On Tue, 20 Dec 2022 02:11:42 +0000
"Zhou, Xiangyun" <xiangyun.z...@intel.com> wrote:
> 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.anan...@huawei.com>
> Sent: Tuesday, December 20, 2022 12:51 AM
> To: Tyler Retzlaff <roret...@linux.microsoft.com>; Zhou, Xiangyun
> <xiangyun.z...@intel.com>
> Cc: dev@dpdk.org; Xu, Bowen <bowen...@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.
>
>
I noticed a couple of other documentation errors here.
The intro comment says this a read-write lock (and it isn't).
Probably copy/paste error.
The user field says is is "core id" but it really is thread id.
