguzik@xxxxxxxxx>
> > > Cc: Gary Guo <gary@xxxxxxxxxxx>
> > > Cc: Jonas Oberhauser <jonas.oberhauser@xxxxxxxxxxxxxxx>
> > > Cc: rcu@xxxxxxxxxxxxxxx
> > > Cc: linux-mm@xxxxxxxxx
> > > Cc: lkmm@xxxxxxxxxxxxxxx
> > > Cc: Nikita Popov <github@xxxxxxxxxx>
> > > Cc: llvm@xxxxxxxxxxxxxxx
> > > ---
> > > 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.

Much better!

Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

> 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
>