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Mutex::unlock - decrement _count inside lock 

`Mutex::unlock` was decrementing the `_count` member after releasing
the mutex, which meant it was unprotected, exposing a race that
could corrupt the count.

This could lead to an assert in `ConditionVariable::wait`, which
checks that the mutex count is one.
pull/12983/head
Kevin Bracey 2020-05-15 19:10:53 +03:00
parent b53dc6695b
commit b52a3b3120
1 changed files with 5 additions and 3 deletions
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8
rtos/source/Mutex.cpp
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@ -123,10 +123,12 @@ bool Mutex::trylock_until(Kernel::Clock::time_point abs_time)
void Mutex::unlock() void Mutex::unlock()
{ {
osStatus status = osMutexRelease(_id); // Count must be adjusted inside the lock. This would leave it incorrect
if (osOK == status) { // on failure, but it only is used for an assert in ConditionVariable,
// and a mutex release failure means MBED_ERROR anyway.
_count--; _count--;
}
osStatus status = osMutexRelease(_id);
if (status != osOK && !mbed_get_error_in_progress()) { if (status != osOK && !mbed_get_error_in_progress()) {
MBED_ERROR1(MBED_MAKE_ERROR(MBED_MODULE_KERNEL, MBED_ERROR_CODE_MUTEX_UNLOCK_FAILED), "Mutex unlock failed", status); MBED_ERROR1(MBED_MAKE_ERROR(MBED_MODULE_KERNEL, MBED_ERROR_CODE_MUTEX_UNLOCK_FAILED), "Mutex unlock failed", status);