docs/lockcnt.txt

   1 DOCUMENTATION FOR LOCKED COUNTERS (aka QemuLockCnt)
   2 ===================================================
   3
   4 QEMU often uses reference counts to track data structures that are being
   5 accessed and should not be freed.  For example, a loop that invoke
   6 callbacks like this is not safe:
   7
   8     QLIST_FOREACH_SAFE(ioh, &io_handlers, next, pioh) {
   9         if (ioh->revents & G_IO_OUT) {
  10             ioh->fd_write(ioh->opaque);
  11         }
  12     }
  13
  14 QLIST_FOREACH_SAFE protects against deletion of the current node (ioh)
  15 by stashing away its "next" pointer.  However, ioh->fd_write could
  16 actually delete the next node from the list.  The simplest way to
  17 avoid this is to mark the node as deleted, and remove it from the
  18 list in the above loop:
  19
  20     QLIST_FOREACH_SAFE(ioh, &io_handlers, next, pioh) {
  21         if (ioh->deleted) {
  22             QLIST_REMOVE(ioh, next);
  23             g_free(ioh);
  24         } else {
  25             if (ioh->revents & G_IO_OUT) {
  26                 ioh->fd_write(ioh->opaque);
  27             }
  28         }
  29     }
  30
  31 If however this loop must also be reentrant, i.e. it is possible that
  32 ioh->fd_write invokes the loop again, some kind of counting is needed:
  33
  34     walking_handlers++;
  35     QLIST_FOREACH_SAFE(ioh, &io_handlers, next, pioh) {
  36         if (ioh->deleted) {
  37             if (walking_handlers == 1) {
  38                 QLIST_REMOVE(ioh, next);
  39                 g_free(ioh);
  40             }
  41         } else {
  42             if (ioh->revents & G_IO_OUT) {
  43                 ioh->fd_write(ioh->opaque);
  44             }
  45         }
  46     }
  47     walking_handlers--;
  48
  49 One may think of using the RCU primitives, rcu_read_lock() and
  50 rcu_read_unlock(); effectively, the RCU nesting count would take
  51 the place of the walking_handlers global variable.  Indeed,
  52 reference counting and RCU have similar purposes, but their usage in
  53 general is complementary:
  54
  55 - reference counting is fine-grained and limited to a single data
  56   structure; RCU delays reclamation of *all* RCU-protected data
  57   structures;
  58
  59 - reference counting works even in the presence of code that keeps
  60   a reference for a long time; RCU critical sections in principle
  61   should be kept short;
  62
  63 - reference counting is often applied to code that is not thread-safe
  64   but is reentrant; in fact, usage of reference counting in QEMU predates
  65   the introduction of threads by many years.  RCU is generally used to
  66   protect readers from other threads freeing memory after concurrent
  67   modifications to a data structure.
  68
  69 - reclaiming data can be done by a separate thread in the case of RCU;
  70   this can improve performance, but also delay reclamation undesirably.
  71   With reference counting, reclamation is deterministic.
  72
  73 This file documents QemuLockCnt, an abstraction for using reference
  74 counting in code that has to be both thread-safe and reentrant.
  75
  76
  77 QemuLockCnt concepts
  78 --------------------
  79
  80 A QemuLockCnt comprises both a counter and a mutex; it has primitives
  81 to increment and decrement the counter, and to take and release the
  82 mutex.  The counter notes how many visits to the data structures are
  83 taking place (the visits could be from different threads, or there could
  84 be multiple reentrant visits from the same thread).  The basic rules
  85 governing the counter/mutex pair then are the following:
  86
  87 - Data protected by the QemuLockCnt must not be freed unless the
  88   counter is zero and the mutex is taken.
  89
  90 - A new visit cannot be started while the counter is zero and the
  91   mutex is taken.
  92
  93 Most of the time, the mutex protects all writes to the data structure,
  94 not just frees, though there could be cases where this is not necessary.
  95
  96 Reads, instead, can be done without taking the mutex, as long as the
  97 readers and writers use the same macros that are used for RCU, for
  98 example atomic_rcu_read, atomic_rcu_set, QLIST_FOREACH_RCU, etc.  This is
  99 because the reads are done outside a lock and a set or QLIST_INSERT_HEAD
 100 can happen concurrently with the read.  The RCU API ensures that the
 101 processor and the compiler see all required memory barriers.
 102
 103 This could be implemented simply by protecting the counter with the
 104 mutex, for example:
 105
 106     // (1)
 107     qemu_mutex_lock(&walking_handlers_mutex);
 108     walking_handlers++;
 109     qemu_mutex_unlock(&walking_handlers_mutex);
 110
 111     ...
 112
 113     // (2)
 114     qemu_mutex_lock(&walking_handlers_mutex);
 115     if (--walking_handlers == 0) {
 116         QLIST_FOREACH_SAFE(ioh, &io_handlers, next, pioh) {
 117             if (ioh->deleted) {
 118                 QLIST_REMOVE(ioh, next);
 119                 g_free(ioh);
 120             }
 121         }
 122     }
 123     qemu_mutex_unlock(&walking_handlers_mutex);
 124
 125 Here, no frees can happen in the code represented by the ellipsis.
 126 If another thread is executing critical section (2), that part of
 127 the code cannot be entered, because the thread will not be able
 128 to increment the walking_handlers variable.  And of course
 129 during the visit any other thread will see a nonzero value for
 130 walking_handlers, as in the single-threaded code.
 131
 132 Note that it is possible for multiple concurrent accesses to delay
 133 the cleanup arbitrarily; in other words, for the walking_handlers
 134 counter to never become zero.  For this reason, this technique is
 135 more easily applicable if concurrent access to the structure is rare.
 136
 137 However, critical sections are easy to forget since you have to do
 138 them for each modification of the counter.  QemuLockCnt ensures that
 139 all modifications of the counter take the lock appropriately, and it
 140 can also be more efficient in two ways:
 141
 142 - it avoids taking the lock for many operations (for example
 143   incrementing the counter while it is non-zero);
 144
 145 - on some platforms, one can implement QemuLockCnt to hold the lock
 146   and the mutex in a single word, making the fast path no more expensive
 147   than simply managing a counter using atomic operations (see
 148   docs/atomics.txt).  This can be very helpful if concurrent access to
 149   the data structure is expected to be rare.
 150
 151
 152 Using the same mutex for frees and writes can still incur some small
 153 inefficiencies; for example, a visit can never start if the counter is
 154 zero and the mutex is taken---even if the mutex is taken by a write,
 155 which in principle need not block a visit of the data structure.
 156 However, these are usually not a problem if any of the following
 157 assumptions are valid:
 158
 159 - concurrent access is possible but rare
 160
 161 - writes are rare
 162
 163 - writes are frequent, but this kind of write (e.g. appending to a
 164   list) has a very small critical section.
 165
 166 For example, QEMU uses QemuLockCnt to manage an AioContext's list of
 167 bottom halves and file descriptor handlers.  Modifications to the list
 168 of file descriptor handlers are rare.  Creation of a new bottom half is
 169 frequent and can happen on a fast path; however: 1) it is almost never
 170 concurrent with a visit to the list of bottom halves; 2) it only has
 171 three instructions in the critical path, two assignments and a smp_wmb().
 172
 173
 174 QemuLockCnt API
 175 ---------------
 176
 177 The QemuLockCnt API is described in include/qemu/thread.h.
 178
 179
 180 QemuLockCnt usage
 181 -----------------
 182
 183 This section explains the typical usage patterns for QemuLockCnt functions.
 184
 185 Setting a variable to a non-NULL value can be done between
 186 qemu_lockcnt_lock and qemu_lockcnt_unlock:
 187
 188     qemu_lockcnt_lock(&xyz_lockcnt);
 189     if (!xyz) {
 190         new_xyz = g_new(XYZ, 1);
 191         ...
 192         atomic_rcu_set(&xyz, new_xyz);
 193     }
 194     qemu_lockcnt_unlock(&xyz_lockcnt);
 195
 196 Accessing the value can be done between qemu_lockcnt_inc and
 197 qemu_lockcnt_dec:
 198
 199     qemu_lockcnt_inc(&xyz_lockcnt);
 200     if (xyz) {
 201         XYZ *p = atomic_rcu_read(&xyz);
 202         ...
 203         /* Accesses can now be done through "p".  */
 204     }
 205     qemu_lockcnt_dec(&xyz_lockcnt);
 206
 207 Freeing the object can similarly use qemu_lockcnt_lock and
 208 qemu_lockcnt_unlock, but you also need to ensure that the count
 209 is zero (i.e. there is no concurrent visit).  Because qemu_lockcnt_inc
 210 takes the QemuLockCnt's lock, the count cannot become non-zero while
 211 the object is being freed.  Freeing an object looks like this:
 212
 213     qemu_lockcnt_lock(&xyz_lockcnt);
 214     if (!qemu_lockcnt_count(&xyz_lockcnt)) {
 215         g_free(xyz);
 216         xyz = NULL;
 217     }
 218     qemu_lockcnt_unlock(&xyz_lockcnt);
 219
 220 If an object has to be freed right after a visit, you can combine
 221 the decrement, the locking and the check on count as follows:
 222
 223     qemu_lockcnt_inc(&xyz_lockcnt);
 224     if (xyz) {
 225         XYZ *p = atomic_rcu_read(&xyz);
 226         ...
 227         /* Accesses can now be done through "p".  */
 228     }
 229     if (qemu_lockcnt_dec_and_lock(&xyz_lockcnt)) {
 230         g_free(xyz);
 231         xyz = NULL;
 232         qemu_lockcnt_unlock(&xyz_lockcnt);
 233     }
 234
 235 QemuLockCnt can also be used to access a list as follows:
 236
 237     qemu_lockcnt_inc(&io_handlers_lockcnt);
 238     QLIST_FOREACH_RCU(ioh, &io_handlers, pioh) {
 239         if (ioh->revents & G_IO_OUT) {
 240             ioh->fd_write(ioh->opaque);
 241         }
 242     }
 243
 244     if (qemu_lockcnt_dec_and_lock(&io_handlers_lockcnt)) {
 245         QLIST_FOREACH_SAFE(ioh, &io_handlers, next, pioh) {
 246             if (ioh->deleted) {
 247                 QLIST_REMOVE(ioh, next);
 248                 g_free(ioh);
 249             }
 250         }
 251         qemu_lockcnt_unlock(&io_handlers_lockcnt);
 252     }
 253
 254 Again, the RCU primitives are used because new items can be added to the
 255 list during the walk.  QLIST_FOREACH_RCU ensures that the processor and
 256 the compiler see the appropriate memory barriers.
 257
 258 An alternative pattern uses qemu_lockcnt_dec_if_lock:
 259
 260     qemu_lockcnt_inc(&io_handlers_lockcnt);
 261     QLIST_FOREACH_SAFE_RCU(ioh, &io_handlers, next, pioh) {
 262         if (ioh->deleted) {
 263             if (qemu_lockcnt_dec_if_lock(&io_handlers_lockcnt)) {
 264                 QLIST_REMOVE(ioh, next);
 265                 g_free(ioh);
 266                 qemu_lockcnt_inc_and_unlock(&io_handlers_lockcnt);
 267             }
 268         } else {
 269             if (ioh->revents & G_IO_OUT) {
 270                 ioh->fd_write(ioh->opaque);
 271             }
 272         }
 273     }
 274     qemu_lockcnt_dec(&io_handlers_lockcnt);
 275
 276 Here you can use qemu_lockcnt_dec instead of qemu_lockcnt_dec_and_lock,
 277 because there is no special task to do if the count goes from 1 to 0.