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

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Message-ID: <9539c551-5c91-42db-8ac1-cff1d6d7c293@huaweicloud.com>
Date: Mon, 30 Sep 2024 13:26:53 +0200
From: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
To: Alan Stern <stern@...land.harvard.edu>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: 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

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.

Best wishes,
   jonas