Re: [PATCH v3 06/20] seqlock: Extend seqcount API with associated locks

[Peter Zijlstra]

On Wed, Jul 08, 2020 at 12:33:14PM +0200, Ahmed S. Darwish wrote:

> > +#define read_seqcount_begin(s)	do_read_seqcount_begin(__to_seqcount_t(s))
> > +
> > +static inline unsigned do_read_seqcount_begin(const seqcount_t *s)
> > +{
> ...
> 
> Hmm, the __to_seqcount_t(s) force cast is not good. It will break the
> arguments type-safety of seqcount macros that do not have either:
> 
>     __associated_lock_is_preemptible() or
>     __assert_associated_lock_held()
> 
> in their path. This basically includes all the read path macros, and
> even some others (e.g. write_seqcount_invalidate()).
> 
> With the suggested force cast above, I can literally *pass anything* to
> read_seqcount_begin() and friends, and the compiler won't say a thing.
> 
> So, I'll restore __to_seqcount_t(s) that to its original implementation:

Right, I figured that the write side would be enough to catch glaring
abuse. But sure.

It's a bummer we didn't upgrade the minimum compiler version to 4.9,
that would've given us _Generic(), which allows one to write this
slightly less verbose I think.

How about something disguisting like this then?


#ifdef CONFIG_LOCKDEP
#define __SEQCOUNT_LOCKDEP(expr)	expr
#else
#define __SEQCOUNT_LOCKDEP(expr)
#endif

typedef seqcount_t * __seqprop_ptr_t;
typedef bool	     __seqprop_preempt_t;
typedef void	     __seqprop_assert_t;

static __always_inline __seqprop_ptr_t
__seqprop_seqcount_ptr(seqcount_t *s)
{
	return s;
}

static __always_inline __seqprop_preempt_t
__seqprop_seqcount_preempt(seqcount_t *s)
{
	return false;
}

static __always_inline __seqprop_assert_t
__seqprop_seqcount_assert(seqcount_t *s)
{
}

#define SEQCOUNT_LOCKTYPE(name, locktype, preempt, lockmember)		\
typedef struct seqcount_##name {					\
	seqcount_t	seqcount;					\
	__SEQCOUNT_LOCKDEP(locktype *lock);				\
} seqcount_##name##_t;							\
									\
static __always_inline void						\
seqcount_##name##_init(seqcount_##name##_t *s, locktype *l)		\
{									\
	seqcount_init(&s->seqcount);					\
	__SEQCOUNT_LOCKDEP(s->lock = l);				\
}									\
									\
static __always_inline __seqprop_ptr_t					\
__seqprop_##name##_ptr(seqcount_##name##_t *s)				\
{									\
	return &s->seqcount;						\
}									\
									\
static __always_inline __seqprop_preempt_t				\
__seqprop_##name##_preempt(seqcount_##name##_t *s)			\
{									\
	return preempt;							\
}									\
									\
static __always_inline __seqprop_assert_t				\
__seqprop_##name##_assert(seqcount_##name##_t *s)			\
{									\
	__SEQCOUNT_LOCKDEP(lockdep_assert_held(s->lockmember));		\
}


#define SEQCNT_LOCKTYPE_ZERO(_name, _lock) {				\
	.seqcount = SEQCNT_ZERO(_name.seqcount),			\
	__SEQCOUNT_LOCKDEP(.lock = (_lock))				\
}

#include <linux/spinlock.h>
#include <linux/ww_mutex.h>

#define __SEQ_RT	IS_BUILTIN(CONFIG_PREEMPT_RT)

SEQCOUNT_LOCKTYPE(raw_spinlock, raw_spinlock_t,	false,		lock)
SEQCOUNT_LOCKTYPE(spinlock,	spinlock_t,	__SEQ_RT,	lock)
SEQCOUNT_LOCKTYPE(rwlock,	rwlock_t,	__SEQ_RT,	lock)
SEQCOUNT_LOCKTYPE(mutex,	struct mutex,	true,		lock)
SEQCOUNT_LOCKTYPE(ww_mutex,	struct ww_mutex,true,		lock->base)

#if (defined(CONFIG_CC_IS_GCC) && CONFIG_GCC_VERSION < 40900) || defined(__CHECKER__)

#define __seqprop_pick(const_expr, s, locktype, prop, otherwise)	\
	__builtin_choose_expr(const_expr,				\
			      __seqprop_##locktype##_##prop((void *)(s)), \
			      otherwise)

extern void __seqprop_invalid(void);

#define __seqprop(s, prop)								\
	__seqprop_pick(__same_type(*(s), seqcount_t), (s), seqcount, prop,		\
	  __seqprop_pick(__same_type(*(s), seqcount_raw_spinlock_t), (s), raw_spinlock, prop, \
	    __seqprop_pick(__same_type(*(s), seqcount_spinlock_t), (s), spinlock, prop,	\
	      __seqprop_pick(__same_type(*(s), seqcount_rwlock_t), (s), rwlock, prop,	\
	        __seqprop_pick(__same_type(*(s), seqcount_mutex_t), (s), mutex, prop,	\
	          __seqprop_pick(__same_type(*(s), seqcount_ww_mutex_t), (s), ww_mutex, prop, \
		    __seqprop_invalid()))))))

#else

#define __seqprop_case(s, locktype, prop) \
	seqcount_##locktype##_t: __seqprop_##locktype##_##prop((void *)s)

#define __seqprop(s, prop)					\
	_Generic(*(s),						\
		 seqcount_t: __seqprop_seqcount_##prop((void*)s),\
		 __seqprop_case((s), raw_spinlock, prop),	\
		 __seqprop_case((s), spinlock, prop),		\
		 __seqprop_case((s), rwlock, prop),		\
		 __seqprop_case((s), mutex, prop),		\
		 __seqprop_case((s), ww_mutex, prop))

#endif

#define __to_seqcount_t(s)			__seqprop(s, ptr)
#define __associated_lock_is_preemptible(s)	__seqprop(s, preempt)
#define __assert_associated_lock_held(s)	__seqprop(s, assert)