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

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Message-ID: <5d7d8a59-57f5-4125-95bb-fda9c193b9cf@huaweicloud.com>
Date: Mon, 30 Sep 2024 19:05:06 +0200
From: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
To: Alan Stern <stern@...land.harvard.edu>
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

Am 9/30/2024 um 6:43 PM schrieb Alan Stern:
> 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? 

That's why I said that it is often allowed to do it. In your case it 
wouldn't, but it is often possible when a and b are non-atomic & 
non-volatile (and haven't escaped, and I believe sometimes even then).

It happens for example here with GCC 14.1.0 -O3:

int fct_hide(void)
{
     int *a, *b;

     do {
         a = READ_ONCE(p);
         asm volatile ("" : : : "memory");
         b = READ_ONCE(p);
     } while (a != b);
     OPTIMIZER_HIDE_VAR(b);
     return *b;
}

         ldr     r1, [r2]
         ldr     r3, [r2]
         cmp     r1, r3
         bne     .L6
         mov     r3, r1   // nay...
         ldr     r0, [r3] // yay!
         bx      lr

Have fun,

    jonas