@ -28,12 +28,6 @@
Usage is the same as C + + thread_local . Declaring a thread local :
static GPR_THREAD_LOCAL ( uint32_t ) foo ;
Initializing a thread local ( must be done at library initialization time ) :
gpr_tls_init ( foo ) ;
Destroying a thread local :
gpr_tls_destroy ( foo ) ;
ALL functions here may be implemented as macros . */
namespace grpc_core {
@ -67,13 +61,43 @@ namespace grpc_core {
template < typename T >
class PthreadTlsImpl : TlsTypeConstrainer < T > {
public :
PthreadTlsImpl ( ) = default ;
PthreadTlsImpl ( const PthreadTlsImpl & ) = delete ;
PthreadTlsImpl & operator = ( const PthreadTlsImpl & ) = delete ;
void Init ( ) { GPR_ASSERT ( 0 = = pthread_key_create ( & key_ , nullptr ) ) ; }
void Destroy ( ) { GPR_ASSERT ( 0 = = pthread_key_delete ( key_ ) ) ; }
// Achtung! This class emulates C++ `thread_local` using pthread keys. Each
// instance of this class is a stand in for a C++ `thread_local`. Think of
// each `thread_local` as a *global* pthread_key_t and a type tag. An
// important consequence of this is that the lifetime of a `pthread_key_t`
// is precisely the lifetime of an instance of this class. To understand why
// this is, consider the following scenario given a fictional implementation
// of this class which creates and destroys its `pthread_key_t` each time
// a given block of code runs (all actions take place on a single thread):
//
// - instance 1 (type tag = T*) is initialized, is assigned `pthread_key_t` 1
// - instance 2 (type tag = int) is initialized, is assigned `pthread_key_t` 2
// - instances 1 and 2 store and retrieve values; all is well
// - instances 1 and 2 are de-initialized; their keys are released to the pool
//
// - another run commences
// - instance 1 receives key 2
// - a value is read from instance 1, it observes a value of type int, but
// interprets it as T*; undefined behavior, kaboom
//
// To properly ensure these invariants are upheld the `pthread_key_t` must be
// `const`, which means it can only be released in the destructor. This is a
// a violation of the style guide, since these objects are always static (see
// footnote) but this code is used in sufficiently narrow circumstances to
// justify the deviation.
//
// https://google.github.io/styleguide/cppguide.html#Static_and_Global_Variables
PthreadTlsImpl ( )
: key_ ( [ ] ( ) {
pthread_key_t key ;
GPR_ASSERT ( 0 = = pthread_key_create ( & key , nullptr ) ) ;
return key ;
} ( ) ) { }
PthreadTlsImpl ( T t ) : PthreadTlsImpl ( ) { * this = t ; }
~ PthreadTlsImpl ( ) { GPR_ASSERT ( 0 = = pthread_key_delete ( key_ ) ) ; }
operator T ( ) const { return Cast < T > ( pthread_getspecific ( key_ ) ) ; }
@ -110,46 +134,18 @@ class PthreadTlsImpl : TlsTypeConstrainer<T> {
return reinterpret_cast < void * > ( uintptr_t ( t ) ) ;
}
pthread_key_t key_ ;
} ;
// This class is never instantiated. It exists to statically ensure that all
// TLS usage is compatible with the most restrictive implementation, allowing
// developers to write correct code regardless of the platform they develop on.
template < typename T >
class TriviallyDestructibleAsserter {
// This type is often used as a global; since the type itself is hidden by the
// macros, enforce compliance with the style guide here rather than at the
// caller. See
// https://google.github.io/styleguide/cppguide.html#Static_and_Global_Variables.
static_assert ( std : : is_trivially_destructible < T > : : value ,
" TLS wrapper must be trivially destructible " ) ;
public :
using Type = T ;
const pthread_key_t key_ ;
} ;
} // namespace grpc_core
# define GPR_THREAD_LOCAL(type) \
grpc_core : : TriviallyDestructibleAsserter < \
grpc_core : : PthreadTlsImpl < type > > : : Type
# define gpr_tls_init(tls) (tls).Init()
# define gpr_tls_destroy(tls) (tls).Destroy()
# define GPR_THREAD_LOCAL(type) grpc_core::PthreadTlsImpl<type>
# else
# define GPR_THREAD_LOCAL(type) \
thread_local grpc_core : : TlsTypeConstrainer < type > : : Type
# define gpr_tls_init(tls) \
do { \
} while ( 0 )
# define gpr_tls_destroy(tls) \
do { \
} while ( 0 )
# endif
# endif /* GRPC_CORE_LIB_GPR_TLS_H */
Tamir Duberstein
4 years ago
committed by
GitHub