diff options
Diffstat (limited to 'src/os_unix.c')
| -rw-r--r-- | src/os_unix.c | 18 |
1 files changed, 15 insertions, 3 deletions
diff --git a/src/os_unix.c b/src/os_unix.c index 602c02386..e33f0f7c5 100644 --- a/src/os_unix.c +++ b/src/os_unix.c @@ -1675,22 +1675,34 @@ int sqlite3UnixSleep(int ms){ ** inMutex the nesting depth of the recursive mutex. The thread ** holding mutexMain can read this variable at any time. ** But is must hold mutexAux to change this variable. Other -** threads must hold mutexAux to read the variable. +** threads must hold mutexAux to read the variable and can +** never write. ** ** mutexOwner The thread id of the thread holding mutexMain. Same ** access rules as for inMutex. ** -** mutexOwnerValid True if the value in mutexOwner is valid. +** mutexOwnerValid True if the value in mutexOwner is valid. The same +** access rules apply as for inMutex. ** ** mutexMain The main mutex. Hold this mutex in order to get exclusive ** access to SQLite data structures. ** ** mutexAux An auxiliary mutex needed to access variables defined above. ** +** Mutexes are always acquired in this order: mutexMain mutexAux. It +** is not necessary to acquire mutexMain in order to get mutexAux - just +** do not attempt to acquire them in the reverse order: mutexAux mutexMain. +** Either get the mutexes with mutexMain first or get mutexAux only. +** +** When running on a platform where the three variables inMutex, mutexOwner, +** and mutexOwnerValid can be set atomically, the mutexAux is not required. +** On many systems, all three are 32-bit integers and writing to a 32-bit +** integer is atomic. I think. But there are no guarantees. So it seems +** safer to protect them using mutexAux. */ static int inMutex = 0; #ifdef SQLITE_UNIX_THREADS -static pthread_t mutexOwner; /* Thread holding the mutex */ +static pthread_t mutexOwner; /* Thread holding mutexMain */ static int mutexOwnerValid = 0; /* True if mutexOwner is valid */ static pthread_mutex_t mutexMain = PTHREAD_MUTEX_INITIALIZER; /* The mutex */ static pthread_mutex_t mutexAux = PTHREAD_MUTEX_INITIALIZER; /* Aux mutex */ |