Copyright (C) 2001 Jason Evans <jasone@FreeBSD.org>. All rights reserved.

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$FreeBSD: head/share/man/man9/sx.9 85392 2001-10-23 22:51:59Z jhb $

.Dd August 14, 2001 .Dt SX 9 .Os .Sh NAME .Nm sx , .Nm sx_init , .Nm sx_destroy , .Nm sx_slock , .Nm sx_xlock , .Nm sx_try_slock , .Nm sx_try_xlock , .Nm sx_sunlock , .Nm sx_xunlock , .Nm sx_try_upgrade , .Nm sx_downgrade , .Nm sx_assert .Nd kernel shared/exclusive lock .Sh SYNOPSIS n sys/param.h n sys/lock.h n sys/mutex.h n sys/sx.h .Ft void .Fn sx_init "struct sx *sx" "const char *description" .Ft void .Fn sx_destroy "struct sx *sx" .Ft void .Fn sx_slock "struct sx *sx" .Ft void .Fn sx_xlock "struct sx *sx" .Ft int .Fn sx_try_slock "struct sx *sx" .Ft int .Fn sx_try_xlock "struct sx *sx" .Ft void .Fn sx_sunlock "struct sx *sx" .Ft void .Fn sx_xunlock "struct sx *sx" .Ft int .Fn sx_try_upgrade "struct sx *sx" .Ft void .Fn sx_downgrade "struct sx *sx" .Ft void .Fn sx_assert "struct sx *sx" "int what" .Sh DESCRIPTION Shared/exclusive locks are used to protect data that are read far more often than they are written. Mutexes are inherently more efficient than shared/exclusive locks, so shared/exclusive locks should be used prudently.

p Shared/exclusive locks are created with .Fn sx_init , where .Fa sx is a pointer to space for a .Vt struct sx , and .Fa description is a pointer to a null-terminated character string that describes the shared/exclusive lock. Shared/exclusive locks are destroyed with .Fn sx_destroy . Threads acquire and release a shared lock by calling .Fn sx_slock or .Fn sx_try_slock and .Fn sx_sunlock . Threads acquire and release an exclusive lock by calling .Fn sx_xlock or .Fn sx_try_xlock and .Fn sx_xunlock . A thread can attempt to upgrade a currently owned shared lock to an exclusive lock by calling .Fn sx_try_upgrade . A thread that owns an exclusive lock can downgrade it to a shared lock by calling .Fn sx_downgrade .

p .Fn sx_try_slock and .Fn 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.

p .Fn 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.

p The .Fn sx_assert function tests specified conditions and panics if they are not met and the kernel is compiled with .Dv INVARIANTS . The following assertions are supported: l -tag -width SX_XLOCKED t Dv SX_LOCKED Assert that the current thread has either a shared or an exclusive lock on the sx lock pointed to by the first argument. t Dv SX_SLOCKED Assert that the current thread has a shared lock on the sx lock pointed to by the first argument. t Dv SX_XLOCKED Assert that the current thread has an exclusive lock on the sx lock pointed to by the first argument. .El

p A thread may not own a shared lock and an exclusive lock simultaneously; attempting to do so will result in deadlock. .Sh SEE ALSO .Xr condvar 9 , .Xr mutex 9 , .Xr sema 9 .Sh BUGS Currently there is no way to assert that a lock is not held. This is not possible in the non-WITNESS case for asserting that this thread does not hold a shared lock. In the non-WITNESS case, the .Dv SX_LOCKED and .Dv SX_SLOCKED assertions merely check that some thread holds a shared lock. They do not ensure that the current thread holds a shared lock.