linux-kernel - Re: [PATCH v1 1/2] powerpc/bitops: Use immediate operand when possible

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Message-ID: <daacce9f-1900-1034-980b-be5a58d6be09@csgroup.eu>
Date:   Wed, 14 Apr 2021 14:42:51 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Segher Boessenkool <segher@...nel.crashing.org>,
        Nicholas Piggin <npiggin@...il.com>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH v1 1/2] powerpc/bitops: Use immediate operand when
 possible



Le 14/04/2021 à 14:24, Segher Boessenkool a écrit :
> On Wed, Apr 14, 2021 at 12:01:21PM +1000, Nicholas Piggin wrote:
>> Would be nice if we could let the compiler deal with it all...
>>
>> static inline unsigned long lr(unsigned long *mem)
>> {
>>          unsigned long val;
>>
>>          /*
>>           * This doesn't clobber memory but want to avoid memory operations
>>           * moving ahead of it
>>           */
>>          asm volatile("ldarx     %0, %y1" : "=r"(val) : "Z"(*mem) : "memory");
>>
>>          return val;
>> }
> 
> (etc.)
> 
> That can not work reliably: the compiler can put random instructions
> between the larx and stcx. this way, and you then do not have guaranteed
> forward progress anymore.  It can put the two in different routines
> (after inlining and other interprocedural optimisations), duplicate
> them, make a different number of copies of them, etc.
> 
> Nothing of that is okay if you want to guarantee forward progress on all
> implementations, and also not if you want to have good performance
> everywhere (or anywhere even).  Unfortunately you have to write all
> larx/stcx. loops as one block of assembler, so that you know exactly
> what instructions will end up in your binary.
> 
> If you don't, it will fail mysteriously after random recompilations, or
> have performance degradations, etc.  You don't want to go there :-)
> 

Could the kernel use GCC builtin atomic functions instead ?

https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html