xref: /freebsd/share/man/man9/rwlock.9 (revision 148a8da8)
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25.\" $FreeBSD$
26.\"
27.Dd November 11, 2017
28.Dt RWLOCK 9
29.Os
30.Sh NAME
31.Nm rwlock ,
32.Nm rw_init ,
33.Nm rw_init_flags ,
34.Nm rw_destroy ,
35.Nm rw_rlock ,
36.Nm rw_wlock ,
37.Nm rw_runlock ,
38.Nm rw_wunlock ,
39.Nm rw_unlock ,
40.Nm rw_try_rlock ,
41.Nm rw_try_upgrade ,
42.Nm rw_try_wlock ,
43.Nm rw_downgrade ,
44.Nm rw_sleep ,
45.Nm rw_initialized ,
46.Nm rw_wowned ,
47.Nm rw_assert ,
48.Nm RW_SYSINIT ,
49.Nm RW_SYSINIT_FLAGS
50.Nd kernel reader/writer lock
51.Sh SYNOPSIS
52.In sys/param.h
53.In sys/lock.h
54.In sys/rwlock.h
55.Ft void
56.Fn rw_init "struct rwlock *rw" "const char *name"
57.Ft void
58.Fn rw_init_flags "struct rwlock *rw" "const char *name" "int opts"
59.Ft void
60.Fn rw_destroy "struct rwlock *rw"
61.Ft void
62.Fn rw_rlock "struct rwlock *rw"
63.Ft void
64.Fn rw_wlock "struct rwlock *rw"
65.Ft int
66.Fn rw_try_rlock "struct rwlock *rw"
67.Ft int
68.Fn rw_try_wlock "struct rwlock *rw"
69.Ft void
70.Fn rw_runlock "struct rwlock *rw"
71.Ft void
72.Fn rw_wunlock "struct rwlock *rw"
73.Ft void
74.Fn rw_unlock "struct rwlock *rw"
75.Ft int
76.Fn rw_try_upgrade "struct rwlock *rw"
77.Ft void
78.Fn rw_downgrade "struct rwlock *rw"
79.Ft int
80.Fn rw_sleep "void *chan" "struct rwlock *rw" "int priority" "const char *wmesg" "int timo"
81.Ft int
82.Fn rw_initialized "const struct rwlock *rw"
83.Ft int
84.Fn rw_wowned "const struct rwlock *rw"
85.Pp
86.Cd "options INVARIANTS"
87.Cd "options INVARIANT_SUPPORT"
88.Ft void
89.Fn rw_assert "const struct rwlock *rw" "int what"
90.In sys/kernel.h
91.Fn RW_SYSINIT "name" "struct rwlock *rw" "const char *desc"
92.Fn RW_SYSINIT_FLAGS "name" "struct rwlock *rw" "const char *desc" "int flags"
93.Sh DESCRIPTION
94Reader/writer locks allow shared access to protected data by multiple threads,
95or exclusive access by a single thread.
96The threads with shared access are known as
97.Em readers
98since they only read the protected data.
99A thread with exclusive access is known as a
100.Em writer
101since it can modify protected data.
102.Pp
103Although reader/writer locks look very similar to
104.Xr sx 9
105locks, their usage pattern is different.
106Reader/writer locks can be treated as mutexes (see
107.Xr mutex 9 )
108with shared/exclusive semantics.
109Unlike
110.Xr sx 9 ,
111an
112.Nm
113can be locked while holding a non-spin mutex, and an
114.Nm
115cannot be held while sleeping.
116The
117.Nm
118locks have priority propagation like mutexes, but priority
119can be propagated only to writers.
120This limitation comes from the fact that readers
121are anonymous.
122Another important property is that readers can always recurse,
123and exclusive locks can be made recursive selectively.
124.Ss Macros and Functions
125.Bl -tag -width indent
126.It Fn rw_init "struct rwlock *rw" "const char *name"
127Initialize structure located at
128.Fa rw
129as reader/writer lock, described by name
130.Fa name .
131The description is used solely for debugging purposes.
132This function must be called before any other operations
133on the lock.
134.It Fn rw_init_flags "struct rwlock *rw" "const char *name" "int opts"
135Initialize the rw lock just like the
136.Fn rw_init
137function, but specifying a set of optional flags to alter the
138behaviour of
139.Fa rw ,
140through the
141.Fa opts
142argument.
143It contains one or more of the following flags:
144.Bl -tag -width ".Dv RW_NOPROFILE"
145.It Dv RW_DUPOK
146Witness should not log messages about duplicate locks being acquired.
147.It Dv RW_NOPROFILE
148Do not profile this lock.
149.It Dv RW_NOWITNESS
150Instruct
151.Xr witness 4
152to ignore this lock.
153.It Dv RW_QUIET
154Do not log any operations for this lock via
155.Xr ktr 4 .
156.It Dv RW_RECURSE
157Allow threads to recursively acquire exclusive locks for
158.Fa rw .
159.It Dv RW_NEW
160If the kernel has been compiled with
161.Cd "option INVARIANTS" ,
162.Fn rw_init_flags
163will assert that the
164.Fa rw
165has not been initialized multiple times without intervening calls to
166.Fn rw_destroy
167unless this option is specified.
168.El
169.It Fn rw_rlock "struct rwlock *rw"
170Lock
171.Fa rw
172as a reader.
173If any thread holds this lock exclusively, the current thread blocks,
174and its priority is propagated to the exclusive holder.
175The
176.Fn rw_rlock
177function can be called when the thread has already acquired reader
178access on
179.Fa rw .
180This is called
181.Dq "recursing on a lock" .
182.It Fn rw_wlock "struct rwlock *rw"
183Lock
184.Fa rw
185as a writer.
186If there are any shared owners of the lock, the current thread blocks.
187The
188.Fn rw_wlock
189function can be called recursively only if
190.Fa rw
191has been initialized with the
192.Dv RW_RECURSE
193option enabled.
194.It Fn rw_try_rlock "struct rwlock *rw"
195Try to lock
196.Fa rw
197as a reader.
198This function will return true if the operation succeeds, otherwise 0
199will be returned.
200.It Fn rw_try_wlock "struct rwlock *rw"
201Try to lock
202.Fa rw
203as a writer.
204This function will return true if the operation succeeds, otherwise 0
205will be returned.
206.It Fn rw_runlock "struct rwlock *rw"
207This function releases a shared lock previously acquired by
208.Fn rw_rlock .
209.It Fn rw_wunlock "struct rwlock *rw"
210This function releases an exclusive lock previously acquired by
211.Fn rw_wlock .
212.It Fn rw_unlock "struct rwlock *rw"
213This function releases a shared lock previously acquired by
214.Fn rw_rlock
215or an exclusive lock previously acquired by
216.Fn rw_wlock .
217.It Fn rw_try_upgrade "struct rwlock *rw"
218Attempt to upgrade a single shared lock to an exclusive lock.
219The current thread must hold a shared lock of
220.Fa rw .
221This will only succeed if the current thread holds the only shared lock on
222.Fa rw ,
223and it only holds a single shared lock.
224If the attempt succeeds
225.Fn rw_try_upgrade
226will return a non-zero value,
227and the current thread will hold an exclusive lock.
228If the attempt fails
229.Fn rw_try_upgrade
230will return zero,
231and the current thread will still hold a shared lock.
232.It Fn rw_downgrade "struct rwlock *rw"
233Convert an exclusive lock into a single shared lock.
234The current thread must hold an exclusive lock of
235.Fa rw .
236.It Fn rw_sleep "void *chan" "struct rwlock *rw" "int priority" "const char *wmesg" "int timo"
237Atomically release
238.Fa rw
239while waiting for an event.
240For more details on the parameters to this function,
241see
242.Xr sleep 9 .
243.It Fn rw_initialized "const struct rwlock *rw"
244This function returns non-zero if
245.Fa rw
246has been initialized, and zero otherwise.
247.It Fn rw_destroy "struct rwlock *rw"
248This functions destroys a lock previously initialized with
249.Fn rw_init .
250The
251.Fa rw
252lock must be unlocked.
253.It Fn rw_wowned "const struct rwlock *rw"
254This function returns a non-zero value if the current thread owns an
255exclusive lock on
256.Fa rw .
257.It Fn rw_assert "const struct rwlock *rw" "int what"
258This function allows assertions specified in
259.Fa what
260to be made about
261.Fa rw .
262If the assertions are not true and the kernel is compiled
263with
264.Cd "options INVARIANTS"
265and
266.Cd "options INVARIANT_SUPPORT" ,
267the kernel will panic.
268Currently the following base assertions are supported:
269.Bl -tag -width ".Dv RA_UNLOCKED"
270.It Dv RA_LOCKED
271Assert that current thread holds either a shared or exclusive lock
272of
273.Fa rw .
274.It Dv RA_RLOCKED
275Assert that current thread holds a shared lock of
276.Fa rw .
277.It Dv RA_WLOCKED
278Assert that current thread holds an exclusive lock of
279.Fa rw .
280.It Dv RA_UNLOCKED
281Assert that current thread holds neither a shared nor exclusive lock of
282.Fa rw .
283.El
284.Pp
285In addition, one of the following optional flags may be specified with
286.Dv RA_LOCKED ,
287.Dv RA_RLOCKED ,
288or
289.Dv RA_WLOCKED :
290.Bl -tag -width ".Dv RA_NOTRECURSED"
291.It Dv RA_RECURSED
292Assert that the current thread holds a recursive lock of
293.Fa rw .
294.It Dv RA_NOTRECURSED
295Assert that the current thread does not hold a recursive lock of
296.Fa rw .
297.El
298.El
299.Sh SEE ALSO
300.Xr locking 9 ,
301.Xr mutex 9 ,
302.Xr panic 9 ,
303.Xr sema 9 ,
304.Xr sx 9
305.Sh HISTORY
306These
307functions appeared in
308.Fx 7.0 .
309.Sh AUTHORS
310.An -nosplit
311The
312.Nm
313facility was written by
314.An "John Baldwin" .
315This manual page was written by
316.An "Gleb Smirnoff" .
317.Sh BUGS
318A kernel without
319.Dv WITNESS
320cannot assert whether the current thread does or does not hold a read lock.
321.Dv RA_LOCKED
322and
323.Dv RA_RLOCKED
324can only assert that
325.Em any
326thread holds a read lock.
327They cannot ensure that the current thread holds a read lock.
328Further,
329.Dv RA_UNLOCKED
330can only assert that the current thread does not hold a write lock.
331.Pp
332Reader/writer is a bit of an awkward name.
333An
334.Nm
335can also be called a
336.Dq Robert Watson
337lock if desired.
338