linux-kernel - Re: [PATCH v2 10/35] nds32: Atomic operations

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Date:   Tue, 28 Nov 2017 12:24:46 +0800
From:   Vincent Chen <deanbo422@...il.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Greentime Hu <green.hu@...il.com>,
        Greentime <greentime@...estech.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        linux-arch <linux-arch@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>,
        Marc Zyngier <marc.zyngier@....com>,
        Rob Herring <robh+dt@...nel.org>,
        Networking <netdev@...r.kernel.org>,
        DTML <devicetree@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        David Howells <dhowells@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        linux-serial@...r.kernel.org, Vincent Chen <vincentc@...estech.com>
Subject: Re: [PATCH v2 10/35] nds32: Atomic operations

2017-11-27 21:57 GMT+08:00 Mark Rutland <mark.rutland@....com>:
> Hi,
>
> On Mon, Nov 27, 2017 at 08:27:57PM +0800, Greentime Hu wrote:
>> +static inline void arch_spin_unlock(arch_spinlock_t * lock)
>> +{
>> +     asm volatile(
>> +             "xor    $r15,  $r15, $r15\n"
>> +             "swi    $r15, [%0]\n"
>> +             :
>> +             :"r"(&lock->lock)
>> +             :"memory");
>> +}
>
> This looks suspicious. There's no clobber for $r15, so isn't this
> corrupting a GPR behind the back of the compiler?
>

Thanks.
$r15 is reserved for assembler.
For safety, compiler always put $r15 in clobber register list by default.


> Shouldn't there be a tmp variable for the register allocation rather
> than hard-coding $r15?
>
>> +static inline void arch_write_unlock(arch_rwlock_t * rw)
>> +{
>> +     asm volatile(
>> +             "xor    $r15, $r15, $r15\n"
>> +             "swi    $r15, [%0]\n"
>> +             :
>> +             :"r"(&rw->lock)
>> +             :"memory","$r15");
>> +}
>
> This time you have a clobber, but it's still not clear to me why you
> don't use a tmp variable and leave the register allocation to the
> compiler.
>

Thanks.
When I recheck the code, I found spinlock.h is not needed due that we
do not support SMP.
So, We decide to remove spinlock.h in the next version patch.

Vincent
> Thanks,
> Mark.