The following code is placed at the beginning of H5private.h:
#ifdef H5_HAVE_THREADSAFE #include <pthread.h> #endif
H5_HAVE_THREADSAFE is defined when the HDF5 library is
compiled with the --enable-threadsafe configuration option. In general,
code for the non-threadsafe version of HDF5 library are placed within
the #else part of the conditional compilation. The exception
to this rule are the changes to the FUNC_ENTER (in
H5private.h), HRETURN and HRETURN_ERROR (in
H5Eprivate.h) macros (see section 3.2).
In the threadsafe implementation, the global library initialization
variable H5_libinit_g is changed to a global structure
consisting of the variable with its associated lock (locks are explained
in section 4.1):
hbool_t H5_libinit_g = FALSE;
becomes
H5_api_t H5_g;
where H5_api_t is
typedef struct H5_api_struct {
H5_mutex_t init_lock; /* API entrance mutex */
hbool_t H5_libinit_g;
} H5_api_t;
All former references to H5_libinit_g in the library are now
made using the macro H5_INIT_GLOBAL. If the threadsafe
library is to be used, the macro is set to H5_g.H5_libinit_g
instead.
A new global boolean variable H5_allow_concurrent_g is used
to determine if multiple threads are allowed to an API call
simultaneously. This is set to FALSE.
All APIs that are allowed to do so have their own local variable that
shadows the global variable and is set to TRUE. In phase 1,
no such APIs exist.
It is defined in H5.c as follows:
hbool_t H5_allow_concurrent_g = FALSE;
The global variable H5_first_init_g of type
pthread_once_t is used to allow only the first thread in the
application process to call an initialization function using
pthread_once. All subsequent calls to
pthread_once by any thread are disregarded.
The call sets up the mutex in the global structure H5_g (see
section 3.1) via an initialization function
H5_first_thread_init. The first thread initialization
function is described in section 4.2.
H5_first_init_g is defined in H5.c as follows:
pthread_once_t H5_first_init_g = PTHREAD_ONCE_INIT;
A global pthread-managed key H5_errstk_key_g is used to
allow pthreads to maintain a separate error stack (of type
H5E_t) for each thread. This is defined in H5.c
as:
pthread_key_t H5_errstk_key_g;
Error stack management is described in section 4.3.
We need to preserve the thread cancellation status of each thread
individually by using a key H5_cancel_key_g. The status is
preserved using a structure (of type H5_cancel_t) which
maintains the cancellability state of the thread before it entered the
library and a count (which works very much like the recursive lock
counter) which keeps track of the number of API calls the thread makes
within the library.
The structure is defined in H5private.h as:
/* cancellability structure */
typedef struct H5_cancel_struct {
int previous_state;
unsigned int cancel_count;
} H5_cancel_t;
Thread cancellation is described in section 4.4.
The FUNC_ENTER macro is now extended to include macro calls
to initialize first threads, disable cancellability and wraps a lock
operation around the checking of the global initialization flag. It
should be noted that the cancellability should be disabled before
acquiring the lock on the library. Doing so otherwise would allow the
possibility that the thread be cancelled just after it has acquired the
lock on the library and in that scenario, if the cleanup routines are not
properly set, the library would be permanently locked out.
The additional macro code and new macro definitions can be found in
Appendix E.1 to E.5. The changes are made in H5private.h.
The HRETURN and HRETURN_ERROR macros are the
counterparts to the FUNC_ENTER macro described in section
3.1. FUNC_LEAVE makes a macro call to HRETURN,
so it is also covered here.
The basic changes to these two macros involve adding macro calls to call an unlock operation and re-enable cancellability if necessary. It should be noted that the cancellability should be re-enabled only after the thread has released the lock to the library. The consequence of doing otherwise would be similar to that described in section 3.1.
The additional macro code and new macro definitions can be found in
Appendix E.9 to E.9. The changes are made in H5Eprivate.h.
A recursive mutex lock m allows a thread t1 to successfully lock m more than once without blocking t1. Another thread t2 will block if t2 tries to lock m while t1 holds the lock to m. If t1 makes k lock calls on m, then it also needs to make k unlock calls on m before it releases the lock.
Our implementation of recursive locks is built on top of a pthread mutex lock (which is not recursive). It makes use of a pthread condition variable to have unsuccessful threads wait on the mutex. Waiting threads are awaken by a signal from the final unlock call made by the thread holding the lock.
Recursive locks are defined to be the following type
(H5private.h):
typedef struct H5_mutex_struct {
pthread_t owner_thread; /* current lock owner */
pthread_mutex_t atomic_lock; /* lock for atomicity of new mechanism */
pthread_cond_t cond_var; /* condition variable */
unsigned int lock_count;
} H5_mutex_t;
Detailed implementation code can be found in Appendix A. The
implementation changes are made in H5TS.c.
Because the mutex lock associated with a recursive lock cannot be
statically initialized, a mechanism is required to initialize the
recursive lock associated with H5_g so that it can be used
for the first time.
The pthreads library allows this through the pthread_once call which as
described in section 3.3 allows only the first thread accessing the
library in an application to initialize H5_g.
In addition to initializing H5_g, it also initializes the
key (see section 3.4) for use with per-thread error stacks (see section
4.3).
The first thread initialization mechanism is implemented as the function
call H5_first_thread_init() in H5TS.c. This is
described in appendix B.
Pthreads allows individual threads to access dynamic and persistent
per-thread data through the use of keys. Each key is associated with
a table that maps threads to data items. Keys can be initialized by
pthread_key_create() in pthreads (see sections 3.4 and 4.2).
Per-thread data items are accessed using a key through the
pthread_getspecific() and pthread_setspecific()
calls to read and write to the association table respectively.
Per-thread error stacks are accessed through the key
H5_errstk_key_g which is initialized by the first thread
initialization call (see section 4.2).
In the non-threadsafe version of the library, there is a global stack
variable H5E_stack_g[1] which is no longer defined in the
threadsafe version. At the same time, the macro call to gain access to
the error stack H5E_get_my_stack is changed from:
#define H5E_get_my_stack() (H5E_stack_g+0)
to:
#define H5E_get_my_stack() H5E_get_stack()
where H5E_get_stack() is a surrogate function that does the
following operations:
H5_errstk_key_g using
pthread_setspecific(). The way we detect if it is the
first time is through pthread_getspecific() which
returns NULL if no previous value is associated with
the thread using the key.pthread_getspecific() returns a non-null value,
then that is the pointer to the error stack associated with the
thread and the stack can be used as usual.
A final change to the error reporting routines is as follows; the current
implementation reports errors to always be detected at thread 0. In the
threadsafe implementation, this is changed to report the number returned
by a call to pthread_self().
The change in code (reflected in H5Eprint of file
H5E.c) is as follows:
#ifdef H5_HAVE_THREADSAFE
fprintf (stream, "HDF5-DIAG: Error detected in thread %d."
,pthread_self());
#else
fprintf (stream, "HDF5-DIAG: Error detected in thread 0.");
#endif
Code for H5E_get_stack() can be found in Appendix C. All the
above changes were made in H5E.c.
To prevent thread cancellations from killing a thread while it is in the library, we maintain per-thread information about the cancellability status of the thread before it entered the library so that we can restore that same status when the thread leaves the library.
By enter and leave the library, we mean the points when a thread makes an API call from a user application and the time that API call returns. Other API or callback function calls made from within that API call are considered within the library.
Because other API calls may be made from within the first API call, we need to maintain a counter to determine which was the first and correspondingly the last return.
When a thread makes an API call, the macro H5_API_SET_CANCEL
calls the worker function H5_cancel_count_inc() which does
the following:
PTHREAD_CANCEL_DISABLE
while storing the previous state into the cancellability structure.
cancel_count is also incremented in this case.
When a thread leaves an API call, the macro
H5_API_UNSET_CANCEL calls the worker function
H5_cancel_count_dec() which does the following:
cancel_count is greater than 1, indicating that the
thread is not yet about to leave the library, then
cancel_count is simply decremented.
H5_cancel_count_inc and H5_cancel_count_dec are
described in Appendix D and may be found in H5TS.c.
Except where stated, all tests involve 16 simultaneous threads that make use of HDF5 API calls without any explicit synchronization typically required in a non-threadsafe environment.
The test program sets up 16 threads to simultaneously create 16 different datasets named from zero to fifteen for a single file and then writing an integer value into that dataset equal to the dataset's named value.
The main thread would join with all 16 threads and attempt to match the resulting HDF5 file with expected results - that each dataset contains the correct value (0 for zero, 1 for one etc ...) and all datasets were correctly created.
The test is implemented in the file ttsafe_dcreate.c.
The error stack test is one in which 16 threads simultaneously try to create datasets with the same name. The result, when properly serialized, should be equivalent to 16 attempts to create the dataset with the same name.
The error stack implementation runs correctly if it reports 15 instances of the dataset name conflict error and finally generates a correct HDF5 containing that single dataset. Each thread should report its own stack of errors with a thread number associated with it.
The test is implemented in the file ttsafe_error.c.
The main idea in thread cancellation safety is as follows; a child thread
is spawned to create and write to a dataset. Following that, it makes a
H5Diterate call on that dataset which activates a callback
function.
A deliberate barrier is invoked at the callback function which waits for both the main and child thread to arrive at that point. After that happens, the main thread proceeds to make a thread cancel call on the child thread while the latter sleeps for 3 seconds before proceeding to write a new value to the dataset.
After the iterate call, the child thread logically proceeds to wait another 3 seconds before writing another newer value to the dataset.
The test is correct if the main thread manages to read the second value at the end of the test. This means that cancellation did not take place until the end of the iteration call despite of the 3 second wait within the iteration callback and the extra dataset write operation. Furthermore, the cancellation should occur before the child can proceed to write the last value into the dataset.
A main thread makes 16 threaded calls to H5Acreate with a
generated name for each attribute. Sixteen attributes should be created
for the single dataset in random (chronological) order and receive values
depending on its generated attribute name (e.g. attrib010 would
receive the value 10).
After joining with all child threads, the main thread proceeds to read each attribute by generated name to see if the value tallies. Failure is detected if the attribute name does not exist (meaning they were never created) or if the wrong values were read back.
void H5_mutex_init(H5_mutex_t *H5_mutex)
{
H5_mutex->owner_thread = NULL;
pthread_mutex_init(&H5_mutex->atomic_lock, NULL);
pthread_cond_init(&H5_mutex->cond_var, NULL);
H5_mutex->lock_count = 0;
}
void H5_mutex_lock(H5_mutex_t *H5_mutex)
{
pthread_mutex_lock(&H5_mutex->atomic_lock);
if (pthread_equal(pthread_self(), H5_mutex->owner_thread)) {
/* already owned by self - increment count */
H5_mutex->lock_count++;
} else {
if (H5_mutex->owner_thread == NULL) {
/* no one else has locked it - set owner and grab lock */
H5_mutex->owner_thread = pthread_self();
H5_mutex->lock_count = 1;
} else {
/* if already locked by someone else */
while (1) {
pthread_cond_wait(&H5_mutex->cond_var, &H5_mutex->atomic_lock);
if (H5_mutex->owner_thread == NULL) {
H5_mutex->owner_thread = pthread_self();
H5_mutex->lock_count = 1;
break;
} /* else do nothing and loop back to wait on condition*/
}
}
}
pthread_mutex_unlock(&H5_mutex->atomic_lock);
}
void H5_mutex_unlock(H5_mutex_t *H5_mutex)
{
pthread_mutex_lock(&H5_mutex->atomic_lock);
H5_mutex->lock_count--;
if (H5_mutex->lock_count == 0) {
H5_mutex->owner_thread = NULL;
pthread_cond_signal(&H5_mutex->cond_var);
}
pthread_mutex_unlock(&H5_mutex->atomic_lock);
}
void H5_first_thread_init(void)
{
/* initialize global API mutex lock */
H5_g.H5_libinit_g = FALSE;
H5_g.init_lock.owner_thread = NULL;
pthread_mutex_init(&H5_g.init_lock.atomic_lock, NULL);
pthread_cond_init(&H5_g.init_lock.cond_var, NULL);
H5_g.init_lock.lock_count = 0;
/* initialize key for thread-specific error stacks */
pthread_key_create(&H5_errstk_key_g, NULL);
/* initialize key for thread cancellability mechanism */
pthread_key_create(&H5_cancel_key_g, NULL);
}
H5E_t *H5E_get_stack(void)
{
H5E_t *estack;
if (estack = pthread_getspecific(H5_errstk_key_g)) {
return estack;
} else {
/* no associated value with current thread - create one */
estack = (H5E_t *)malloc(sizeof(H5E_t));
pthread_setspecific(H5_errstk_key_g, (void *)estack);
return estack;
}
}
void H5_cancel_count_inc(void)
{
H5_cancel_t *cancel_counter;
if (cancel_counter = pthread_getspecific(H5_cancel_key_g)) {
/* do nothing here */
} else {
/*
* first time thread calls library - create new counter and
* associate with key
*/
cancel_counter = (H5_cancel_t *)malloc(sizeof(H5_cancel_t));
cancel_counter->cancel_count = 0;
pthread_setspecific(H5_cancel_key_g, (void *)cancel_counter);
}
if (cancel_counter->cancel_count == 0) {
/* thread entering library */
pthread_setcancelstate(PTHREAD_CANCEL_DISABLE,
&(cancel_counter->previous_state));
}
cancel_counter->cancel_count++;
}
void H5_cancel_count_dec(void)
{
H5_cancel_t *cancel_counter = pthread_getspecific(H5_cancel_key_g);
if (cancel_counter->cancel_count == 1)
pthread_setcancelstate(cancel_counter->previous_state, NULL);
cancel_counter->cancel_count--;
}
FUNC_ENTER
/* Initialize the library */ \
H5_FIRST_THREAD_INIT \
H5_API_UNSET_CANCEL \
H5_API_LOCK_BEGIN \
if (!(H5_INIT_GLOBAL)) { \
H5_INIT_GLOBAL = TRUE; \
if (H5_init_library() < 0) { \
HRETURN_ERROR (H5E_FUNC, H5E_CANTINIT, err, \
"library initialization failed"); \
} \
} \
H5_API_LOCK_END \
:
:
:
H5_FIRST_THREAD_INIT
/* Macro for first thread initialization */
#define H5_FIRST_THREAD_INIT \
pthread_once(&H5_first_init_g, H5_first_thread_init);
H5_API_UNSET_CANCEL
#define H5_API_UNSET_CANCEL \
if (H5_IS_API(__func__)) { \
H5_cancel_count_inc(); \
}
H5_API_LOCK_BEGIN
#define H5_API_LOCK_BEGIN \
if (H5_IS_API(__func__)) { \
H5_mutex_lock(&H5_g.init_lock);
H5_API_LOCK_END#define H5_API_LOCK_END }
HRETURN and HRETURN_ERROR
:
:
H5_API_UNLOCK_BEGIN \
H5_API_UNLOCK_END \
H5_API_SET_CANCEL \
return ret_val; \
}
H5_API_UNLOCK_BEGIN
#define H5_API_UNLOCK_BEGIN \
if (H5_IS_API(__func__)) { \
H5_mutex_unlock(&H5_g.init_lock);
H5_API_UNLOCK_END#define H5_API_UNLOCK_END }
H5_API_SET_CANCEL
#define H5_API_SET_CANCEL \
if (H5_IS_API(__func__)) { \
H5_cancel_count_dec(); \
}