Re: [PATCH] rcu: replace _________p1 with __UNIQUE_ID(rcu)

["Paul E. McKenney" <paulmck@kernel.org>]

On Mon, Sep 13, 2021 at 04:04:30PM -0700, Paul E. McKenney wrote:
> On Mon, Sep 13, 2021 at 04:41:31AM +0800, Chun-Hung Tseng wrote:
> > This commit replaced _________p1 with __UNIQUE_ID(rcu), which
> > generates unique variable names during compilation. Necessary
> > modifications due to the changes in the RCU macros have also been
> > reflected in this commit.
> > 
> > The same idea is used for the min/max macros (commit 589a978 and commit
> > e9092d0), which aims to reduce variable shadowing issues caused by hard
> > coded variable names.
> > 
> > Signed-off-by: Jim Huang <jserv@ccns.ncku.edu.tw>
> > Signed-off-by: Chun-Hung Tseng <henrybear327@gmail.com>
> 
> OK, I will bite...
> 
> > ---
> >  include/linux/rcupdate.h | 44 +++++++++++++++++++++++-----------------
> >  include/linux/srcu.h     |  3 ++-
> >  2 files changed, 27 insertions(+), 20 deletions(-)
> > 
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index 434d12fe2d4f..a5ab20822040 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -370,39 +370,41 @@ static inline void rcu_preempt_sleep_check(void) { }
> >   * Converts @p from an __rcu pointer to a __kernel pointer.
> >   * This allows an __rcu pointer to be used with xchg() and friends.
> >   */
> > -#define unrcu_pointer(p)						\
> > +#define __unrcu_pointer(p, local)					\
> >  ({									\
> > -	typeof(*p) *_________p1 = (typeof(*p) *__force)(p);		\
> > +	typeof(*p) *local = (typeof(*p) *__force)(p);			\
> 
> Why not like this?
> 
> 	typeof(*p) *__UNIQUE_ID(rcu) = (typeof(*p) *__force)(p);	\
> 
> Then we don't need the extra argument and the changes to the calls.
> 
> So what C-preprocessor subtlety am I missing?

Never mind!!!  My suggested approach would generate a unique name at
every use, except on non-gcc/non-clang compilers, which would obviously
not do what we want.

							Thanx, Paul

> >  	rcu_check_sparse(p, __rcu);					\
> > -	((typeof(*p) __force __kernel *)(_________p1)); 		\
> > +	((typeof(*p) __force __kernel *)(local)); 			\
> >  })
> > +#define unrcu_pointer(p) __unrcu_pointer(p, __UNIQUE_ID(rcu))
> >  
> > -#define __rcu_access_pointer(p, space) \
> > +#define __rcu_access_pointer(p, local, space) \
> >  ({ \
> > -	typeof(*p) *_________p1 = (typeof(*p) *__force)READ_ONCE(p); \
> > +	typeof(*p) *local = (typeof(*p) *__force)READ_ONCE(p); \
> >  	rcu_check_sparse(p, space); \
> > -	((typeof(*p) __force __kernel *)(_________p1)); \
> > +	((typeof(*p) __force __kernel *)(local)); \
> >  })
> > -#define __rcu_dereference_check(p, c, space) \
> > +#define __rcu_dereference_check(p, local, c, space) \
> >  ({ \
> >  	/* Dependency order vs. p above. */ \
> > -	typeof(*p) *________p1 = (typeof(*p) *__force)READ_ONCE(p); \
> > +	typeof(*p) *local = (typeof(*p) *__force)READ_ONCE(p); \
> >  	RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_check() usage"); \
> >  	rcu_check_sparse(p, space); \
> > -	((typeof(*p) __force __kernel *)(________p1)); \
> > +	((typeof(*p) __force __kernel *)(local)); \
> >  })
> > -#define __rcu_dereference_protected(p, c, space) \
> > +#define __rcu_dereference_protected(p, local, c, space) \
> >  ({ \
> >  	RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_protected() usage"); \
> >  	rcu_check_sparse(p, space); \
> >  	((typeof(*p) __force __kernel *)(p)); \
> >  })
> > -#define rcu_dereference_raw(p) \
> > +#define __rcu_dereference_raw(p, local) \
> >  ({ \
> >  	/* Dependency order vs. p above. */ \
> > -	typeof(p) ________p1 = READ_ONCE(p); \
> > -	((typeof(*p) __force __kernel *)(________p1)); \
> > +	typeof(p) local = READ_ONCE(p); \
> > +	((typeof(*p) __force __kernel *)(local)); \
> >  })
> > +#define rcu_dereference_raw(p) __rcu_dereference_raw(p, __UNIQUE_ID(rcu))
> >  
> >  /**
> >   * RCU_INITIALIZER() - statically initialize an RCU-protected global variable
> > @@ -489,7 +491,7 @@ do {									      \
> >   * when tearing down multi-linked structures after a grace period
> >   * has elapsed.
> >   */
> > -#define rcu_access_pointer(p) __rcu_access_pointer((p), __rcu)
> > +#define rcu_access_pointer(p) __rcu_access_pointer((p), __UNIQUE_ID(rcu), __rcu)
> >  
> >  /**
> >   * rcu_dereference_check() - rcu_dereference with debug checking
> > @@ -525,7 +527,8 @@ do {									      \
> >   * annotated as __rcu.
> >   */
> >  #define rcu_dereference_check(p, c) \
> > -	__rcu_dereference_check((p), (c) || rcu_read_lock_held(), __rcu)
> > +	__rcu_dereference_check((p), __UNIQUE_ID(rcu), \
> > +				(c) || rcu_read_lock_held(), __rcu)
> >  
> >  /**
> >   * rcu_dereference_bh_check() - rcu_dereference_bh with debug checking
> > @@ -540,7 +543,8 @@ do {									      \
> >   * rcu_read_lock() but also rcu_read_lock_bh() into account.
> >   */
> >  #define rcu_dereference_bh_check(p, c) \
> > -	__rcu_dereference_check((p), (c) || rcu_read_lock_bh_held(), __rcu)
> > +	__rcu_dereference_check((p), __UNIQUE_ID(rcu), \
> > +				(c) || rcu_read_lock_bh_held(), __rcu)
> >  
> >  /**
> >   * rcu_dereference_sched_check() - rcu_dereference_sched with debug checking
> > @@ -555,7 +559,8 @@ do {									      \
> >   * only rcu_read_lock() but also rcu_read_lock_sched() into account.
> >   */
> >  #define rcu_dereference_sched_check(p, c) \
> > -	__rcu_dereference_check((p), (c) || rcu_read_lock_sched_held(), \
> > +	__rcu_dereference_check((p), __UNIQUE_ID(rcu), \
> > +				(c) || rcu_read_lock_sched_held(), \
> >  				__rcu)
> >  
> >  /*
> > @@ -565,7 +570,8 @@ do {									      \
> >   * The no-tracing version of rcu_dereference_raw() must not call
> >   * rcu_read_lock_held().
> >   */
> > -#define rcu_dereference_raw_check(p) __rcu_dereference_check((p), 1, __rcu)
> > +#define rcu_dereference_raw_check(p) \
> > +	__rcu_dereference_check((p), __UNIQUE_ID(rcu), 1, __rcu)
> >  
> >  /**
> >   * rcu_dereference_protected() - fetch RCU pointer when updates prevented
> > @@ -584,7 +590,7 @@ do {									      \
> >   * but very ugly failures.
> >   */
> >  #define rcu_dereference_protected(p, c) \
> > -	__rcu_dereference_protected((p), (c), __rcu)
> > +	__rcu_dereference_protected((p), __UNIQUE_ID(rcu), (c), __rcu)
> >  
> >  
> >  /**
> > diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> > index e6011a9975af..01226e4d960a 100644
> > --- a/include/linux/srcu.h
> > +++ b/include/linux/srcu.h
> > @@ -117,7 +117,8 @@ static inline int srcu_read_lock_held(const struct srcu_struct *ssp)
> >   * lockdep_is_held() calls.
> >   */
> >  #define srcu_dereference_check(p, ssp, c) \
> > -	__rcu_dereference_check((p), (c) || srcu_read_lock_held(ssp), __rcu)
> > +	__rcu_dereference_check((p), __UNIQUE_ID(rcu), \
> > +				(c) || srcu_read_lock_held(ssp), __rcu)
> >  
> >  /**
> >   * srcu_dereference - fetch SRCU-protected pointer for later dereferencing
> > -- 
> > 2.25.1
> >