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

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Message-ID: <6ae2461a-e509-4c4d-8959-ae17eb214419@huaweicloud.com>
Date: Mon, 7 Oct 2024 13:54:43 +0200
From: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 'Alan Stern' <stern@...land.harvard.edu>,
 David Laight <David.Laight@...lab.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
 "linux-kernel@...r.kernel.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" <maged.michael@...il.com>,
 Mateusz Guzik <mjguzik@...il.com>, Gary Guo <gary@...yguo.net>,
 "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
 "linux-mm@...ck.org" <linux-mm@...ck.org>,
 "lkmm@...ts.linux.dev" <lkmm@...ts.linux.dev>
Subject: Re: [PATCH 1/2] compiler.h: Introduce ptr_eq() to preserve address
 dependency

Am 10/3/2024 um 3:23 PM schrieb Mathieu Desnoyers:
> What _does_ work however are the following two approaches:
> 
> 1) Perform the equality check on the original variables, creating
> new versions (with OPTIMIZER_HIDE_VAR) of both variables for the
> rest of their use, therefore making sure the pointer dereference
> are not derived from versions of the variables which were compared
> with another pointer. (as suggested by Boqun)

This should not be guaranteed to work, because right after the 
comparison the compiler can do b=a, then it doesn't matter how much you 
hide afterwards.

However it might work if you escape the addresses of a and b first, in 
which case the compiler will not do b=a anymore, but it might force the 
compiler to put a and b on the stack, which has some performance impact.

> 
> 2) Perform the equality check on the versions resulting of hiding
> both variables, making sure those versions of the variables are
> not dereferenced afterwards. (as suggested by Linus)
> 

jonas