Based on kernel version 4.15. Page generated on 2018-01-29 10:00 EST.
1 The errseq_t datatype 2 ===================== 3 An errseq_t is a way of recording errors in one place, and allowing any 4 number of "subscribers" to tell whether it has changed since a previous 5 point where it was sampled. 6 7 The initial use case for this is tracking errors for file 8 synchronization syscalls (fsync, fdatasync, msync and sync_file_range), 9 but it may be usable in other situations. 10 11 It's implemented as an unsigned 32-bit value. The low order bits are 12 designated to hold an error code (between 1 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 Note that there is a risk of collisions if new errors are being recorded 17 frequently, since we have so few bits to use as a counter. 18 19 To mitigate this, the bit between the error value and counter is used as 20 a flag to tell whether the value has been sampled since a new value was 21 recorded. That allows us to avoid bumping the counter if no one has 22 sampled it since the last time an error was recorded. 23 24 Thus we end up with a value that looks something like this:: 25 26 bit: 31..13 12 11..0 27 +-----------------+----+----------------+ 28 | counter | SF | errno | 29 +-----------------+----+----------------+ 30 31 The general idea is for "watchers" to sample an errseq_t value and keep 32 it as a running cursor. That value can later be used to tell whether 33 any new errors have occurred since that sampling was done, and atomically 34 record the state at the time that it was checked. This allows us to 35 record errors in one place, and then have a number of "watchers" that 36 can tell whether the value has changed since they last checked it. 37 38 A new errseq_t should always be zeroed out. An errseq_t value of all zeroes 39 is the special (but common) case where there has never been an error. An all 40 zero value thus serves as the "epoch" if one wishes to know whether there 41 has ever been an error set since it was first initialized. 42 43 API usage 44 ========= 45 Let me tell you a story about a worker drone. Now, he's a good worker 46 overall, but the company is a little...management heavy. He has to 47 report to 77 supervisors today, and tomorrow the "big boss" is coming in 48 from out of town and he's sure to test the poor fellow too. 49 50 They're all handing him work to do -- so much he can't keep track of who 51 handed him what, but that's not really a big problem. The supervisors 52 just want to know when he's finished all of the work they've handed him so 53 far and whether he made any mistakes since they last asked. 54 55 He might have made the mistake on work they didn't actually hand him, 56 but he can't keep track of things at that level of detail, all he can 57 remember is the most recent mistake that he made. 58 59 Here's our worker_drone representation:: 60 61 struct worker_drone { 62 errseq_t wd_err; /* for recording errors */ 63 }; 64 65 Every day, the worker_drone starts out with a blank slate:: 66 67 struct worker_drone wd; 68 69 wd.wd_err = (errseq_t)0; 70 71 The supervisors come in and get an initial read for the day. They 72 don't care about anything that happened before their watch begins:: 73 74 struct supervisor { 75 errseq_t s_wd_err; /* private "cursor" for wd_err */ 76 spinlock_t s_wd_err_lock; /* protects s_wd_err */ 77 } 78 79 struct supervisor su; 80 81 su.s_wd_err = errseq_sample(&wd.wd_err); 82 spin_lock_init(&su.s_wd_err_lock); 83 84 Now they start handing him tasks to do. Every few minutes they ask him to 85 finish up all of the work they've handed him so far. Then they ask him 86 whether he made any mistakes on any of it:: 87 88 spin_lock(&su.su_wd_err_lock); 89 err = errseq_check_and_advance(&wd.wd_err, &su.s_wd_err); 90 spin_unlock(&su.su_wd_err_lock); 91 92 Up to this point, that just keeps returning 0. 93 94 Now, the owners of this company are quite miserly and have given him 95 substandard equipment with which to do his job. Occasionally it 96 glitches and he makes a mistake. He sighs a heavy sigh, and marks it 97 down:: 98 99 errseq_set(&wd.wd_err, -EIO); 100 101 ...and then gets back to work. The supervisors eventually poll again 102 and they each get the error when they next check. Subsequent calls will 103 return 0, until another error is recorded, at which point it's reported 104 to each of them once. 105 106 Note that the supervisors can't tell how many mistakes he made, only 107 whether one was made since they last checked, and the latest value 108 recorded. 109 110 Occasionally the big boss comes in for a spot check and asks the worker 111 to do a one-off job for him. He's not really watching the worker 112 full-time like the supervisors, but he does need to know whether a 113 mistake occurred while his job was processing. 114 115 He can just sample the current errseq_t in the worker, and then use that 116 to tell whether an error has occurred later:: 117 118 errseq_t since = errseq_sample(&wd.wd_err); 119 /* submit some work and wait for it to complete */ 120 err = errseq_check(&wd.wd_err, since); 121 122 Since he's just going to discard "since" after that point, he doesn't 123 need to advance it here. He also doesn't need any locking since it's 124 not usable by anyone else. 125 126 Serializing errseq_t cursor updates 127 =================================== 128 Note that the errseq_t API does not protect the errseq_t cursor during a 129 check_and_advance_operation. Only the canonical error code is handled 130 atomically. In a situation where more than one task might be using the 131 same errseq_t cursor at the same time, it's important to serialize 132 updates to that cursor. 133 134 If that's not done, then it's possible for the cursor to go backward 135 in which case the same error could be reported more than once. 136 137 Because of this, it's often advantageous to first do an errseq_check to 138 see if anything has changed, and only later do an 139 errseq_check_and_advance after taking the lock. e.g.:: 140 141 if (errseq_check(&wd.wd_err, READ_ONCE(su.s_wd_err)) { 142 /* su.s_wd_err is protected by s_wd_err_lock */ 143 spin_lock(&su.s_wd_err_lock); 144 err = errseq_check_and_advance(&wd.wd_err, &su.s_wd_err); 145 spin_unlock(&su.s_wd_err_lock); 146 } 147 148 That avoids the spinlock in the common case where nothing has changed 149 since the last time it was checked.
