|SX(9)||Kernel Developer's Manual||SX(9)|
— kernel shared/exclusive lock
sx *sx, const char
sx *sx, const char
*chan, struct sx
*sx, int priority,
const char *wmesg,
struct thread *
struct sx *sx);
struct sx *sx, int
struct sx *sx,
Shared/exclusive locks are used to protect data that are read far more often than they are written. Shared/exclusive locks do not implement priority propagation like mutexes and reader/writer locks to prevent priority inversions, so shared/exclusive locks should be used prudently.
Shared/exclusive locks are created with either
where sx is a pointer to space for a
struct sx, and description is a
pointer to a null-terminated character string that describes the
shared/exclusive lock. The opts argument to
sx_init_flags() specifies a set of optional flags to
alter the behavior of sx. It contains one or more of
the following flags:
- Disable adaptive spinning, rather than sleeping, for lock operations while
an exclusive lock holder is executing on another CPU. Adaptive spinning is
the default unless the kernel is compiled with
- Witness should not log messages about duplicate locks being acquired.
- Instruct witness(4) to ignore this lock.
- Do not profile this lock.
- Allow threads to recursively acquire exclusive locks for sx.
- Do not log any operations for this lock via ktr(4).
- If the kernel has been compiled with
sx_init() will assert that the sx has not been initialized multiple times without intervening calls to
sx_destroy() unless this option is specified.
Shared/exclusive locks are destroyed with
The lock sx must not be locked by any thread when it
Threads acquire and release a shared lock by calling
Threads acquire and release an exclusive lock by calling
A thread can attempt to upgrade a currently held shared lock to an exclusive
lock by calling
sx_try_upgrade(). A thread that has
an exclusive lock can downgrade it to a shared lock by calling
will return 0 if the shared/exclusive lock cannot be acquired immediately;
otherwise the shared/exclusive lock will be acquired and a non-zero value
will be returned.
will return 0 if the shared lock cannot be upgraded to an exclusive lock
immediately; otherwise the exclusive lock will be acquired and a non-zero
value will be returned.
do the same as their normal versions but performing an interruptible sleep.
They return a non-zero value if the sleep has been interrupted by a signal
or an interrupt, otherwise 0.
A thread can atomically release a shared/exclusive
lock while waiting for an event by calling
For more details on the parameters to this function, see
When compiled with
options INVARIANT_SUPPORT, the
function tests sx for the assertions specified in
what, and panics if they are not met. One of the
following assertions must be specified:
- Assert that the current thread has either a shared or an exclusive lock on the sx lock pointed to by the first argument.
- Assert that the current thread has a shared lock on the sx lock pointed to by the first argument.
- Assert that the current thread has an exclusive lock on the sx lock pointed to by the first argument.
- Assert that the current thread has no lock on the sx lock pointed to by the first argument.
In addition, one of the following optional assertions may be
included with either an
- Assert that the current thread has a recursed lock on sx.
- Assert that the current thread does not have a recursed lock on sx.
will return a pointer to the thread which currently holds an exclusive lock
on sx. If no thread holds an exclusive lock on
NULL is returned
will return non-zero if the current thread holds the exclusive lock;
otherwise, it will return zero.
For ease of programming,
is provided as a macro frontend to the respective functions,
Algorithms that are aware of what state the lock is in should use either of
the two specific functions for a minor performance benefit.
macro is used to generate a call to the
routine at system startup in order to initialize a given
sx lock. The parameters are the same as
sx_init() but with an additional argument,
name, that is used in generating unique variable names
for the related structures associated with the lock and the sysinit
A thread may not hold both a shared lock and an exclusive lock on the same lock simultaneously; attempting to do so will result in deadlock.
A thread may hold a shared or exclusive lock on an
sx lock while sleeping. As a result, an
sx lock may not be acquired while holding a mutex.
Otherwise, if one thread slept while holding an
lock while another thread blocked on the same
lock after acquiring a mutex, then the second thread would effectively end
up sleeping while holding a mutex, which is not allowed.
A kernel without
WITNESS cannot assert
whether the current thread does or does not hold a shared lock.
only assert that
holds a shared lock. They cannot ensure that the current thread holds a
shared lock. Further,
SA_UNLOCKED can only assert
that the current thread does not hold an exclusive lock.
|March 28, 2016||Debian|