C++20 report error at file rte_spinlock.h

[Zhou, Xiangyun]

Thanks very much  for Konstantin and Tyler's analyzing.

I agree that removal of 'volatile' is enough. A spinlock has already used to protect these variables.

-----Original Message-----
From: Konstantin Ananyev <konstantin.ananyev at huawei.com> 
Sent: Tuesday, December 20, 2022 12:51 AM
To: Tyler Retzlaff <roretzla at linux.microsoft.com>; Zhou, Xiangyun <xiangyun.zhou at intel.com>
Cc: dev at dpdk.org; Xu, Bowen <bowen.xu at intel.com>
Subject: RE: C++20 report error at file rte_spinlock.h


> On Tue, Dec 13, 2022 at 06:11:06AM +0000, Zhou, Xiangyun wrote:
> > Dear dpdk dev,
> >
> > I'm using dpdk 21.11 LTS, when compile my program with CPP flag 
> > "-std=c++20", the compiler report below errors. After checking file
> rte_spinlock.h, I think the error report by compiler is valid, there 
> should be a potential issue when using functions 
> rte_spinlock_recursive_lock, rte_spinlock_recursive_unlock and 
> rte_spinlock_recursive_trylock in multi-thread, we could either remove "volatile" definition to ask users to handle the multi-thread issue, or using atomic operatings instead of self-increment and self-decrement.
> >
> >
> > /home/dpdk/lib/eal/include/generic/rte_spinlock.h:221:12: error: 
> > increment of object of volatile-qualified type 'volatile int' is
> deprecated [-Werror,-Wdeprecated-volatile]
> >         slr->count++;
> >                   ^
> > /home/dpdk/lib/eal/include/generic/rte_spinlock.h:231:6: error: 
> > decrement of object of volatile-qualified type 'volatile int' is
> deprecated [-Werror,-Wdeprecated-volatile]
> >         if (--(slr->count) == 0) {
> >             ^
> > /home/dpdk/lib/eal/include/generic/rte_spinlock.h:255:12: error: 
> > increment of object of volatile-qualified type 'volatile int' is
> deprecated [-Werror,-Wdeprecated-volatile]
> >         slr->count++;
> >
> 
> i have work in progress to optionally use standard atomics but in the 
> meantime the correct thing to do here is to use the gcc builtins that 
> match the requirements of the c++11 memory model.
> 
> the code should be converted to use __atomic_fetch_{add,sub} or 
> __atomic_{add,sub}_fetch as appropriate.
> 
> ty.

>From looking at the code, I don't think it is necessary:
both 'user' and 'count' supposed to be protected by 'sl'.
In fact, it looks safe just to remove 'volatile' qualifier here.