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39 .Nm lwkt_token_uninit ,
43 .Nm lwkt_token_pool_lookup ,
49 .Fn lwkt_token_init "struct lwkt_token *tok" "int mpsafe"
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.
83 is 0 the MP lock is taken before acquiring the token and released after
87 function is called to de-initialize one.
88 Before using a token, it must be initialized.
92 function attempts to acquire a token.
93 If it is unsuccessful, the calling thread blocks.
96 does the same thing; however, if it cannot acquire the token, it returns 0
100 function releases a previously acquired soft token.
103 .Fn lwkt_token_pool_lookup
104 function takes an address and maps it to one of a number of statically
107 .Fn lwkt_getpooltoken
108 function acquires a token associated with an address.
109 Use these two functions when tokens must protect a data structure,
110 but the structure can be deallocated.
111 Pool tokens do not need to be initialized.
113 A simple example of using a token to protect access to a data structure:
115 /* Data structure to be protected */
116 struct protected_data {
117 struct lwkt_token tok;
121 struct protected_data pdata;
123 /* Called early in boot */
127 lwkt_token_init(&pdata.tok ,1);
132 * A silly kthread; it uses a token to protect pdata.data.
140 * Get the soft token.
142 lwkt_gettoken(&pdata.tok);
144 local = pdata.data++;
145 tsleep(pdata, 0, "sleep", 0);
147 * While we are asleep, we do not hold the token. When we
148 * awake here, we will hold the token again, but we may not
149 * depend on local reflecting pdata.data.
159 lwkt_reltoken(&pdata.tok);
163 An example using pool tokens:
165 struct dynamic_data {
170 * Use a token to protect a reference count in a dynamic structure.
171 * Embedding a token in the structure would be inappropriate, since
172 * another thread may attempt to take the token after we have freed
173 * the object but before we have removed all external references to it.
176 kfunction(struct dynamic_data *dynptr)
178 struct lwkt_token *tok;
181 * Get a token from the associated with the address of dynptr
183 tok = lwkt_getpooltoken(dynptr);
185 if (dynptr->ref == 0)
189 * Release the token via its reference, as above
195 Soft tokens are not released when a thread is preempted; they are only released
196 when a thread explicitly blocks, such as via
203 blocks while attempting to acquire a token, all currently-held tokens will
204 be released till a thread can acquire all of them again.
206 When tokens are held and
208 is used, tokens are not released until blocking happens - that is until the
214 The LWKT Token implementation is in
215 .Pa /sys/kern/lwkt_token.c .
223 LWKT tokens first appeared in
228 implementation was written by