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39 .Nm lwkt_token_uninit ,
43 .Nm lwkt_token_pool_lookup ,
49 .Fn lwkt_token_init "struct lwkt_token *tok" "char *desc"
51 .Fn lwkt_token_uninit "struct lwkt_token *tok"
53 .Fn lwkt_gettoken "struct lwkt_token *tok"
55 .Fn lwkt_trytoken "struct lwkt_token *tok"
57 .Fn lwkt_reltoken "struct lwkt_token *tok"
58 .Ft struct lwkt_token *
59 .Fn lwkt_token_pool_lookup "void *ptr"
60 .Ft struct lwkt_token *
61 .Fn lwkt_getpooltoken "void *ptr"
64 A soft token is a lock which is only held while a thread is running.
65 If a thread explicitly blocks, all its tokens are released, and reacquired
66 when the thread resumes.
67 While a thread blocks, the conditions protected by a soft token
68 may change and may need to be reevaluated on wakeup.
70 Tokens may be taken recursively.
71 However, tokens must be released in the reverse order they were acquired.
73 The pool token interface exists to allow using tokens with data structures
74 which may be deallocated.
75 It allows getting a token reference from an address, which
76 is implemented by a set of statically allocated tokens and a hash function.
80 function is called to initialize a token. The
82 argument specifies the wait string displayed when waiting for the token.
85 function is called to de-initialize one.
86 Before using a token, it must be initialized.
90 function attempts to acquire a token.
91 If it is unsuccessful, the calling thread blocks.
94 does the same thing; however, if it cannot acquire the token, it returns 0
98 function releases a previously acquired soft token.
101 .Fn lwkt_token_pool_lookup
102 function takes an address and maps it to one of a number of statically
105 .Fn lwkt_getpooltoken
106 function acquires a token associated with an address.
107 Use these two functions when tokens must protect a data structure,
108 but the structure can be deallocated.
109 Pool tokens do not need to be initialized.
111 A simple example of using a token to protect access to a data structure:
113 /* Data structure to be protected */
114 struct protected_data {
115 struct lwkt_token tok;
119 struct protected_data pdata;
121 /* Called early in boot */
125 lwkt_token_init(&pdata.tok, "example");
130 * A silly kthread; it uses a token to protect pdata.data.
138 * Get the soft token.
140 lwkt_gettoken(&pdata.tok);
142 local = pdata.data++;
143 tsleep(pdata, 0, "sleep", 0);
145 * While we are asleep, we do not hold the token. When we
146 * awake here, we will hold the token again, but we may not
147 * depend on local reflecting pdata.data.
157 lwkt_reltoken(&pdata.tok);
161 An example using pool tokens:
163 struct dynamic_data {
168 * Use a token to protect a reference count in a dynamic structure.
169 * Embedding a token in the structure would be inappropriate, since
170 * another thread may attempt to take the token after we have freed
171 * the object but before we have removed all external references to it.
174 kfunction(struct dynamic_data *dynptr)
176 struct lwkt_token *tok;
179 * Get a token from the associated with the address of dynptr
181 tok = lwkt_getpooltoken(dynptr);
183 if (dynptr->ref == 0)
187 * Release the token via its reference, as above
193 Soft tokens are not released when a thread is preempted; they are only released
194 when a thread explicitly blocks, such as via
201 blocks while attempting to acquire a token, all currently-held tokens will
202 be released till a thread can acquire all of them again.
204 When tokens are held and
206 is used, tokens are not released until blocking happens - that is until the
212 The LWKT Token implementation is in
213 .Pa /sys/kern/lwkt_token.c .
221 LWKT tokens first appeared in
226 implementation was written by