projects/linux·blob v4.13-rc4 · refs

208 lines · 6.5 KiB

1#include <linux/err.h>
2#include <linux/bug.h>
3#include <linux/atomic.h>
4#include <linux/errseq.h>
5
6/*
7 * An errseq_t is a way of recording errors in one place, and allowing any
8 * number of "subscribers" to tell whether it has changed since a previous
9 * point where it was sampled.
10 *
11 * It's implemented as an unsigned 32-bit value. The low order bits are
12 * designated to hold an error code (between 0 and -MAX_ERRNO). The upper bits
13 * are used as a counter. This is done with atomics instead of locking so that
14 * these functions can be called from any context.
15 *
16 * The general idea is for consumers to sample an errseq_t value. That value
17 * can later be used to tell whether any new errors have occurred since that
18 * sampling was done.
19 *
20 * Note that there is a risk of collisions if new errors are being recorded
21 * frequently, since we have so few bits to use as a counter.
22 *
23 * To mitigate this, one bit is used as a flag to tell whether the value has
24 * been sampled since a new value was recorded. That allows us to avoid bumping
25 * the counter if no one has sampled it since the last time an error was
26 * recorded.
27 *
28 * A new errseq_t should always be zeroed out. A errseq_t value of all zeroes
29 * is the special (but common) case where there has never been an error. An all
30 * zero value thus serves as the "epoch" if one wishes to know whether there
31 * has ever been an error set since it was first initialized.
32 */
33
34/* The low bits are designated for error code (max of MAX_ERRNO) */
35#define ERRSEQ_SHIFT ilog2(MAX_ERRNO + 1)
36
37/* This bit is used as a flag to indicate whether the value has been seen */
38#define ERRSEQ_SEEN (1 << ERRSEQ_SHIFT)
39
40/* The lowest bit of the counter */
41#define ERRSEQ_CTR_INC (1 << (ERRSEQ_SHIFT + 1))
42
43/**
44 * __errseq_set - set a errseq_t for later reporting
45 * @eseq: errseq_t field that should be set
46 * @err: error to set
47 *
48 * This function sets the error in *eseq, and increments the sequence counter
49 * if the last sequence was sampled at some point in the past.
50 *
51 * Any error set will always overwrite an existing error.
52 *
53 * Most callers will want to use the errseq_set inline wrapper to efficiently
54 * handle the common case where err is 0.
55 *
56 * We do return an errseq_t here, primarily for debugging purposes. The return
57 * value should not be used as a previously sampled value in later calls as it
58 * will not have the SEEN flag set.
59 */
60errseq_t __errseq_set(errseq_t *eseq, int err)
61{
62 errseq_t cur, old;
63
64 /* MAX_ERRNO must be able to serve as a mask */
65 BUILD_BUG_ON_NOT_POWER_OF_2(MAX_ERRNO + 1);
66
67 /*
68 * Ensure the error code actually fits where we want it to go. If it
69 * doesn't then just throw a warning and don't record anything. We
70 * also don't accept zero here as that would effectively clear a
71 * previous error.
72 */
73 old = READ_ONCE(*eseq);
74
75 if (WARN(unlikely(err == 0 || (unsigned int)-err > MAX_ERRNO),
76 "err = %d\n", err))
77 return old;
78
79 for (;;) {
80 errseq_t new;
81
82 /* Clear out error bits and set new error */
83 new = (old & ~(MAX_ERRNO|ERRSEQ_SEEN)) | -err;
84
85 /* Only increment if someone has looked at it */
86 if (old & ERRSEQ_SEEN)
87 new += ERRSEQ_CTR_INC;
88
89 /* If there would be no change, then call it done */
90 if (new == old) {
91 cur = new;
92 break;
93 }
94
95 /* Try to swap the new value into place */
96 cur = cmpxchg(eseq, old, new);
97
98 /*
99 * Call it success if we did the swap or someone else beat us
100 * to it for the same value.
101 */
102 if (likely(cur == old || cur == new))
103 break;
104
105 /* Raced with an update, try again */
106 old = cur;
107 }
108 return cur;
109}
110EXPORT_SYMBOL(__errseq_set);
111
112/**
113 * errseq_sample - grab current errseq_t value
114 * @eseq: pointer to errseq_t to be sampled
115 *
116 * This function allows callers to sample an errseq_t value, marking it as
117 * "seen" if required.
118 */
119errseq_t errseq_sample(errseq_t *eseq)
120{
121 errseq_t old = READ_ONCE(*eseq);
122 errseq_t new = old;
123
124 /*
125 * For the common case of no errors ever having been set, we can skip
126 * marking the SEEN bit. Once an error has been set, the value will
127 * never go back to zero.
128 */
129 if (old != 0) {
130 new |= ERRSEQ_SEEN;
131 if (old != new)
132 cmpxchg(eseq, old, new);
133 }
134 return new;
135}
136EXPORT_SYMBOL(errseq_sample);
137
138/**
139 * errseq_check - has an error occurred since a particular sample point?
140 * @eseq: pointer to errseq_t value to be checked
141 * @since: previously-sampled errseq_t from which to check
142 *
143 * Grab the value that eseq points to, and see if it has changed "since"
144 * the given value was sampled. The "since" value is not advanced, so there
145 * is no need to mark the value as seen.
146 *
147 * Returns the latest error set in the errseq_t or 0 if it hasn't changed.
148 */
149int errseq_check(errseq_t *eseq, errseq_t since)
150{
151 errseq_t cur = READ_ONCE(*eseq);
152
153 if (likely(cur == since))
154 return 0;
155 return -(cur & MAX_ERRNO);
156}
157EXPORT_SYMBOL(errseq_check);
158
159/**
160 * errseq_check_and_advance - check an errseq_t and advance to current value
161 * @eseq: pointer to value being checked and reported
162 * @since: pointer to previously-sampled errseq_t to check against and advance
163 *
164 * Grab the eseq value, and see whether it matches the value that "since"
165 * points to. If it does, then just return 0.
166 *
167 * If it doesn't, then the value has changed. Set the "seen" flag, and try to
168 * swap it into place as the new eseq value. Then, set that value as the new
169 * "since" value, and return whatever the error portion is set to.
170 *
171 * Note that no locking is provided here for concurrent updates to the "since"
172 * value. The caller must provide that if necessary. Because of this, callers
173 * may want to do a lockless errseq_check before taking the lock and calling
174 * this.
175 */
176int errseq_check_and_advance(errseq_t *eseq, errseq_t *since)
177{
178 int err = 0;
179 errseq_t old, new;
180
181 /*
182 * Most callers will want to use the inline wrapper to check this,
183 * so that the common case of no error is handled without needing
184 * to take the lock that protects the "since" value.
185 */
186 old = READ_ONCE(*eseq);
187 if (old != *since) {
188 /*
189 * Set the flag and try to swap it into place if it has
190 * changed.
191 *
192 * We don't care about the outcome of the swap here. If the
193 * swap doesn't occur, then it has either been updated by a
194 * writer who is altering the value in some way (updating
195 * counter or resetting the error), or another reader who is
196 * just setting the "seen" flag. Either outcome is OK, and we
197 * can advance "since" and return an error based on what we
198 * have.
199 */
200 new = old | ERRSEQ_SEEN;
201 if (new != old)
202 cmpxchg(eseq, old, new);
203 *since = new;
204 err = -(new & MAX_ERRNO);
205 }
206 return err;
207}
208EXPORT_SYMBOL(errseq_check_and_advance);