Re: [PATCH v2 1/1] Move kfree_call_rcu() to slab_common.c

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Fri, Dec 22, 2017 at 09:17:04AM +0800, Boqun Feng wrote:
> Hi Shoaib,
> 
> On Thu, Dec 21, 2017 at 02:32:50PM -0800, rao.shoaib@oracle.com wrote:
> > From: Rao Shoaib <rao.shoaib@oracle.com>
> > 
> > This patch moves kfree_call_rcu() and related macros out of rcu code.
> > A new function call_rcu_lazy() is created for calling __call_rcu() with
> > the lazy flag. kfree_call_rcu() in the tiny implementation remains unchanged.
> > 
> 
> Mind to explain why you want to do this in the commit log?

I am too close to this one, so I need you guys to hash this out.  ;-)

> > V2: Addresses the noise generated by checkpatch
> > 
> > Signed-off-by: Rao Shoaib <rao.shoaib@oracle.com>
> > ---
> >  include/linux/rcupdate.h | 43 +++----------------------------------------
> >  include/linux/rcutree.h  |  2 --
> >  include/linux/slab.h     | 44 ++++++++++++++++++++++++++++++++++++++++++++
> >  kernel/rcu/tree.c        | 24 ++++++++++--------------
> >  mm/slab_common.c         | 10 ++++++++++
> >  5 files changed, 67 insertions(+), 56 deletions(-)
> > 
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index a6ddc42..23ed728 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -55,6 +55,9 @@ void call_rcu(struct rcu_head *head, rcu_callback_t func);
> >  #define	call_rcu	call_rcu_sched
> >  #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
> >  
> > +/* only for use by kfree_call_rcu() */
> > +void call_rcu_lazy(struct rcu_head *head, rcu_callback_t func);
> > +
> >  void call_rcu_bh(struct rcu_head *head, rcu_callback_t func);
> >  void call_rcu_sched(struct rcu_head *head, rcu_callback_t func);
> >  void synchronize_sched(void);
> > @@ -838,45 +841,6 @@ static inline notrace void rcu_read_unlock_sched_notrace(void)
> >  #define __is_kfree_rcu_offset(offset) ((offset) < 4096)
> >  
> >  /*
> > - * Helper macro for kfree_rcu() to prevent argument-expansion eyestrain.
> > - */
> > -#define __kfree_rcu(head, offset) \
> > -	do { \
> > -		BUILD_BUG_ON(!__is_kfree_rcu_offset(offset)); \
> > -		kfree_call_rcu(head, (rcu_callback_t)(unsigned long)(offset)); \
> > -	} while (0)
> > -
> > -/**
> > - * kfree_rcu() - kfree an object after a grace period.
> > - * @ptr:	pointer to kfree
> > - * @rcu_head:	the name of the struct rcu_head within the type of @ptr.
> > - *
> > - * Many rcu callbacks functions just call kfree() on the base structure.
> > - * These functions are trivial, but their size adds up, and furthermore
> > - * when they are used in a kernel module, that module must invoke the
> > - * high-latency rcu_barrier() function at module-unload time.
> > - *
> > - * The kfree_rcu() function handles this issue.  Rather than encoding a
> > - * function address in the embedded rcu_head structure, kfree_rcu() instead
> > - * encodes the offset of the rcu_head structure within the base structure.
> > - * Because the functions are not allowed in the low-order 4096 bytes of
> > - * kernel virtual memory, offsets up to 4095 bytes can be accommodated.
> > - * If the offset is larger than 4095 bytes, a compile-time error will
> > - * be generated in __kfree_rcu().  If this error is triggered, you can
> > - * either fall back to use of call_rcu() or rearrange the structure to
> > - * position the rcu_head structure into the first 4096 bytes.
> > - *
> > - * Note that the allowable offset might decrease in the future, for example,
> > - * to allow something like kmem_cache_free_rcu().
> > - *
> > - * The BUILD_BUG_ON check must not involve any function calls, hence the
> > - * checks are done in macros here.
> > - */
> > -#define kfree_rcu(ptr, rcu_head)					\
> > -	__kfree_rcu(&((ptr)->rcu_head), offsetof(typeof(*(ptr)), rcu_head))
> > -
> > -
> > -/*
> >   * Place this after a lock-acquisition primitive to guarantee that
> >   * an UNLOCK+LOCK pair acts as a full barrier.  This guarantee applies
> >   * if the UNLOCK and LOCK are executed by the same CPU or if the
> > @@ -888,5 +852,4 @@ static inline notrace void rcu_read_unlock_sched_notrace(void)
> >  #define smp_mb__after_unlock_lock()	do { } while (0)
> >  #endif /* #else #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE */
> >  
> > -
> >  #endif /* __LINUX_RCUPDATE_H */
> > diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
> > index 37d6fd3..7746b19 100644
> > --- a/include/linux/rcutree.h
> > +++ b/include/linux/rcutree.h
> > @@ -48,8 +48,6 @@ void synchronize_rcu_bh(void);
> >  void synchronize_sched_expedited(void);
> >  void synchronize_rcu_expedited(void);
> >  
> > -void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func);
> > -
> >  /**
> >   * synchronize_rcu_bh_expedited - Brute-force RCU-bh grace period
> >   *
> > diff --git a/include/linux/slab.h b/include/linux/slab.h
> > index 50697a1..a71f6a78 100644
> > --- a/include/linux/slab.h
> > +++ b/include/linux/slab.h
> > @@ -342,6 +342,50 @@ void *__kmalloc(size_t size, gfp_t flags) __assume_kmalloc_alignment __malloc;
> >  void *kmem_cache_alloc(struct kmem_cache *, gfp_t flags) __assume_slab_alignment __malloc;
> >  void kmem_cache_free(struct kmem_cache *, void *);
> >  
> > +void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func);
> > +
> > +/* Helper macro for kfree_rcu() to prevent argument-expansion eyestrain. */
> > +#define __kfree_rcu(head, offset) \
> > +	do { \
> > +		unsigned long __o = (unsigned long)offset; \
> > +		BUILD_BUG_ON(!__is_kfree_rcu_offset(__o)); \
> > +		kfree_call_rcu(head, (rcu_callback_t)(__o)); \
> > +	} while (0)
> > +
> > +/**
> > + * kfree_rcu() - kfree an object after a grace period.
> > + * @ptr:	pointer to kfree
> > + * @rcu_head:	the name of the struct rcu_head within the type of @ptr.
> 
> So you already rename this parameter to 'rcu_head_name', and...
> 
> > + *
> > + * Many rcu callbacks functions just call kfree() on the base structure.
> > + * These functions are trivial, but their size adds up, and furthermore
> > + * when they are used in a kernel module, that module must invoke the
> > + * high-latency rcu_barrier() function at module-unload time.
> > + *
> > + * The kfree_rcu() function handles this issue.  Rather than encoding a
> > + * function address in the embedded rcu_head structure, kfree_rcu() instead
> > + * encodes the offset of the rcu_head structure within the base structure.
> > + * Because the functions are not allowed in the low-order 4096 bytes of
> > + * kernel virtual memory, offsets up to 4095 bytes can be accommodated.
> > + * If the offset is larger than 4095 bytes, a compile-time error will
> > + * be generated in __kfree_rcu().  If this error is triggered, you can
> > + * either fall back to use of call_rcu() or rearrange the structure to
> > + * position the rcu_head structure into the first 4096 bytes.
> > + *
> > + * Note that the allowable offset might decrease in the future, for example,
> > + * to allow something like kmem_cache_free_rcu().
> > + *
> > + * The BUILD_BUG_ON check must not involve any function calls, hence the
> > + * checks are done in macros here.
> > + */
> > +#define kfree_rcu(ptr, rcu_head_name)	\
> > +	do { \
> > +		typeof(ptr) __ptr = ptr;	\
> > +		unsigned long __off = offsetof(typeof(*(__ptr)), \
> > +						      rcu_head_name); \
> > +		struct rcu_head *__rptr = (void *)__ptr + __off; \
> > +		__kfree_rcu(__rptr, __off); \
> > +	} while (0)
> 
> why do you want to open code this?

He needs it to be a macro so that offsetof() will work properly.

> >  /*
> >   * Bulk allocation and freeing operations. These are accelerated in an
> >   * allocator specific way to avoid taking locks repeatedly or building
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index f9c0ca2..7d2830f 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -3180,6 +3180,16 @@ void call_rcu_sched(struct rcu_head *head, rcu_callback_t func)
> >  }
> >  EXPORT_SYMBOL_GPL(call_rcu_sched);
> >  
> > +/* Queue an RCU callback for lazy invocation after a grace period.
> > + * Currently there is no way of tagging the lazy RCU callbacks in the
> > + * list of pending callbacks. Until then, this function may only be
> > + * called from kfree_call_rcu().
> > + */
> > +void call_rcu_lazy(struct rcu_head *head, rcu_callback_t func)
> > +{
> > +	__call_rcu(head, func, &rcu_sched_state, -1, 1);
> 
> Why do you switch this from rcu_state_p to rcu_sched_state? Have you
> checked your changes don't break PREEMPT=y kernel? Did I miss something
> subtle?

Good catch, this would be a problem on PREEMPT=y kernels.  I agree with
Boqun's implicit suggestion of using rcu_state_p.

							Thanx, Paul

> Regards,
> Boqun
> 
> > +}
> > +
> >  /**
> >   * call_rcu_bh() - Queue an RCU for invocation after a quicker grace period.
> >   * @head: structure to be used for queueing the RCU updates.
> > @@ -3209,20 +3219,6 @@ void call_rcu_bh(struct rcu_head *head, rcu_callback_t func)
> >  EXPORT_SYMBOL_GPL(call_rcu_bh);
> >  
> >  /*
> > - * Queue an RCU callback for lazy invocation after a grace period.
> > - * This will likely be later named something like "call_rcu_lazy()",
> > - * but this change will require some way of tagging the lazy RCU
> > - * callbacks in the list of pending callbacks. Until then, this
> > - * function may only be called from __kfree_rcu().
> > - */
> > -void kfree_call_rcu(struct rcu_head *head,
> > -		    rcu_callback_t func)
> > -{
> > -	__call_rcu(head, func, rcu_state_p, -1, 1);
> > -}
> > -EXPORT_SYMBOL_GPL(kfree_call_rcu);
> > -
> > -/*
> >   * Because a context switch is a grace period for RCU-sched and RCU-bh,
> >   * any blocking grace-period wait automatically implies a grace period
> >   * if there is only one CPU online at any point time during execution
> > diff --git a/mm/slab_common.c b/mm/slab_common.c
> > index c8cb367..0d8a63b 100644
> > --- a/mm/slab_common.c
> > +++ b/mm/slab_common.c
> > @@ -1483,6 +1483,16 @@ void kzfree(const void *p)
> >  }
> >  EXPORT_SYMBOL(kzfree);
> >  
> > +/*
> > + * Queue Memory to be freed by RCU after a grace period.
> > + */
> > +void kfree_call_rcu(struct rcu_head *head,
> > +		    rcu_callback_t func)
> > +{
> > +	call_rcu_lazy(head, func);
> > +}
> > +EXPORT_SYMBOL_GPL(kfree_call_rcu);
> > +
> >  /* Tracepoints definitions. */
> >  EXPORT_TRACEPOINT_SYMBOL(kmalloc);
> >  EXPORT_TRACEPOINT_SYMBOL(kmem_cache_alloc);
> > -- 
> > 2.7.4
> > 
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> 

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