allow but NOT DEMAND this. If one thread has acquired the shared lock , other thread can acquire the shared lock too. but only CAN. in some case system not let another thread enter to lock, despite it also request share access. one case: if another rthread request exclusive acess - he begin wait and after this - any thread which acquire even shared access to lock - also begin wait
If lock_shared is called by a thread that already owns the mutex in any mode (exclusive or shared), the behavior is undefined.
and
Shared mode SRW locks should not be acquired recursively as this can lead to deadlocks when combined with exclusive acquisition.
why is this ? because if between 2 calls to lock_shared ( AcquireSRWLockShared ) another thread call AcquireSRWLockExclusive - the second call is block.
the code in example make assumption that ALL threads can enter to lock at once. that if one thread enter to lock in shared mode, another thread also ALWAYS can enter to lock in shared mode (if no exclusive requests). but i not view clear formalization of such requirement. and we must not based on this.
i be will add next rule:
thread inside lock must not wait on another thread to enter this lock
this is obvivius for exlusive access, but not obvivous to shared. but must be cleare stated along with the recursive rule ( should not be acquired recursively as this can lead to deadlocks, even in shared mode)