|Main index||Section 9||Options|
lockmgr_assert(const struct lock *lkp, int what);
|A pointer to the lock to initialize.|
|The priority passed to sleep(9).|
|The lock message. This is used for both debugging output and sleep(9).|
|The timeout value passed to sleep(9).|
|The flags the lock is to be initialized with:|
|LK_ADAPTIVE||Enable adaptive spinning for this lock if the kernel is compiled with the ADAPTIVE_LOCKMGRS option.|
|Allow recursive exclusive locks.|
|LK_NOPROFILE||Disable lock profiling for this lock.|
|LK_NOSHARE||Allow exclusive locks only.|
|LK_NOWITNESS||Instruct witness(4) to ignore this lock.|
|LK_NODUP||witness(4) should log messages about duplicate locks being acquired.|
|LK_QUIET||Disable ktr(4) logging for this lock.|
|LK_TIMELOCK||Use timo during a sleep; otherwise, 0 is used.|
The lockdestroy() function is used to destroy a lock, and while it is called in a number of places in the kernel, it currently does nothing.
The lockmgr() and lockmgr_rw() functions handle general locking functionality within the kernel, including support for shared and exclusive locks, and recursion. lockmgr() and lockmgr_rw() are also able to upgrade and downgrade locks.
Their arguments are:
|A pointer to the lock to manipulate.|
|Flags indicating what action is to be taken.|
|LK_SHARED||Acquire a shared lock. If an exclusive lock is currently held, EDEADLK will be returned.|
|LK_EXCLUSIVE||Acquire an exclusive lock. If an exclusive lock is already held, and LK_CANRECURSE is not set, the system will panic(9).|
|LK_DOWNGRADE||Downgrade exclusive lock to a shared lock. Downgrading a shared lock is not permitted. If an exclusive lock has been recursed, the system will panic(9).|
|LK_UPGRADE||Upgrade a shared lock to an exclusive lock. If this call fails, the shared lock is lost, even if the LK_NOWAIT flag is specified. During the upgrade, the shared lock could be temporarily dropped. Attempts to upgrade an exclusive lock will cause a panic(9).|
|LK_TRYUPGRADE||Try to upgrade a shared lock to an exclusive lock. The failure to upgrade does not result in the dropping of the shared lock ownership.|
|LK_RELEASE||Release the lock. Releasing a lock that is not held can cause a panic(9).|
|LK_DRAIN||Wait for all activity on the lock to end, then mark it decommissioned. This is used before freeing a lock that is part of a piece of memory that is about to be freed. (As documented in <sys/lockmgr.h>)|
|LK_SLEEPFAIL||Fail if operation has slept.|
|LK_NOWAIT||Do not allow the call to sleep. This can be used to test the lock.|
|LK_NOWITNESS||Skip the witness(4) checks for this instance.|
|LK_CANRECURSE||Allow recursion on an exclusive lock. For every lock there must be a release.|
|LK_INTERLOCK||Unlock the interlock (which should be locked already).|
postpones serving further shared requests for shared-locked lock if there is
exclusive waiter, to avoid exclusive lock starvation.
But, if the thread requesting the shared lock already owns a shared lockmgr
lock, the request is granted even in presence of the parallel exclusive lock
request, which is done to avoid deadlocks with recursive shared acquisition.
The LK_NODDLKTREAT flag can only be used by code which requests shared non-recursive lock. The flag allows exclusive requests to preempt the current shared request even if the current thread owns shared locks. This is safe since shared lock is guaranteed to not recurse, and is used when thread is known to held unrelated shared locks, to not cause unnecessary starvation. An example is vp locking in VFS lookup(9), when dvp is already locked.
|An interlock mutex for controlling group access to the lock. If LK_INTERLOCK is specified, lockmgr() and lockmgr_rw() assume ilk is currently owned and not recursed, and will return it unlocked. See mtx_assert(9).|
The lockmgr_args() and lockmgr_args_rw() function work like lockmgr() and lockmgr_rw() but accepting a wmesg, timo and prio on a per-instance basis. The specified values will override the default ones, but this can still be used passing, respectively, LK_WMESG_DEFAULT, LK_PRIO_DEFAULT and LK_TIMO_DEFAULT.
The lockmgr_disown() function switches the owner from the current thread to be LK_KERNPROC, if the lock is already held.
The lockmgr_printinfo() function prints debugging information about the lock. It is used primarily by VOP_PRINT(9) functions.
The lockmgr_recursed() function returns true if the lock is recursed, 0 otherwise.
The lockmgr_waiters() function returns true if the lock has waiters, 0 otherwise.
The lockstatus() function returns the status of the lock in relation to the current thread.
When compiled with options INVARIANTS and options INVARIANT_SUPPORT, the lockmgr_assert() function tests lkp for the assertions specified in what, and panics if they are not met. One of the following assertions must be specified:
|KA_LOCKED||Assert that the current thread has either a shared or an exclusive lock on the lkp lock pointed to by the first argument.|
|KA_SLOCKED||Assert that the current thread has a shared lock on the lkp lock pointed to by the first argument.|
|KA_XLOCKED||Assert that the current thread has an exclusive lock on the lkp lock pointed to by the first argument.|
|Assert that the current thread has no lock on the lkp lock pointed to by the first argument.|
In addition, one of the following optional assertions can be used with either an KA_LOCKED, KA_SLOCKED, or KA_XLOCKED assertion:
|KA_RECURSED||Assert that the current thread has a recursed lock on lkp.|
|Assert that the current thread does not have a recursed lock on lkp.|
The lockstatus() function returns:
|An exclusive lock is held by the current thread.|
|An exclusive lock is held by someone other than the current thread.|
|LK_SHARED||A shared lock is held.|
|0||The lock is not held by anyone.|
|LK_FORCEUPGRADE was requested and another thread had already requested a lock upgrade.|
|LK_NOWAIT was set, and a sleep would have been required, or LK_TRYUPGRADE operation was not able to upgrade the lock.|
|LK_SLEEPFAIL was set and lockmgr() or lockmgr_rw() did sleep.|
|PCATCH was set in the lock priority, and a signal was delivered during a sleep. Note the ERESTART error below.|
|PCATCH was set in the lock priority, a signal was delivered during a sleep, and the system call is to be restarted.|
|a non-zero timeout was given, and the timeout expired.|
Upgrade attempts that fail result in the loss of the lock that is currently held. Also, it is invalid to upgrade an exclusive lock, and a panic(9) will be the result of trying.
|LOCK (9)||November 2, 2014|
|Main index||Section 9||Options|
|“||Ken Thompson has an automobile which he helped design. Unlike most automobiles, it has neither speedometer, nor gas gauge, nor any of the other numerous idiot lights which plague the modern driver. Rather, if the driver makes a mistake, a giant “?” lights up in the center of the dashboard. “The experienced driver,” says Thompson, “will usually know what's wrong.”||”|