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 #include <linux/log2.h>
7 
8 /*
9  * An errseq_t is a way of recording errors in one place, and allowing any
10  * number of "subscribers" to tell whether it has changed since a previous
11  * point where it was sampled.
12  *
13  * It's implemented as an unsigned 32-bit value. The low order bits are
14  * designated to hold an error code (between 0 and -MAX_ERRNO). The upper bits
15  * are used as a counter. This is done with atomics instead of locking so that
16  * these functions can be called from any context.
17  *
18  * The general idea is for consumers to sample an errseq_t value. That value
19  * can later be used to tell whether any new errors have occurred since that
20  * sampling was done.
21  *
22  * Note that there is a risk of collisions if new errors are being recorded
23  * frequently, since we have so few bits to use as a counter.
24  *
25  * To mitigate this, one bit is used as a flag to tell whether the value has
26  * been sampled since a new value was recorded. That allows us to avoid bumping
27  * the counter if no one has sampled it since the last time an error was
28  * recorded.
29  *
30  * A new errseq_t should always be zeroed out.  A errseq_t value of all zeroes
31  * is the special (but common) case where there has never been an error. An all
32  * zero value thus serves as the "epoch" if one wishes to know whether there
33  * has ever been an error set since it was first initialized.
34  */
35 
36 /* The low bits are designated for error code (max of MAX_ERRNO) */
37 #define ERRSEQ_SHIFT		ilog2(MAX_ERRNO + 1)
38 
39 /* This bit is used as a flag to indicate whether the value has been seen */
40 #define ERRSEQ_SEEN		(1 << ERRSEQ_SHIFT)
41 
42 /* The lowest bit of the counter */
43 #define ERRSEQ_CTR_INC		(1 << (ERRSEQ_SHIFT + 1))
44 
45 /**
46  * errseq_set - set a errseq_t for later reporting
47  * @eseq: errseq_t field that should be set
48  * @err: error to set (must be between -1 and -MAX_ERRNO)
49  *
50  * This function sets the error in @eseq, and increments the sequence counter
51  * if the last sequence was sampled at some point in the past.
52  *
53  * Any error set will always overwrite an existing error.
54  *
55  * Return: The previous value, primarily for debugging purposes. The
56  * return value should not be used as a previously sampled value in later
57  * calls as it will not have the SEEN flag set.
58  */
errseq_set(errseq_t * eseq,int err)59 errseq_t errseq_set(errseq_t *eseq, int err)
60 {
61 	errseq_t cur, old;
62 
63 	/* MAX_ERRNO must be able to serve as a mask */
64 	BUILD_BUG_ON_NOT_POWER_OF_2(MAX_ERRNO + 1);
65 
66 	/*
67 	 * Ensure the error code actually fits where we want it to go. If it
68 	 * doesn't then just throw a warning and don't record anything. We
69 	 * also don't accept zero here as that would effectively clear a
70 	 * previous error.
71 	 */
72 	old = READ_ONCE(*eseq);
73 
74 	if (WARN(unlikely(err == 0 || (unsigned int)-err > MAX_ERRNO),
75 				"err = %d\n", err))
76 		return old;
77 
78 	for (;;) {
79 		errseq_t new;
80 
81 		/* Clear out error bits and set new error */
82 		new = (old & ~(MAX_ERRNO|ERRSEQ_SEEN)) | -err;
83 
84 		/* Only increment if someone has looked at it */
85 		if (old & ERRSEQ_SEEN)
86 			new += ERRSEQ_CTR_INC;
87 
88 		/* If there would be no change, then call it done */
89 		if (new == old) {
90 			cur = new;
91 			break;
92 		}
93 
94 		/* Try to swap the new value into place */
95 		cur = cmpxchg(eseq, old, new);
96 
97 		/*
98 		 * Call it success if we did the swap or someone else beat us
99 		 * to it for the same value.
100 		 */
101 		if (likely(cur == old || cur == new))
102 			break;
103 
104 		/* Raced with an update, try again */
105 		old = cur;
106 	}
107 	return cur;
108 }
109 EXPORT_SYMBOL(errseq_set);
110 
111 /**
112  * errseq_sample() - Grab current errseq_t value.
113  * @eseq: Pointer to errseq_t to be sampled.
114  *
115  * This function allows callers to initialise their errseq_t variable.
116  * If the error has been "seen", new callers will not see an old error.
117  * If there is an unseen error in @eseq, the caller of this function will
118  * see it the next time it checks for an error.
119  *
120  * Context: Any context.
121  * Return: The current errseq value.
122  */
errseq_sample(errseq_t * eseq)123 errseq_t errseq_sample(errseq_t *eseq)
124 {
125 	errseq_t old = READ_ONCE(*eseq);
126 
127 	/* If nobody has seen this error yet, then we can be the first. */
128 	if (!(old & ERRSEQ_SEEN))
129 		old = 0;
130 	return old;
131 }
132 EXPORT_SYMBOL(errseq_sample);
133 
134 /**
135  * errseq_check() - Has an error occurred since a particular sample point?
136  * @eseq: Pointer to errseq_t value to be checked.
137  * @since: Previously-sampled errseq_t from which to check.
138  *
139  * Grab the value that eseq points to, and see if it has changed @since
140  * the given value was sampled. The @since value is not advanced, so there
141  * is no need to mark the value as seen.
142  *
143  * Return: The latest error set in the errseq_t or 0 if it hasn't changed.
144  */
errseq_check(errseq_t * eseq,errseq_t since)145 int errseq_check(errseq_t *eseq, errseq_t since)
146 {
147 	errseq_t cur = READ_ONCE(*eseq);
148 
149 	if (likely(cur == since))
150 		return 0;
151 	return -(cur & MAX_ERRNO);
152 }
153 EXPORT_SYMBOL(errseq_check);
154 
155 /**
156  * errseq_check_and_advance() - Check an errseq_t and advance to current value.
157  * @eseq: Pointer to value being checked and reported.
158  * @since: Pointer to previously-sampled errseq_t to check against and advance.
159  *
160  * Grab the eseq value, and see whether it matches the value that @since
161  * points to. If it does, then just return 0.
162  *
163  * If it doesn't, then the value has changed. Set the "seen" flag, and try to
164  * swap it into place as the new eseq value. Then, set that value as the new
165  * "since" value, and return whatever the error portion is set to.
166  *
167  * Note that no locking is provided here for concurrent updates to the "since"
168  * value. The caller must provide that if necessary. Because of this, callers
169  * may want to do a lockless errseq_check before taking the lock and calling
170  * this.
171  *
172  * Return: Negative errno if one has been stored, or 0 if no new error has
173  * occurred.
174  */
errseq_check_and_advance(errseq_t * eseq,errseq_t * since)175 int errseq_check_and_advance(errseq_t *eseq, errseq_t *since)
176 {
177 	int err = 0;
178 	errseq_t old, new;
179 
180 	/*
181 	 * Most callers will want to use the inline wrapper to check this,
182 	 * so that the common case of no error is handled without needing
183 	 * to take the lock that protects the "since" value.
184 	 */
185 	old = READ_ONCE(*eseq);
186 	if (old != *since) {
187 		/*
188 		 * Set the flag and try to swap it into place if it has
189 		 * changed.
190 		 *
191 		 * We don't care about the outcome of the swap here. If the
192 		 * swap doesn't occur, then it has either been updated by a
193 		 * writer who is altering the value in some way (updating
194 		 * counter or resetting the error), or another reader who is
195 		 * just setting the "seen" flag. Either outcome is OK, and we
196 		 * can advance "since" and return an error based on what we
197 		 * have.
198 		 */
199 		new = old | ERRSEQ_SEEN;
200 		if (new != old)
201 			cmpxchg(eseq, old, new);
202 		*since = new;
203 		err = -(new & MAX_ERRNO);
204 	}
205 	return err;
206 }
207 EXPORT_SYMBOL(errseq_check_and_advance);
208