Is Parallel Programming Hard, And, If So, What Can You Do About It?

8.2. REFERENCE COUNTING 75ure. Otherwise, if the attempt is successful, lines 14-15 exit the read-side critical section and return apointer to the file structure.The fcheck_files() primitive is a helper functionfor fget(). It uses the rcu_dereference()primitive to safely fetch an RCU-protected pointerfor later dereferencing (this emits a memory barrieron CPUs such as DEC Alpha in which data dependenciesdo not enforce memory ordering). Line 22uses rcu_dereference() to fetch a pointer to thistask’scurrentfile-descriptortable, andline24checksto see if the specified file descriptor is in range. If so,line 25 fetches the pointer to the file structure, againusing the rcu_dereference() primitive. Line 26then returns a pointer to the file structure or NULLin case of failure.The fput() primitive releases a reference to a filestructure. Line 31 atomically decrements the referencecount,and, iftheresultwaszero,line32invokesthecall_rcu()primitives inordertofreeupthefilestructure (via the file_free_rcu() function specifiedin call_rcu()’s second argument), but onlyafter all currently-executing RCU read-side criticalsections complete. The time period required for allcurrently-executing RCU read-side critical sectionsto complete is termed a “grace period”. Note thatthe atomic_dec_and_test() primitive contains amemory barrier. This memory barrier is not necessaryin this example, since the structure cannotbe destroyed until the RCU read-side critical sectioncompletes, but in Linux, all atomic operations thatreturn a result must by definition contain memorybarriers.Once the grace period completes, the file_free_rcu() function obtains a pointer to the file structureon line 39, and frees it on line 40.This approach is also used by Linux’s virtualmemorysystem, see get_page_unless_zero() andput_page_testzero() for page structures as wellas try_to_unuse() and mmput() for memory-mapstructures.8.2.2 Linux Primitives SupportingReference CountingThe Linux-kernel primitives used in the above examplesare summarized in the following list.• atomic t Type definition for 32-bit quantityto be manipulated atomically.• void atomic dec(atomic t *var); Atomicallydecrements the referenced variable withoutnecessarily issuing a memory barrier or disablingcompiler optimizations.• int atomic dec and test(atomic t*var); Atomically decrements the referencedvariable, returning TRUE if the resultis zero. Issues a memory barrier and disablescompiler optimizations that might otherwisemove memory references across this primitive.• void atomic inc(atomic t *var); Atomicallyincrements the referenced variable withoutnecessarily issuing a memory barrier or disablingcompiler optimizations.• int atomic inc not zero(atomic t*var); Atomically increments the referencedvariable, but only if the value isnon-zero, and returning TRUE if the incrementoccurred. Issues a memory barrier and disablescompiler optimizations that might otherwisemove memory references across this primitive.• int atomic read(atomic t *var); Returnsthe integer value of the referenced variable.This is not an atomic operation, and it neitherissues memory barriers nor disables compileroptimizations.• void atomic set(atomic t *var, intval); Sets the value of the referencedatomic variable to “val”. This is not anatomic operation, and it neither issues memorybarriers nor disables compiler optimizations.• void call rcu(struct rcu head*head, void (*func)(struct rcu head*head)); Invokes func(head) some timeafter all currently executing RCU readsidecritical sections complete, however, thecall_rcu() primitive returns immediately.Note that head is normally a field withinan RCU-protected data structure, and thatfunc is normally a function that frees up thisdata structure. The time interval between theinvocation of call_rcu() and the invocation offunc is termed a “grace period”. Any intervalof time containing a grace period is itself agrace period.• type *container of(p, type, f); Given apointer “p” to a field “f” within a structure ofthe specified type, return a pointer to the structure.• void rcu read lock(void); Marks the beginningof an RCU read-side critical section.• void rcu read unlock(void); Marks theend of an RCU read-side critical section. RCUread-side critical sections may be nested.