linux-kernel - Re: [PATCH v3 06/13] epoll: introduce helpers for adding/removing events to uring

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Date:   Fri, 31 May 2019 15:11:27 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Roman Penyaev <rpenyaev@...e.de>
Cc:     azat@...event.org, akpm@...ux-foundation.org,
        viro@...iv.linux.org.uk, torvalds@...ux-foundation.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 06/13] epoll: introduce helpers for adding/removing
 events to uring

On Fri, May 31, 2019 at 01:24:07PM +0200, Roman Penyaev wrote:
> On 2019-05-31 11:55, Peter Zijlstra wrote:
> > On Thu, May 16, 2019 at 10:58:03AM +0200, Roman Penyaev wrote:
> > > +#define atomic_set_unless_zero(ptr, flags)			\
> > > +({								\
> > > +	typeof(ptr) _ptr = (ptr);				\
> > > +	typeof(flags) _flags = (flags);				\
> > > +	typeof(*_ptr) _old, _val = READ_ONCE(*_ptr);		\
> > > +								\
> > > +	for (;;) {						\
> > > +		if (!_val)					\
> > > +			break;					\
> > > +		_old = cmpxchg(_ptr, _val, _flags);		\
> > > +		if (_old == _val)				\
> > > +			break;					\
> > > +		_val = _old;					\
> > > +	}							\
> > > +	_val;							\
> > > +})
> > 
> > > +#define atomic_or_with_mask(ptr, flags, mask)			\
> > > +({								\
> > > +	typeof(ptr) _ptr = (ptr);				\
> > > +	typeof(flags) _flags = (flags);				\
> > > +	typeof(flags) _mask = (mask);				\
> > > +	typeof(*_ptr) _old, _new, _val = READ_ONCE(*_ptr);	\
> > > +								\
> > > +	for (;;) {						\
> > > +		_new = (_val & ~_mask) | _flags;		\
> > > +		_old = cmpxchg(_ptr, _val, _new);		\
> > > +		if (_old == _val)				\
> > > +			break;					\
> > > +		_val = _old;					\
> > > +	}							\
> > > +	_val;							\
> > > +})
> > 
> > Don't call them atomic_*() if they're not part of the atomic_t
> > interface.
> 
> Can we add those two?  Or keep it local is better?

Afaict you're using them on the user visible values; we should not put
atomic_t into shared memory.

Your interface isn't compatible with those 'funny' architectures like
parisc etc. Since you expect userspace to do atomic ops on these
variables too. It is not a one-way interface ...