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Jeff Layton84cbada2017-07-06 07:02:24 -04001#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/**
Jeff Layton3acdfd22017-07-24 06:22:15 -040044 * errseq_set - set a errseq_t for later reporting
Jeff Layton84cbada2017-07-06 07:02:24 -040045 * @eseq: errseq_t field that should be set
Jeff Layton3acdfd22017-07-24 06:22:15 -040046 * @err: error to set (must be between -1 and -MAX_ERRNO)
Jeff Layton84cbada2017-07-06 07:02:24 -040047 *
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 *
Jeff Layton3acdfd22017-07-24 06:22:15 -040053 * We do return the latest value here, primarily for debugging purposes. The
54 * return value should not be used as a previously sampled value in later calls
55 * as it will not have the SEEN flag set.
Jeff Layton84cbada2017-07-06 07:02:24 -040056 */
Jeff Layton3acdfd22017-07-24 06:22:15 -040057errseq_t errseq_set(errseq_t *eseq, int err)
Jeff Layton84cbada2017-07-06 07:02:24 -040058{
59 errseq_t cur, old;
60
61 /* MAX_ERRNO must be able to serve as a mask */
62 BUILD_BUG_ON_NOT_POWER_OF_2(MAX_ERRNO + 1);
63
64 /*
65 * Ensure the error code actually fits where we want it to go. If it
66 * doesn't then just throw a warning and don't record anything. We
67 * also don't accept zero here as that would effectively clear a
68 * previous error.
69 */
70 old = READ_ONCE(*eseq);
71
72 if (WARN(unlikely(err == 0 || (unsigned int)-err > MAX_ERRNO),
73 "err = %d\n", err))
74 return old;
75
76 for (;;) {
77 errseq_t new;
78
79 /* Clear out error bits and set new error */
80 new = (old & ~(MAX_ERRNO|ERRSEQ_SEEN)) | -err;
81
82 /* Only increment if someone has looked at it */
83 if (old & ERRSEQ_SEEN)
84 new += ERRSEQ_CTR_INC;
85
86 /* If there would be no change, then call it done */
87 if (new == old) {
88 cur = new;
89 break;
90 }
91
92 /* Try to swap the new value into place */
93 cur = cmpxchg(eseq, old, new);
94
95 /*
96 * Call it success if we did the swap or someone else beat us
97 * to it for the same value.
98 */
99 if (likely(cur == old || cur == new))
100 break;
101
102 /* Raced with an update, try again */
103 old = cur;
104 }
105 return cur;
106}
Jeff Layton3acdfd22017-07-24 06:22:15 -0400107EXPORT_SYMBOL(errseq_set);
Jeff Layton84cbada2017-07-06 07:02:24 -0400108
109/**
110 * errseq_sample - grab current errseq_t value
111 * @eseq: pointer to errseq_t to be sampled
112 *
113 * This function allows callers to sample an errseq_t value, marking it as
114 * "seen" if required.
115 */
116errseq_t errseq_sample(errseq_t *eseq)
117{
118 errseq_t old = READ_ONCE(*eseq);
119 errseq_t new = old;
120
121 /*
122 * For the common case of no errors ever having been set, we can skip
123 * marking the SEEN bit. Once an error has been set, the value will
124 * never go back to zero.
125 */
126 if (old != 0) {
127 new |= ERRSEQ_SEEN;
128 if (old != new)
129 cmpxchg(eseq, old, new);
130 }
131 return new;
132}
133EXPORT_SYMBOL(errseq_sample);
134
135/**
136 * errseq_check - has an error occurred since a particular sample point?
137 * @eseq: pointer to errseq_t value to be checked
138 * @since: previously-sampled errseq_t from which to check
139 *
140 * Grab the value that eseq points to, and see if it has changed "since"
141 * the given value was sampled. The "since" value is not advanced, so there
142 * is no need to mark the value as seen.
143 *
144 * Returns the latest error set in the errseq_t or 0 if it hasn't changed.
145 */
146int errseq_check(errseq_t *eseq, errseq_t since)
147{
148 errseq_t cur = READ_ONCE(*eseq);
149
150 if (likely(cur == since))
151 return 0;
152 return -(cur & MAX_ERRNO);
153}
154EXPORT_SYMBOL(errseq_check);
155
156/**
157 * errseq_check_and_advance - check an errseq_t and advance to current value
158 * @eseq: pointer to value being checked and reported
159 * @since: pointer to previously-sampled errseq_t to check against and advance
160 *
161 * Grab the eseq value, and see whether it matches the value that "since"
162 * points to. If it does, then just return 0.
163 *
164 * If it doesn't, then the value has changed. Set the "seen" flag, and try to
165 * swap it into place as the new eseq value. Then, set that value as the new
166 * "since" value, and return whatever the error portion is set to.
167 *
168 * Note that no locking is provided here for concurrent updates to the "since"
169 * value. The caller must provide that if necessary. Because of this, callers
170 * may want to do a lockless errseq_check before taking the lock and calling
171 * this.
172 */
173int errseq_check_and_advance(errseq_t *eseq, errseq_t *since)
174{
175 int err = 0;
176 errseq_t old, new;
177
178 /*
179 * Most callers will want to use the inline wrapper to check this,
180 * so that the common case of no error is handled without needing
181 * to take the lock that protects the "since" value.
182 */
183 old = READ_ONCE(*eseq);
184 if (old != *since) {
185 /*
186 * Set the flag and try to swap it into place if it has
187 * changed.
188 *
189 * We don't care about the outcome of the swap here. If the
190 * swap doesn't occur, then it has either been updated by a
191 * writer who is altering the value in some way (updating
192 * counter or resetting the error), or another reader who is
193 * just setting the "seen" flag. Either outcome is OK, and we
194 * can advance "since" and return an error based on what we
195 * have.
196 */
197 new = old | ERRSEQ_SEEN;
198 if (new != old)
199 cmpxchg(eseq, old, new);
200 *since = new;
201 err = -(new & MAX_ERRNO);
202 }
203 return err;
204}
205EXPORT_SYMBOL(errseq_check_and_advance);