Re: Kernel Recursive mutexes 10.4.9 ++

On Jul 18, 2007, at 8:54 PM, Erez Kaplan wrote:

On Jul 18, 2007, at 10:00 PM, Terry Lambert wrote:

On Jul 18, 2007, at 2:37 AM, Erez Kaplan wrote:

Hi,

10.4.9+ 10.5

How do I obtain a Kernel Recursive mutexe?

The available api does not support this.

lck_mtx_alloc_init(gmutex_grp_m, LCK_ATTR_NULL)

You don't; this is what reference and use counts are for.

Can you explain a bit more or give reference to an example.

For example, all thread reentrant FSs will take an I/O reference on a vnode, and use that reference to prevent the FS from being unmounted  out from under an operation in progress, rather than holding a lock over the complete operation (thus unnecessarily serializing all other I/O to the vnode).  Here is a simplified design pattern:

some function(object)
lock mutex
atomic increment reference count
unlock mutex

... long duration operation ...

lock mutex
atomic decrement reference count
unlock mutex

/* wake up destroy routine; destroy routine may fail acquisition race */
if !reference count
wakeup(object)

...

destroy(object)
lock mutex
again:
/* Don't destroy the object while someone is using it */
if reference count
msleep(mutex, object)
goto again /* avoid race */

/* last reference gone, OK to kill it */
destroy object
unlock mutex

-- Terry