sx_downgrade (9)

Leading comments

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

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
1. Redistributions of source code must retain the above copyright
   notice(s), this list of conditions and the following disclaimer as
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2. Redistributions in binary ...

(The comments found at the beginning of the groff file "man9/sx_downgrade.9freebsd".)

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 - kernel shared/exclusive lock

SYNOPSIS

In sys/param.h In sys/lock.h In sys/sx.h Ft void Fn sx_init struct sx *sx const char *description Ft void Fn sx_init_flags struct sx *sx const char *description int opts 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_slock_sig struct sx *sx Ft int Fn sx_xlock_sig 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 void Fn sx_unlock struct sx *sx Ft int Fn sx_try_upgrade struct sx *sx Ft void Fn sx_downgrade struct sx *sx Ft int Fn sx_sleep void *chan struct sx *sx int priority const char *wmesg int timo Ft struct thread * Fn sx_xholder struct sx *sx Ft int Fn sx_xlocked const struct sx *sx

options INVARIANTS options INVARIANT_SUPPORT Ft void Fn sx_assert const struct sx *sx int what In sys/kernel.h Fn SX_SYSINIT name struct sx *sx const char *description

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 Fn sx_init or Fn sx_init_flags 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. The Fa opts argument to Fn sx_init_flags specifies a set of optional flags to alter the behavior of Fa sx . It contains one or more of the following flags:

SX_NOADAPTIVE

If the kernel is not compiled with options NO_ADAPTIVE_SX then lock operations for Fa sx will spin instead of sleeping while an exclusive lock holder is executing on another CPU.

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 Fa sx .

SX_QUIET

Do not log any operations for this lock via ktr(4).

Shared/exclusive locks are destroyed with Fn sx_destroy . The lock Fa sx must not be locked by any thread when it is destroyed.

Threads acquire and release a shared lock by calling Fn sx_slock , Fn sx_slock_sig or Fn sx_try_slock and Fn sx_sunlock or Fn sx_unlock . Threads acquire and release an exclusive lock by calling Fn sx_xlock , Fn sx_xlock_sig or Fn sx_try_xlock and Fn sx_xunlock or Fn sx_unlock . A thread can attempt to upgrade a currently held shared lock to an exclusive lock by calling Fn sx_try_upgrade . A thread that has an exclusive lock can downgrade it to a shared lock by calling Fn sx_downgrade .

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.

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.

Fn sx_slock_sig and Fn 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 Fn sx_sleep . For more details on the parameters to this function, see sleep(9).

When compiled with options INVARIANTS and options INVARIANT_SUPPORT the Fn sx_assert function tests Fa sx for the assertions specified in Fa 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 Vt sx lock pointed to by the first argument.

SA_SLOCKED

Assert that the current thread has a shared lock on the Vt sx lock pointed to by the first argument.

SA_XLOCKED

Assert that the current thread has an exclusive lock on the Vt sx lock pointed to by the first argument.

SA_UNLOCKED

Assert that the current thread has no lock on the Vt 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 Fa sx .

SA_NOTRECURSED

Assert that the current thread does not have a recursed lock on Fa sx .

Fn sx_xholder will return a pointer to the thread which currently holds an exclusive lock on Fa sx . If no thread holds an exclusive lock on Fa sx , then NULL is returned instead.

Fn sx_xlocked will return non-zero if the current thread holds the exclusive lock; otherwise, it will return zero.

For ease of programming, Fn sx_unlock is provided as a macro frontend to the respective functions, Fn sx_sunlock and Fn 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 Fn SX_SYSINIT macro is used to generate a call to the Fn sx_sysinit routine at system startup in order to initialize a given Fa sx lock. The parameters are the same as Fn sx_init but with an additional argument, Fa name , that is used in generating unique variable names for the related structures associated with the lock and the sysinit routine.

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 ifconfig lock while sleeping. As a result, an ifconfig lock may not be acquired while holding a mutex. Otherwise, if one thread slept while holding an ifconfig lock while another thread blocked on the same ifconfig lock after acquiring a mutex, then the second thread would effectively end up sleeping while holding a mutex, which is not allowed.

SEE ALSO

locking(9), lock(9), mutex(9), panic(9), rwlock(9), sema(9)

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 SA_LOCKED and SA_SLOCKED assertions merely check that some thread holds a shared lock. They do not ensure that the current thread holds a shared lock.