when i link foo.c (see attached) with -lpthread and run it, i get the following incorrect ouput: --------------------------------------------------------------- thread 1 created thread 2 created main thread waiting ... thread 1 acquiring read lock thread 1 read lock acquisition succeeded thread 2 acquiring read lock thread 2 read lock acquisition succeeded thread 1 releasing read lock thread 1 read lock release succeeded thread 1 acquiring write lock thread 1 write lock acquisition succeeded /**** error ****/ thread 1 releasing write lock thread 1 write lock release succeeded thread 1 dying ... thread 2 releasing read lock thread 2 read lock release failed thread 2 acquiring write lock thread 2 write lock acquisition succeeded thread 2 releasing write lock thread 2 write lock release succeeded thread 2 dying ... --------------------------------------------------------------- at the marked point, thread 1 should have blocked because thread 2 still holds the read lock.
i think the reason for this is that when thread 1 acquires the read lock, it is the first reader, so it acquires rwlock->rw_mutex_rw. however, when it releases the read lock, it is not the last reader, so it did not release rwlock->rw_mutex_rw. so when it tries to acquire the write lock, it succeeds because the mutex is recursive and thread 1 still owns it. i changed the implementation to use a mutex and a conditional variable (see attached patch), and foo.c gives the following correct output. --------------------------------------------------------------- thread 1 created thread 2 created main thread waiting ... thread 1 acquiring read lock thread 1 read lock acquisition succeeded thread 2 acquiring read lock thread 2 read lock acquisition succeeded thread 1 releasing read lock thread 1 read lock release succeeded thread 1 acquiring write lock thread 2 releasing read lock thread 1 write lock acquisition succeeded thread 1 releasing write lock thread 1 write lock release succeeded thread 1 dying ... thread 2 read lock release succeeded thread 2 acquiring write lock thread 2 write lock acquisition succeeded thread 2 releasing write lock thread 2 write lock release succeeded thread 2 dying ... --------------------------------------------------------------- sincerely, xiaoguang li
pth_rwlock.diff
Description: pth_rwlock.diff
foo.c
Description: foo.c
