linux-kernel - Re: [PATCH 1/2] compiler.h: Introduce ptr

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Message-ID: <2cdda043-1ad9-40cf-a157-0c16a0ffb046@rowland.harvard.edu>
Date: Mon, 30 Sep 2024 12:43:53 -0400
From: Alan Stern <stern@...land.harvard.edu>
To: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Will Deacon <will@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Boqun Feng <boqun.feng@...il.com>, John Stultz <jstultz@...gle.com>,
	Neeraj Upadhyay <Neeraj.Upadhyay@....com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Joel Fernandes <joel@...lfernandes.org>,
	Josh Triplett <josh@...htriplett.org>,
	Uladzislau Rezki <urezki@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>, Ingo Molnar <mingo@...hat.com>,
	Waiman Long <longman@...hat.com>,
	Mark Rutland <mark.rutland@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Vlastimil Babka <vbabka@...e.cz>, maged.michael@...il.com,
	Mateusz Guzik <mjguzik@...il.com>, Gary Guo <gary@...yguo.net>,
	rcu@...r.kernel.org, linux-mm@...ck.org, lkmm@...ts.linux.dev
Subject: Re: [PATCH 1/2] compiler.h: Introduce ptr_eq() to preserve address
 dependency

On Mon, Sep 30, 2024 at 01:26:53PM +0200, Jonas Oberhauser wrote:
> 
> 
> Am 9/28/2024 um 4:49 PM schrieb Alan Stern:
> > On Sat, Sep 28, 2024 at 09:51:27AM -0400, Mathieu Desnoyers wrote:
> > > Compiler CSE and SSA GVN optimizations can cause the address dependency
> > > of addresses returned by rcu_dereference to be lost when comparing those
> > > pointers with either constants or previously loaded pointers.
> > > 
> > > Introduce ptr_eq() to compare two addresses while preserving the address
> > > dependencies for later use of the address. It should be used when
> > > comparing an address returned by rcu_dereference().
> > > 
> > > This is needed to prevent the compiler CSE and SSA GVN optimizations
> > > from replacing the registers holding @a or @b based on their
> > 
> > "Replacing" isn't the right word.  What the compiler does is use one
> > rather than the other.  Furthermore, the compiler can play these games
> > even with values that aren't in registers.
> > 
> > You should just say: "... from using @a (or @b) in places where the
> > source refers to @b (or @a) (based on the fact that after the
> > comparison, the two are known to be equal), which does not ..."
> 
> I should also point out that it is not enough to prevent the compiler from
> using @a instead of @b.
> 
> It must also be prevented from assigning @b=@a, which it is often allowed to
> do after finding @a==@b.

Wouldn't that be a bug?  Consider this litmus test:

int x = 0;
int y = 45;
int z = 0;

void P0(int *x, int *y, int *z) {
	int r1, r2;

	r1 = READ_ONCE(*x);
	r2 = READ_ONCE(*y);
	if (r1 == r2) {
		WRITE_ONCE(*z, 1);
		// L1: WRITE_ONCE(*y, r1);
	}
}

void P1(int *x, int *y) {
	int r3;

	WRITE_ONCE(*x, 45);
	WRITE_ONCE(*y, 56);
	r3 = READ_ONCE(*y);
}

exists (z=1 /\ 1:r3=45)		(* Not allowed *)

If the compiler were to make the extra assignment (basically, 
uncommenting the line marked L1) then the exists clause could be 
satisfied.  That would indicate there's a bug in the compiler.

Alan