From 4a557a5d1a6145ea586dc9b17a9b4e5190c9c017 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Thu, 30 Jun 2022 09:34:10 -0700 Subject: [PATCH] sparse: introduce conditional lock acquire function attribute The kernel tends to try to avoid conditional locking semantics because it makes it harder to think about and statically check locking rules, but we do have a few fundamental locking primitives that take locks conditionally - most obviously the 'trylock' functions. That has always been a problem for 'sparse' checking for locking imbalance, and we've had a special '__cond_lock()' macro that we've used to let sparse know how the locking works: # define __cond_lock(x,c) ((c) ? ({ __acquire(x); 1; }) : 0) so that you can then use this to tell sparse that (for example) the spinlock trylock macro ends up acquiring the lock when it succeeds, but not when it fails: #define raw_spin_trylock(lock) __cond_lock(lock, _raw_spin_trylock(lock)) and then sparse can follow along the locking rules when you have code like if (!spin_trylock(&dentry->d_lock)) return LRU_SKIP; .. sparse sees that the lock is held here.. spin_unlock(&dentry->d_lock); and sparse ends up happy about the lock contexts. However, this '__cond_lock()' use does result in very ugly header files, and requires you to basically wrap the real function with that macro that uses '__cond_lock'. Which has made PeterZ NAK things that try to fix sparse warnings over the years [1]. To solve this, there is now a very experimental patch to sparse that basically does the exact same thing as '__cond_lock()' did, but using a function attribute instead. That seems to make PeterZ happy [2]. Note that this does not replace existing use of '__cond_lock()', but only exposes the new proposed attribute and uses it for the previously unannotated 'refcount_dec_and_lock()' family of functions. For existing sparse installations, this will make no difference (a negative output context was ignored), but if you have the experimental sparse patch it will make sparse now understand code that uses those functions, the same way '__cond_lock()' makes sparse understand the very similar 'atomic_dec_and_lock()' uses that have the old '__cond_lock()' annotations. Note that in some cases this will silence existing context imbalance warnings. But in other cases it may end up exposing new sparse warnings for code that sparse just didn't see the locking for at all before. This is a trial, in other words. I'd expect that if it ends up being successful, and new sparse releases end up having this new attribute, we'll migrate the old-style '__cond_lock()' users to use the new-style '__cond_acquires' function attribute. The actual experimental sparse patch was posted in [3]. Link: https://lore.kernel.org/all/20130930134434.GC12926@twins.programming.kicks-ass.net/ [1] Link: https://lore.kernel.org/all/Yr60tWxN4P568x3W@worktop.programming.kicks-ass.net/ [2] Link: https://lore.kernel.org/all/CAHk-=wjZfO9hGqJ2_hGQG3U_XzSh9_XaXze=HgPdvJbgrvASfA@mail.gmail.com/ [3] Acked-by: Peter Zijlstra Cc: Alexander Aring Cc: Luc Van Oostenryck Signed-off-by: Linus Torvalds --- include/linux/compiler_types.h | 2 ++ include/linux/refcount.h | 6 +++--- 2 files changed, 5 insertions(+), 3 deletions(-) diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h index d08dfcb0ac68..4f2a819fd60a 100644 --- a/include/linux/compiler_types.h +++ b/include/linux/compiler_types.h @@ -24,6 +24,7 @@ static inline void __chk_io_ptr(const volatile void __iomem *ptr) { } /* context/locking */ # define __must_hold(x) __attribute__((context(x,1,1))) # define __acquires(x) __attribute__((context(x,0,1))) +# define __cond_acquires(x) __attribute__((context(x,0,-1))) # define __releases(x) __attribute__((context(x,1,0))) # define __acquire(x) __context__(x,1) # define __release(x) __context__(x,-1) @@ -50,6 +51,7 @@ static inline void __chk_io_ptr(const volatile void __iomem *ptr) { } /* context/locking */ # define __must_hold(x) # define __acquires(x) +# define __cond_acquires(x) # define __releases(x) # define __acquire(x) (void)0 # define __release(x) (void)0 diff --git a/include/linux/refcount.h b/include/linux/refcount.h index b8a6e387f8f9..a62fcca97486 100644 --- a/include/linux/refcount.h +++ b/include/linux/refcount.h @@ -361,9 +361,9 @@ static inline void refcount_dec(refcount_t *r) extern __must_check bool refcount_dec_if_one(refcount_t *r); extern __must_check bool refcount_dec_not_one(refcount_t *r); -extern __must_check bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock); -extern __must_check bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock); +extern __must_check bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock) __cond_acquires(lock); +extern __must_check bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock) __cond_acquires(lock); extern __must_check bool refcount_dec_and_lock_irqsave(refcount_t *r, spinlock_t *lock, - unsigned long *flags); + unsigned long *flags) __cond_acquires(lock); #endif /* _LINUX_REFCOUNT_H */