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AP_Param: fixed race in param count handling

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This commit is contained in:
Andrew Tridgell 2020-04-19 09:24:54 +10:00
parent 65e790a48c
commit db0d3c5e89
2 changed files with 30 additions and 7 deletions
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32
libraries/AP_Param/AP_Param.cpp
View File

@ -73,6 +73,9 @@ uint16_t AP_Param::_num_vars;
// cached parameter count // cached parameter count
uint16_t AP_Param::_parameter_count; uint16_t AP_Param::_parameter_count;
uint16_t AP_Param::_count_marker;
uint16_t AP_Param::_count_marker_done;
HAL_Semaphore AP_Param::_count_sem;
// storage and naming information about all types that can be saved // storage and naming information about all types that can be saved
const AP_Param::Info *AP_Param::_var_info; const AP_Param::Info *AP_Param::_var_info;
@ -2325,19 +2328,33 @@ void AP_Param::send_parameter(const char *name, enum ap_var_type var_type, uint8
uint16_t AP_Param::count_parameters(void) uint16_t AP_Param::count_parameters(void)
{ {
// if we haven't cached the parameter count yet... // if we haven't cached the parameter count yet...
uint16_t ret = _parameter_count; WITH_SEMAPHORE(_count_sem);
if (0 == ret) { if (_parameter_count != 0 &&
_count_marker == _count_marker_done) {
return _parameter_count;
}
/*
cope with another thread invalidating the count while we are
counting
*/
uint8_t limit = 4;
while ((_parameter_count == 0 ||
_count_marker != _count_marker_done) &&
limit--) {
AP_Param *vp; AP_Param *vp;
AP_Param::ParamToken token; AP_Param::ParamToken token;
uint16_t count = 0;
uint16_t marker = _count_marker;
for (vp = AP_Param::first(&token, nullptr); for (vp = AP_Param::first(&token, nullptr);
vp != nullptr; vp != nullptr;
vp = AP_Param::next_scalar(&token, nullptr)) { vp = AP_Param::next_scalar(&token, nullptr)) {
ret++; count++;
} }
_parameter_count = ret; _parameter_count = count;
_count_marker_done = marker;
} }
return ret; return _parameter_count;
} }
/* /*
@ -2345,7 +2362,10 @@ uint16_t AP_Param::count_parameters(void)
*/ */
void AP_Param::invalidate_count(void) void AP_Param::invalidate_count(void)
{ {
_parameter_count = 0; // we don't take the semaphore here as we don't want to block. The
// not-equal test is strong enough to ensure we get the right
// answer
_count_marker++;
} }
/* /*

5
libraries/AP_Param/AP_Param.h
View File

@ -485,7 +485,7 @@ public:
// set frame type flags. Used to unhide frame specific parameters // set frame type flags. Used to unhide frame specific parameters
static void set_frame_type_flags(uint16_t flags_to_set) { static void set_frame_type_flags(uint16_t flags_to_set) {
_parameter_count = 0; invalidate_count();
_frame_type_flags |= flags_to_set; _frame_type_flags |= flags_to_set;
} }
@ -674,6 +674,9 @@ private:
static StorageAccess _storage; static StorageAccess _storage;
static uint16_t _num_vars; static uint16_t _num_vars;
static uint16_t _parameter_count; static uint16_t _parameter_count;
static uint16_t _count_marker;
static uint16_t _count_marker_done;
static HAL_Semaphore _count_sem;
static const struct Info * _var_info; static const struct Info * _var_info;
/* /*