SX(9) FreeBSD Kernel Developer's Manual SX(9)
NAME
sx, sx_init, sx_init_flags, sx_destroy, sx_slock, sx_xlock, sx_slock_sig,
sx_xlock_sig, sx_try_slock, sx_try_xlock, sx_sunlock, sx_xunlock,
sx_unlock, sx_try_upgrade, sx_downgrade, sx_sleep, sx_xholder,
sx_xlocked, sx_assert, SX_SYSINIT, SX_SYSINIT_FLAGS - kernel
shared/exclusive lock
SYNOPSIS
#include <sys/param.h>
#include <sys/lock.h>
#include <sys/sx.h>
void
sx_init(struct sx *sx, const char *description);
void
sx_init_flags(struct sx *sx, const char *description, int opts);
void
sx_destroy(struct sx *sx);
void
sx_slock(struct sx *sx);
void
sx_xlock(struct sx *sx);
int
sx_slock_sig(struct sx *sx);
int
sx_xlock_sig(struct sx *sx);
int
sx_try_slock(struct sx *sx);
int
sx_try_xlock(struct sx *sx);
void
sx_sunlock(struct sx *sx);
void
sx_xunlock(struct sx *sx);
void
sx_unlock(struct sx *sx);
int
sx_try_upgrade(struct sx *sx);
void
sx_downgrade(struct sx *sx);
int
sx_sleep(void *chan, struct sx *sx, int priority, const char *wmesg,
int timo);
struct thread *
sx_xholder(struct sx *sx);
int
sx_xlocked(const struct sx *sx);
options INVARIANTS
options INVARIANT_SUPPORT
void
sx_assert(const struct sx *sx, int what);
#include <sys/kernel.h>
SX_SYSINIT(name, struct sx *sx, const char *desc);
SX_SYSINIT_FLAGS(name, struct sx *sx, const char *desc, int flags);
DESCRIPTION
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 sx_init() or
sx_init_flags() 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:
SX_DUPOK Witness should not log messages about duplicate locks being
acquired.
SX_NOWITNESS Instruct witness(4) to ignore this lock.
SX_NOPROFILE Do not profile this lock.
SX_RECURSE Allow threads to recursively acquire exclusive locks for
sx.
SX_QUIET Do not log any operations for this lock via ktr(4).
SX_NEW If the kernel has been compiled with options INVARIANTS,
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 sx_destroy(). The lock sx must
not be locked by any thread when it is destroyed.
Threads acquire and release a shared lock by calling sx_slock(),
sx_slock_sig() or sx_try_slock() and sx_sunlock() or sx_unlock().
Threads acquire and release an exclusive lock by calling sx_xlock(),
sx_xlock_sig() or sx_try_xlock() and sx_xunlock() or sx_unlock(). 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
sx_downgrade().
sx_try_slock() and sx_try_xlock() 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.
sx_try_upgrade() 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.
sx_slock_sig() and sx_xlock_sig() 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 sx_sleep(). For more details on the parameters to
this function, see sleep(9).
When compiled with options INVARIANTS and options INVARIANT_SUPPORT, the
sx_assert() function tests sx for the assertions specified in what, and
panics if they are not met. One of the following assertions must be
specified:
SA_LOCKED Assert that the current thread has either a shared or an
exclusive lock on the sx lock pointed to by the first
argument.
SA_SLOCKED Assert that the current thread has a shared lock on the
sx lock pointed to by the first argument.
SA_XLOCKED Assert that the current thread has an exclusive lock on
the sx lock pointed to by the first argument.
SA_UNLOCKED 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 SA_LOCKED, SA_SLOCKED, or SA_XLOCKED assertion:
SA_RECURSED Assert that the current thread has a recursed lock on
sx.
SA_NOTRECURSED Assert that the current thread does not have a
recursed lock on sx.
sx_xholder() will return a pointer to the thread which currently holds an
exclusive lock on sx. If no thread holds an exclusive lock on sx, then
NULL is returned instead.
sx_xlocked() will return non-zero if the current thread holds the
exclusive lock; otherwise, it will return zero.
For ease of programming, sx_unlock() is provided as a macro frontend to
the respective functions, sx_sunlock() and sx_xunlock(). Algorithms that
are aware of what state the lock is in should use either of the two
specific functions for a minor performance benefit.
The SX_SYSINIT() macro is used to generate a call to the sx_sysinit()
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 routine. The
SX_SYSINIT_FLAGS() macro can similarly be used to initialize a given sx
lock using sx_init_flags().
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.
CONTEXT
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 sx lock while
another thread blocked on the same sx lock after acquiring a mutex, then
the second thread would effectively end up sleeping while holding a
mutex, which is not allowed.
SEE ALSO
lock(9), locking(9), mutex(9), panic(9), rwlock(9), sema(9)
BUGS
A kernel without WITNESS cannot assert whether the current thread does or
does not hold a shared lock. SA_LOCKED and SA_SLOCKED can only assert
that any thread 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.
FreeBSD 13.1-RELEASE-p6 November 11, 2017 FreeBSD 13.1-RELEASE-p6
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