include/linux/rcu_segcblist.h

   1 /* SPDX-License-Identifier: GPL-2.0+ */
   2 /*
   3  * RCU segmented callback lists
   4  *
   5  * This seemingly RCU-private file must be available to SRCU users
   6  * because the size of the TREE SRCU srcu_struct structure depends
   7  * on these definitions.
   8  *
   9  * Copyright IBM Corporation, 2017
  10  *
  11  * Authors: Paul E. McKenney <paulmck@linux.net.ibm.com>
  12  */
  13
  14 #ifndef __INCLUDE_LINUX_RCU_SEGCBLIST_H
  15 #define __INCLUDE_LINUX_RCU_SEGCBLIST_H
  16
  17 #include <linux/types.h>
  18 #include <linux/atomic.h>
  19
  20 /* Simple unsegmented callback lists. */
  21 struct rcu_cblist {
  22         struct rcu_head *head;
  23         struct rcu_head **tail;
  24         long len;
  25 };
  26
  27 #define RCU_CBLIST_INITIALIZER(n) { .head = NULL, .tail = &n.head }
  28
  29 /* Complicated segmented callback lists.  ;-) */
  30
  31 /*
  32  * Index values for segments in rcu_segcblist structure.
  33  *
  34  * The segments are as follows:
  35  *
  36  * [head, *tails[RCU_DONE_TAIL]):
  37  *      Callbacks whose grace period has elapsed, and thus can be invoked.
  38  * [*tails[RCU_DONE_TAIL], *tails[RCU_WAIT_TAIL]):
  39  *      Callbacks waiting for the current GP from the current CPU's viewpoint.
  40  * [*tails[RCU_WAIT_TAIL], *tails[RCU_NEXT_READY_TAIL]):
  41  *      Callbacks that arrived before the next GP started, again from
  42  *      the current CPU's viewpoint.  These can be handled by the next GP.
  43  * [*tails[RCU_NEXT_READY_TAIL], *tails[RCU_NEXT_TAIL]):
  44  *      Callbacks that might have arrived after the next GP started.
  45  *      There is some uncertainty as to when a given GP starts and
  46  *      ends, but a CPU knows the exact times if it is the one starting
  47  *      or ending the GP.  Other CPUs know that the previous GP ends
  48  *      before the next one starts.
  49  *
  50  * Note that RCU_WAIT_TAIL cannot be empty unless RCU_NEXT_READY_TAIL is also
  51  * empty.
  52  *
  53  * The ->gp_seq[] array contains the grace-period number at which the
  54  * corresponding segment of callbacks will be ready to invoke.  A given
  55  * element of this array is meaningful only when the corresponding segment
  56  * is non-empty, and it is never valid for RCU_DONE_TAIL (whose callbacks
  57  * are already ready to invoke) or for RCU_NEXT_TAIL (whose callbacks have
  58  * not yet been assigned a grace-period number).
  59  */
  60 #define RCU_DONE_TAIL           0       /* Also RCU_WAIT head. */
  61 #define RCU_WAIT_TAIL           1       /* Also RCU_NEXT_READY head. */
  62 #define RCU_NEXT_READY_TAIL     2       /* Also RCU_NEXT head. */
  63 #define RCU_NEXT_TAIL           3
  64 #define RCU_CBLIST_NSEGS        4
  65
  66
  67 /*
  68  *                     ==NOCB Offloading state machine==
  69  *
  70  *
  71  *  ----------------------------------------------------------------------------
  72  *  |                              SEGCBLIST_RCU_CORE                          |
  73  *  |                                                                          |
  74  *  |  Callbacks processed by rcu_core() from softirqs or local                |
  75  *  |  rcuc kthread, without holding nocb_lock.                                |
  76  *  ----------------------------------------------------------------------------
  77  *                                         |
  78  *                                         v
  79  *  ----------------------------------------------------------------------------
  80  *  |       SEGCBLIST_RCU_CORE | SEGCBLIST_LOCKING | SEGCBLIST_OFFLOADED       |
  81  *  |                                                                          |
  82  *  | Callbacks processed by rcu_core() from softirqs or local                 |
  83  *  | rcuc kthread, while holding nocb_lock. Waking up CB and GP kthreads.     |
  84  *  ----------------------------------------------------------------------------
  85  *                                         |
  86  *                                         v
  87  *  ----------------------------------------------------------------------------
  88  *  |        SEGCBLIST_RCU_CORE | SEGCBLIST_LOCKING | SEGCBLIST_OFFLOADED      |
  89  *  |                              + unparked CB kthread                       |
  90  *  |                                                                          |
  91  *  | CB kthread got unparked and processes callbacks concurrently with        |
  92  *  | rcu_core(), holding nocb_lock.                                           |
  93  *  ---------------------------------------------------------------------------
  94  *                                         |
  95  *                                         v
  96  *  ---------------------------------------------------------------------------|
  97  *  |                           SEGCBLIST_RCU_CORE |                           |
  98  *  |                           SEGCBLIST_LOCKING |                            |
  99  *  |                           SEGCBLIST_OFFLOADED |                          |
 100  *  |                           SEGCBLIST_KTHREAD_GP                           |
 101  *  |                           + unparked CB kthread                          |
 102  *  |                                                                          |
 103  *  | GP kthread woke up and acknowledged nocb_lock.                           |
 104  *  ---------------------------------------- -----------------------------------
 105  *                                         |
 106  *                                         v
 107  *  |--------------------------------------------------------------------------|
 108  *  |                           SEGCBLIST_LOCKING |                            |
 109  *  |                           SEGCBLIST_OFFLOADED |                          |
 110  *  |                           SEGCBLIST_KTHREAD_GP |                         |
 111  *  |                           + unparked CB kthread                          |
 112  *  |                                                                          |
 113  *  |   Kthreads handle callbacks holding nocb_lock, local rcu_core() stops    |
 114  *  |   handling callbacks. Enable bypass queueing.                            |
 115  *  ----------------------------------------------------------------------------
 116  */
 117
 118
 119
 120 /*
 121  *                       ==NOCB De-Offloading state machine==
 122  *
 123  *
 124  *  |--------------------------------------------------------------------------|
 125  *  |                           SEGCBLIST_LOCKING    |                         |
 126  *  |                           SEGCBLIST_OFFLOADED  |                         |
 127  *  |                           SEGCBLIST_KTHREAD_GP                           |
 128  *  |                           + unparked CB kthread                          |
 129  *  |                                                                          |
 130  *  |   CB/GP kthreads handle callbacks holding nocb_lock, local rcu_core()    |
 131  *  |   ignores callbacks. Bypass enqueue is enabled.                          |
 132  *  ----------------------------------------------------------------------------
 133  *                                      |
 134  *                                      v
 135  *  |--------------------------------------------------------------------------|
 136  *  |                           SEGCBLIST_RCU_CORE   |                         |
 137  *  |                           SEGCBLIST_LOCKING    |                         |
 138  *  |                           SEGCBLIST_OFFLOADED  |                         |
 139  *  |                           SEGCBLIST_KTHREAD_GP                           |
 140  *  |                           + unparked CB kthread                          |
 141  *  |                                                                          |
 142  *  |   CB/GP kthreads handle callbacks holding nocb_lock, local rcu_core()    |
 143  *  |   handles callbacks concurrently. Bypass enqueue is disabled.            |
 144  *  |   Invoke RCU core so we make sure not to preempt it in the middle with   |
 145  *  |   leaving some urgent work unattended within a jiffy.                    |
 146  *  ----------------------------------------------------------------------------
 147  *                                      |
 148  *                                      v
 149  *  |--------------------------------------------------------------------------|
 150  *  |                           SEGCBLIST_RCU_CORE   |                         |
 151  *  |                           SEGCBLIST_LOCKING    |                         |
 152  *  |                           SEGCBLIST_KTHREAD_GP                           |
 153  *  |                           + unparked CB kthread                          |
 154  *  |                                                                          |
 155  *  |   CB/GP kthreads and local rcu_core() handle callbacks concurrently      |
 156  *  |   holding nocb_lock. Wake up GP kthread if necessary.                    |
 157  *  ----------------------------------------------------------------------------
 158  *                                      |
 159  *                                      v
 160  *  |--------------------------------------------------------------------------|
 161  *  |                           SEGCBLIST_RCU_CORE   |                         |
 162  *  |                           SEGCBLIST_LOCKING    |                         |
 163  *  |                           + unparked CB kthread                          |
 164  *  |                                                                          |
 165  *  |   GP kthread woke up and acknowledged the fact that SEGCBLIST_OFFLOADED  |
 166  *  |   got cleared. The callbacks from the target CPU will be ignored from the|
 167  *  |   GP kthread loop.                                                       |
 168  *  ----------------------------------------------------------------------------
 169  *                                      |
 170  *                                      v
 171  *  ----------------------------------------------------------------------------
 172  *  |                SEGCBLIST_RCU_CORE | SEGCBLIST_LOCKING                    |
 173  *  |                          + parked CB kthread                             |
 174  *  |                                                                          |
 175  *  | CB kthread is parked. Callbacks processed by rcu_core() from softirqs or |
 176  *  | local rcuc kthread, while holding nocb_lock.                             |
 177  *  ----------------------------------------------------------------------------
 178  *                                      |
 179  *                                      v
 180  *  ----------------------------------------------------------------------------
 181  *  |                         SEGCBLIST_RCU_CORE                               |
 182  *  |                                                                          |
 183  *  |  Callbacks processed by rcu_core() from softirqs or local                |
 184  *  |  rcuc kthread, without holding nocb_lock.                                |
 185  *  ----------------------------------------------------------------------------
 186  */
 187 #define SEGCBLIST_ENABLED       BIT(0)
 188 #define SEGCBLIST_OFFLOADED     BIT(1)
 189
 190 struct rcu_segcblist {
 191         struct rcu_head *head;
 192         struct rcu_head **tails[RCU_CBLIST_NSEGS];
 193         unsigned long gp_seq[RCU_CBLIST_NSEGS];
 194 #ifdef CONFIG_RCU_NOCB_CPU
 195         atomic_long_t len;
 196 #else
 197         long len;
 198 #endif
 199         long seglen[RCU_CBLIST_NSEGS];
 200         u8 flags;
 201 };
 202
 203 #define RCU_SEGCBLIST_INITIALIZER(n) \
 204 { \
 205         .head = NULL, \
 206         .tails[RCU_DONE_TAIL] = &n.head, \
 207         .tails[RCU_WAIT_TAIL] = &n.head, \
 208         .tails[RCU_NEXT_READY_TAIL] = &n.head, \
 209         .tails[RCU_NEXT_TAIL] = &n.head, \
 210 }
 211
 212 #endif /* __INCLUDE_LINUX_RCU_SEGCBLIST_H */