1 // SPDX-License-Identifier: GPL-2.0
2 #include <linux/err.h>
3 #include <linux/bug.h>
4 #include <linux/atomic.h>
5 #include <linux/errseq.h>
6
7 /*
8 * An errseq_t is a way of recording errors in one place, and allowing any
9 * number of "subscribers" to tell whether it has changed since a previous
10 * point where it was sampled.
11 *
12 * It's implemented as an unsigned 32-bit value. The low order bits are
13 * designated to hold an error code (between 0 and -MAX_ERRNO). The upper bits
14 * are used as a counter. This is done with atomics instead of locking so that
15 * these functions can be called from any context.
16 *
17 * The general idea is for consumers to sample an errseq_t value. That value
18 * can later be used to tell whether any new errors have occurred since that
19 * sampling was done.
20 *
21 * Note that there is a risk of collisions if new errors are being recorded
22 * frequently, since we have so few bits to use as a counter.
23 *
24 * To mitigate this, one bit is used as a flag to tell whether the value has
25 * been sampled since a new value was recorded. That allows us to avoid bumping
26 * the counter if no one has sampled it since the last time an error was
27 * recorded.
28 *
29 * A new errseq_t should always be zeroed out. A errseq_t value of all zeroes
30 * is the special (but common) case where there has never been an error. An all
31 * zero value thus serves as the "epoch" if one wishes to know whether there
32 * has ever been an error set since it was first initialized.
33 */
34
35 /* The low bits are designated for error code (max of MAX_ERRNO) */
36 #define ERRSEQ_SHIFT ilog2(MAX_ERRNO + 1)
37
38 /* This bit is used as a flag to indicate whether the value has been seen */
39 #define ERRSEQ_SEEN (1 << ERRSEQ_SHIFT)
40
41 /* The lowest bit of the counter */
42 #define ERRSEQ_CTR_INC (1 << (ERRSEQ_SHIFT + 1))
43
44 /**
45 * errseq_set - set a errseq_t for later reporting
46 * @eseq: errseq_t field that should be set
47 * @err: error to set (must be between -1 and -MAX_ERRNO)
48 *
49 * This function sets the error in @eseq, and increments the sequence counter
50 * if the last sequence was sampled at some point in the past.
51 *
52 * Any error set will always overwrite an existing error.
53 *
54 * Return: The previous value, primarily for debugging purposes. The
55 * return value should not be used as a previously sampled value in later
56 * calls as it will not have the SEEN flag set.
57 */
errseq_set(errseq_t * eseq,int err)58 errseq_t errseq_set(errseq_t *eseq, int err)
59 {
60 errseq_t cur, old;
61
62 /* MAX_ERRNO must be able to serve as a mask */
63 BUILD_BUG_ON_NOT_POWER_OF_2(MAX_ERRNO + 1);
64
65 /*
66 * Ensure the error code actually fits where we want it to go. If it
67 * doesn't then just throw a warning and don't record anything. We
68 * also don't accept zero here as that would effectively clear a
69 * previous error.
70 */
71 old = READ_ONCE(*eseq);
72
73 if (WARN(unlikely(err == 0 || (unsigned int)-err > MAX_ERRNO),
74 "err = %d\n", err))
75 return old;
76
77 for (;;) {
78 errseq_t new;
79
80 /* Clear out error bits and set new error */
81 new = (old & ~(MAX_ERRNO|ERRSEQ_SEEN)) | -err;
82
83 /* Only increment if someone has looked at it */
84 if (old & ERRSEQ_SEEN)
85 new += ERRSEQ_CTR_INC;
86
87 /* If there would be no change, then call it done */
88 if (new == old) {
89 cur = new;
90 break;
91 }
92
93 /* Try to swap the new value into place */
94 cur = cmpxchg(eseq, old, new);
95
96 /*
97 * Call it success if we did the swap or someone else beat us
98 * to it for the same value.
99 */
100 if (likely(cur == old || cur == new))
101 break;
102
103 /* Raced with an update, try again */
104 old = cur;
105 }
106 return cur;
107 }
108 EXPORT_SYMBOL(errseq_set);
109
110 /**
111 * errseq_sample() - Grab current errseq_t value.
112 * @eseq: Pointer to errseq_t to be sampled.
113 *
114 * This function allows callers to initialise their errseq_t variable.
115 * If the error has been "seen", new callers will not see an old error.
116 * If there is an unseen error in @eseq, the caller of this function will
117 * see it the next time it checks for an error.
118 *
119 * Context: Any context.
120 * Return: The current errseq value.
121 */
errseq_sample(errseq_t * eseq)122 errseq_t errseq_sample(errseq_t *eseq)
123 {
124 errseq_t old = READ_ONCE(*eseq);
125
126 /* If nobody has seen this error yet, then we can be the first. */
127 if (!(old & ERRSEQ_SEEN))
128 old = 0;
129 return old;
130 }
131 EXPORT_SYMBOL(errseq_sample);
132
133 /**
134 * errseq_check() - Has an error occurred since a particular sample point?
135 * @eseq: Pointer to errseq_t value to be checked.
136 * @since: Previously-sampled errseq_t from which to check.
137 *
138 * Grab the value that eseq points to, and see if it has changed @since
139 * the given value was sampled. The @since value is not advanced, so there
140 * is no need to mark the value as seen.
141 *
142 * Return: The latest error set in the errseq_t or 0 if it hasn't changed.
143 */
errseq_check(errseq_t * eseq,errseq_t since)144 int errseq_check(errseq_t *eseq, errseq_t since)
145 {
146 errseq_t cur = READ_ONCE(*eseq);
147
148 if (likely(cur == since))
149 return 0;
150 return -(cur & MAX_ERRNO);
151 }
152 EXPORT_SYMBOL(errseq_check);
153
154 /**
155 * errseq_check_and_advance() - Check an errseq_t and advance to current value.
156 * @eseq: Pointer to value being checked and reported.
157 * @since: Pointer to previously-sampled errseq_t to check against and advance.
158 *
159 * Grab the eseq value, and see whether it matches the value that @since
160 * points to. If it does, then just return 0.
161 *
162 * If it doesn't, then the value has changed. Set the "seen" flag, and try to
163 * swap it into place as the new eseq value. Then, set that value as the new
164 * "since" value, and return whatever the error portion is set to.
165 *
166 * Note that no locking is provided here for concurrent updates to the "since"
167 * value. The caller must provide that if necessary. Because of this, callers
168 * may want to do a lockless errseq_check before taking the lock and calling
169 * this.
170 *
171 * Return: Negative errno if one has been stored, or 0 if no new error has
172 * occurred.
173 */
errseq_check_and_advance(errseq_t * eseq,errseq_t * since)174 int errseq_check_and_advance(errseq_t *eseq, errseq_t *since)
175 {
176 int err = 0;
177 errseq_t old, new;
178
179 /*
180 * Most callers will want to use the inline wrapper to check this,
181 * so that the common case of no error is handled without needing
182 * to take the lock that protects the "since" value.
183 */
184 old = READ_ONCE(*eseq);
185 if (old != *since) {
186 /*
187 * Set the flag and try to swap it into place if it has
188 * changed.
189 *
190 * We don't care about the outcome of the swap here. If the
191 * swap doesn't occur, then it has either been updated by a
192 * writer who is altering the value in some way (updating
193 * counter or resetting the error), or another reader who is
194 * just setting the "seen" flag. Either outcome is OK, and we
195 * can advance "since" and return an error based on what we
196 * have.
197 */
198 new = old | ERRSEQ_SEEN;
199 if (new != old)
200 cmpxchg(eseq, old, new);
201 *since = new;
202 err = -(new & MAX_ERRNO);
203 }
204 return err;
205 }
206 EXPORT_SYMBOL(errseq_check_and_advance);
207