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39 .Nm lwkt_token_uninit ,
43 .Nm lwkt_token_pool_lookup ,
44 .Nm lwkt_getpooltoken ,
45 .\".Nm lwkt_relpooltoken ,
51 .Fn lwkt_token_init "struct lwkt_token *tok" "char *desc"
53 .Fn lwkt_token_uninit "struct lwkt_token *tok"
55 .Fn lwkt_gettoken "struct lwkt_token *tok"
57 .Fn lwkt_trytoken "struct lwkt_token *tok"
59 .Fn lwkt_reltoken "struct lwkt_token *tok"
60 .Ft struct lwkt_token *
61 .Fn lwkt_token_pool_lookup "void *ptr"
62 .Ft struct lwkt_token *
63 .Fn lwkt_getpooltoken "void *ptr"
65 .Fn lwkt_token_swap "void"
67 A soft token is a lock which is only held while a thread is running.
68 If a thread explicitly blocks, all its tokens are released, and reacquired
69 when the thread resumes.
70 While a thread blocks, the conditions protected by a soft token
71 may change and may need to be reevaluated on wakeup.
73 Tokens may be taken recursively.
74 However, tokens must be released in the reverse order they were acquired.
76 The pool token interface exists to allow using tokens with data structures
77 which may be deallocated.
78 It allows getting a token reference from an address, which
79 is implemented by a set of statically allocated tokens and a hash function.
83 function is called to initialize a token.
86 argument specifies the wait string displayed when waiting for the token.
89 function is called to de-initialize one.
90 Before using a token, it must be initialized.
94 function attempts to acquire a token.
95 If it is unsuccessful, the calling thread blocks.
98 does the same thing; however, if it cannot acquire the token, it returns 0
102 function releases a previously acquired soft token.
105 .Fn lwkt_token_pool_lookup
106 function takes an address and maps it to one of a number of statically
109 .Fn lwkt_getpooltoken
110 function acquires a token associated with an address.
111 Use these two functions when tokens must protect a data structure,
112 but the structure can be deallocated.
113 Pool tokens do not need to be initialized.
117 function swaps the two most recently acquired tokens; this allows release of
119 This function should not be called when less than two tokens are held.
121 A simple example of using a token to protect access to a data structure:
123 /* Data structure to be protected */
124 struct protected_data {
125 struct lwkt_token tok;
129 struct protected_data pdata;
131 /* Called early in boot */
135 lwkt_token_init(&pdata.tok, "example");
140 * A silly kthread; it uses a token to protect pdata.data.
148 * Get the soft token.
150 lwkt_gettoken(&pdata.tok);
152 local = pdata.data++;
153 tsleep(pdata, 0, "sleep", 0);
155 * While we are asleep, we do not hold the token. When we
156 * awake here, we will hold the token again, but we may not
157 * depend on local reflecting pdata.data.
167 lwkt_reltoken(&pdata.tok);
171 An example using pool tokens:
173 struct dynamic_data {
178 * Use a token to protect a reference count in a dynamic structure.
179 * Embedding a token in the structure would be inappropriate, since
180 * another thread may attempt to take the token after we have freed
181 * the object but before we have removed all external references to it.
184 kfunction(struct dynamic_data *dynptr)
186 struct lwkt_token *tok;
189 * Get a token from the associated with the address of dynptr
191 tok = lwkt_getpooltoken(dynptr);
193 if (dynptr->ref == 0)
197 * Release the token via its reference, as above
203 Soft tokens are not released when a thread is preempted; they are only released
204 when a thread explicitly blocks, such as via
211 blocks while attempting to acquire a token, all currently-held tokens will
212 be released till a thread can acquire all of them again.
214 When tokens are held and
216 is used, tokens are not released until blocking happens - that is until the
222 The LWKT Token implementation is in
223 .Pa /sys/kern/lwkt_token.c .
231 LWKT tokens first appeared in
236 implementation was written by