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procsignal.c
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1 /*-------------------------------------------------------------------------
2  *
3  * procsignal.c
4  * Routines for interprocess signaling
5  *
6  *
7  * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
8  * Portions Copyright (c) 1994, Regents of the University of California
9  *
10  * IDENTIFICATION
11  * src/backend/storage/ipc/procsignal.c
12  *
13  *-------------------------------------------------------------------------
14  */
15 #include "postgres.h"
16 
17 #include <signal.h>
18 #include <unistd.h>
19 
20 #include "access/parallel.h"
21 #include "commands/async.h"
22 #include "miscadmin.h"
23 #include "pgstat.h"
24 #include "replication/walsender.h"
25 #include "storage/ipc.h"
26 #include "storage/latch.h"
27 #include "storage/proc.h"
28 #include "storage/shmem.h"
29 #include "storage/sinval.h"
30 #include "tcop/tcopprot.h"
31 
32 /*
33  * The SIGUSR1 signal is multiplexed to support signaling multiple event
34  * types. The specific reason is communicated via flags in shared memory.
35  * We keep a boolean flag for each possible "reason", so that different
36  * reasons can be signaled to a process concurrently. (However, if the same
37  * reason is signaled more than once nearly simultaneously, the process may
38  * observe it only once.)
39  *
40  * Each process that wants to receive signals registers its process ID
41  * in the ProcSignalSlots array. The array is indexed by backend ID to make
42  * slot allocation simple, and to avoid having to search the array when you
43  * know the backend ID of the process you're signaling. (We do support
44  * signaling without backend ID, but it's a bit less efficient.)
45  *
46  * The flags are actually declared as "volatile sig_atomic_t" for maximum
47  * portability. This should ensure that loads and stores of the flag
48  * values are atomic, allowing us to dispense with any explicit locking.
49  *
50  * pss_signalFlags are intended to be set in cases where we don't need to
51  * keep track of whether or not the target process has handled the signal,
52  * but sometimes we need confirmation, as when making a global state change
53  * that cannot be considered complete until all backends have taken notice
54  * of it. For such use cases, we set a bit in pss_barrierCheckMask and then
55  * increment the current "barrier generation"; when the new barrier generation
56  * (or greater) appears in the pss_barrierGeneration flag of every process,
57  * we know that the message has been received everywhere.
58  */
59 typedef struct
60 {
61  pid_t pss_pid;
62  sig_atomic_t pss_signalFlags[NUM_PROCSIGNALS];
66 
67 /*
68  * Information that is global to the entire ProcSignal system can be stored
69  * here.
70  *
71  * psh_barrierGeneration is the highest barrier generation in existence.
72  */
73 typedef struct
74 {
78 
79 /*
80  * We reserve a slot for each possible BackendId, plus one for each
81  * possible auxiliary process type. (This scheme assumes there is not
82  * more than one of any auxiliary process type at a time.)
83  */
84 #define NumProcSignalSlots (MaxBackends + NUM_AUXPROCTYPES)
85 
86 /* Check whether the relevant type bit is set in the flags. */
87 #define BARRIER_SHOULD_CHECK(flags, type) \
88  (((flags) & (((uint32) 1) << (uint32) (type))) != 0)
89 
91 static volatile ProcSignalSlot *MyProcSignalSlot = NULL;
92 
93 static bool CheckProcSignal(ProcSignalReason reason);
94 static void CleanupProcSignalState(int status, Datum arg);
95 static void ProcessBarrierPlaceholder(void);
96 
97 /*
98  * ProcSignalShmemSize
99  * Compute space needed for procsignal's shared memory
100  */
101 Size
103 {
104  Size size;
105 
106  size = mul_size(NumProcSignalSlots, sizeof(ProcSignalSlot));
107  size = add_size(size, offsetof(ProcSignalHeader, psh_slot));
108  return size;
109 }
110 
111 /*
112  * ProcSignalShmemInit
113  * Allocate and initialize procsignal's shared memory
114  */
115 void
117 {
118  Size size = ProcSignalShmemSize();
119  bool found;
120 
121  ProcSignal = (ProcSignalHeader *)
122  ShmemInitStruct("ProcSignal", size, &found);
123 
124  /* If we're first, initialize. */
125  if (!found)
126  {
127  int i;
128 
129  pg_atomic_init_u64(&ProcSignal->psh_barrierGeneration, 0);
130 
131  for (i = 0; i < NumProcSignalSlots; ++i)
132  {
133  ProcSignalSlot *slot = &ProcSignal->psh_slot[i];
134 
135  slot->pss_pid = 0;
136  MemSet(slot->pss_signalFlags, 0, sizeof(slot->pss_signalFlags));
139  }
140  }
141 }
142 
143 /*
144  * ProcSignalInit
145  * Register the current process in the procsignal array
146  *
147  * The passed index should be my BackendId if the process has one,
148  * or MaxBackends + aux process type if not.
149  */
150 void
151 ProcSignalInit(int pss_idx)
152 {
153  volatile ProcSignalSlot *slot;
154  uint64 barrier_generation;
155 
156  Assert(pss_idx >= 1 && pss_idx <= NumProcSignalSlots);
157 
158  slot = &ProcSignal->psh_slot[pss_idx - 1];
159 
160  /* sanity check */
161  if (slot->pss_pid != 0)
162  elog(LOG, "process %d taking over ProcSignal slot %d, but it's not empty",
163  MyProcPid, pss_idx);
164 
165  /* Clear out any leftover signal reasons */
166  MemSet(slot->pss_signalFlags, 0, NUM_PROCSIGNALS * sizeof(sig_atomic_t));
167 
168  /*
169  * Initialize barrier state. Since we're a brand-new process, there
170  * shouldn't be any leftover backend-private state that needs to be
171  * updated. Therefore, we can broadcast the latest barrier generation and
172  * disregard any previously-set check bits.
173  *
174  * NB: This only works if this initialization happens early enough in the
175  * startup sequence that we haven't yet cached any state that might need
176  * to be invalidated. That's also why we have a memory barrier here, to be
177  * sure that any later reads of memory happen strictly after this.
178  */
180  barrier_generation =
182  pg_atomic_write_u64(&slot->pss_barrierGeneration, barrier_generation);
184 
185  /* Mark slot with my PID */
186  slot->pss_pid = MyProcPid;
187 
188  /* Remember slot location for CheckProcSignal */
189  MyProcSignalSlot = slot;
190 
191  /* Set up to release the slot on process exit */
193 }
194 
195 /*
196  * CleanupProcSignalState
197  * Remove current process from ProcSignal mechanism
198  *
199  * This function is called via on_shmem_exit() during backend shutdown.
200  */
201 static void
203 {
204  int pss_idx = DatumGetInt32(arg);
205  volatile ProcSignalSlot *slot;
206 
207  slot = &ProcSignal->psh_slot[pss_idx - 1];
208  Assert(slot == MyProcSignalSlot);
209 
210  /*
211  * Clear MyProcSignalSlot, so that a SIGUSR1 received after this point
212  * won't try to access it after it's no longer ours (and perhaps even
213  * after we've unmapped the shared memory segment).
214  */
215  MyProcSignalSlot = NULL;
216 
217  /* sanity check */
218  if (slot->pss_pid != MyProcPid)
219  {
220  /*
221  * don't ERROR here. We're exiting anyway, and don't want to get into
222  * infinite loop trying to exit
223  */
224  elog(LOG, "process %d releasing ProcSignal slot %d, but it contains %d",
225  MyProcPid, pss_idx, (int) slot->pss_pid);
226  return; /* XXX better to zero the slot anyway? */
227  }
228 
229  /*
230  * Make this slot look like it's absorbed all possible barriers, so that
231  * no barrier waits block on it.
232  */
234 
235  slot->pss_pid = 0;
236 }
237 
238 /*
239  * SendProcSignal
240  * Send a signal to a Postgres process
241  *
242  * Providing backendId is optional, but it will speed up the operation.
243  *
244  * On success (a signal was sent), zero is returned.
245  * On error, -1 is returned, and errno is set (typically to ESRCH or EPERM).
246  *
247  * Not to be confused with ProcSendSignal
248  */
249 int
250 SendProcSignal(pid_t pid, ProcSignalReason reason, BackendId backendId)
251 {
252  volatile ProcSignalSlot *slot;
253 
254  if (backendId != InvalidBackendId)
255  {
256  slot = &ProcSignal->psh_slot[backendId - 1];
257 
258  /*
259  * Note: Since there's no locking, it's possible that the target
260  * process detaches from shared memory and exits right after this
261  * test, before we set the flag and send signal. And the signal slot
262  * might even be recycled by a new process, so it's remotely possible
263  * that we set a flag for a wrong process. That's OK, all the signals
264  * are such that no harm is done if they're mistakenly fired.
265  */
266  if (slot->pss_pid == pid)
267  {
268  /* Atomically set the proper flag */
269  slot->pss_signalFlags[reason] = true;
270  /* Send signal */
271  return kill(pid, SIGUSR1);
272  }
273  }
274  else
275  {
276  /*
277  * BackendId not provided, so search the array using pid. We search
278  * the array back to front so as to reduce search overhead. Passing
279  * InvalidBackendId means that the target is most likely an auxiliary
280  * process, which will have a slot near the end of the array.
281  */
282  int i;
283 
284  for (i = NumProcSignalSlots - 1; i >= 0; i--)
285  {
286  slot = &ProcSignal->psh_slot[i];
287 
288  if (slot->pss_pid == pid)
289  {
290  /* the above note about race conditions applies here too */
291 
292  /* Atomically set the proper flag */
293  slot->pss_signalFlags[reason] = true;
294  /* Send signal */
295  return kill(pid, SIGUSR1);
296  }
297  }
298  }
299 
300  errno = ESRCH;
301  return -1;
302 }
303 
304 /*
305  * EmitProcSignalBarrier
306  * Send a signal to every Postgres process
307  *
308  * The return value of this function is the barrier "generation" created
309  * by this operation. This value can be passed to WaitForProcSignalBarrier
310  * to wait until it is known that every participant in the ProcSignal
311  * mechanism has absorbed the signal (or started afterwards).
312  *
313  * Note that it would be a bad idea to use this for anything that happens
314  * frequently, as interrupting every backend could cause a noticeable
315  * performance hit.
316  *
317  * Callers are entitled to assume that this function will not throw ERROR
318  * or FATAL.
319  */
320 uint64
322 {
323  uint32 flagbit = 1 << (uint32) type;
324  uint64 generation;
325 
326  /*
327  * Set all the flags.
328  *
329  * Note that pg_atomic_fetch_or_u32 has full barrier semantics, so this is
330  * totally ordered with respect to anything the caller did before, and
331  * anything that we do afterwards. (This is also true of the later call to
332  * pg_atomic_add_fetch_u64.)
333  */
334  for (int i = 0; i < NumProcSignalSlots; i++)
335  {
336  volatile ProcSignalSlot *slot = &ProcSignal->psh_slot[i];
337 
339  }
340 
341  /*
342  * Increment the generation counter.
343  */
344  generation =
346 
347  /*
348  * Signal all the processes, so that they update their advertised barrier
349  * generation.
350  *
351  * Concurrency is not a problem here. Backends that have exited don't
352  * matter, and new backends that have joined since we entered this
353  * function must already have current state, since the caller is
354  * responsible for making sure that the relevant state is entirely visible
355  * before calling this function in the first place. We still have to wake
356  * them up - because we can't distinguish between such backends and older
357  * backends that need to update state - but they won't actually need to
358  * change any state.
359  */
360  for (int i = NumProcSignalSlots - 1; i >= 0; i--)
361  {
362  volatile ProcSignalSlot *slot = &ProcSignal->psh_slot[i];
363  pid_t pid = slot->pss_pid;
364 
365  if (pid != 0)
366  {
367  /* see SendProcSignal for details */
368  slot->pss_signalFlags[PROCSIG_BARRIER] = true;
369  kill(pid, SIGUSR1);
370  }
371  }
372 
373  return generation;
374 }
375 
376 /*
377  * WaitForProcSignalBarrier - wait until it is guaranteed that all changes
378  * requested by a specific call to EmitProcSignalBarrier() have taken effect.
379  *
380  * We expect that the barrier will normally be absorbed very quickly by other
381  * backends, so we start by waiting just 1/8 of a second and then back off
382  * by a factor of two every time we time out, to a maximum wait time of
383  * 1 second.
384  */
385 void
386 WaitForProcSignalBarrier(uint64 generation)
387 {
388  long timeout = 125L;
389 
390  Assert(generation <= pg_atomic_read_u64(&ProcSignal->psh_barrierGeneration));
391 
392  for (int i = NumProcSignalSlots - 1; i >= 0; i--)
393  {
394  volatile ProcSignalSlot *slot = &ProcSignal->psh_slot[i];
395  uint64 oldval;
396 
397  oldval = pg_atomic_read_u64(&slot->pss_barrierGeneration);
398  while (oldval < generation)
399  {
400  int events;
401 
403 
404  events =
409 
410  oldval = pg_atomic_read_u64(&slot->pss_barrierGeneration);
411  if (events & WL_TIMEOUT)
412  timeout = Min(timeout * 2, 1000L);
413  }
414  }
415 
416  /*
417  * The caller is probably calling this function because it wants to read
418  * the shared state or perform further writes to shared state once all
419  * backends are known to have absorbed the barrier. However, the read of
420  * pss_barrierGeneration was performed unlocked; insert a memory barrier
421  * to separate it from whatever follows.
422  */
424 }
425 
426 /*
427  * Handle receipt of an interrupt indicating a global barrier event.
428  *
429  * All the actual work is deferred to ProcessProcSignalBarrier(), because we
430  * cannot safely access the barrier generation inside the signal handler as
431  * 64bit atomics might use spinlock based emulation, even for reads. As this
432  * routine only gets called when PROCSIG_BARRIER is sent that won't cause a
433  * lot fo unnecessary work.
434  */
435 static void
437 {
438  InterruptPending = true;
440  /* latch will be set by procsignal_sigusr1_handler */
441 }
442 
443 /*
444  * Perform global barrier related interrupt checking.
445  *
446  * Any backend that participates in ProcSignal signaling must arrange to
447  * call this function periodically. It is called from CHECK_FOR_INTERRUPTS(),
448  * which is enough for normal backends, but not necessarily for all types of
449  * background processes.
450  */
451 void
453 {
454  uint64 local_gen;
455  uint64 shared_gen;
456  uint32 flags;
457 
458  Assert(MyProcSignalSlot);
459 
460  /* Exit quickly if there's no work to do. */
462  return;
463  ProcSignalBarrierPending = false;
464 
465  /*
466  * It's not unlikely to process multiple barriers at once, before the
467  * signals for all the barriers have arrived. To avoid unnecessary work in
468  * response to subsequent signals, exit early if we already have processed
469  * all of them.
470  */
471  local_gen = pg_atomic_read_u64(&MyProcSignalSlot->pss_barrierGeneration);
472  shared_gen = pg_atomic_read_u64(&ProcSignal->psh_barrierGeneration);
473 
474  Assert(local_gen <= shared_gen);
475 
476  if (local_gen == shared_gen)
477  return;
478 
479  /*
480  * Get and clear the flags that are set for this backend. Note that
481  * pg_atomic_exchange_u32 is a full barrier, so we're guaranteed that the
482  * read of the barrier generation above happens before we atomically
483  * extract the flags, and that any subsequent state changes happen
484  * afterward.
485  */
486  flags = pg_atomic_exchange_u32(&MyProcSignalSlot->pss_barrierCheckMask, 0);
487 
488  /*
489  * Process each type of barrier. It's important that nothing we call from
490  * here throws an error, because pss_barrierCheckMask has already been
491  * cleared. If we jumped out of here before processing all barrier types,
492  * then we'd forget about the need to do so later.
493  *
494  * NB: It ought to be OK to call the barrier-processing functions
495  * unconditionally, but it's more efficient to call only the ones that
496  * might need us to do something based on the flags.
497  */
500 
501  /*
502  * State changes related to all types of barriers that might have been
503  * emitted have now been handled, so we can update our notion of the
504  * generation to the one we observed before beginning the updates. If
505  * things have changed further, it'll get fixed up when this function is
506  * next called.
507  */
508  pg_atomic_write_u64(&MyProcSignalSlot->pss_barrierGeneration, shared_gen);
509 }
510 
511 static void
513 {
514  /*
515  * XXX. This is just a placeholder until the first real user of this
516  * machinery gets committed. Rename PROCSIGNAL_BARRIER_PLACEHOLDER to
517  * PROCSIGNAL_BARRIER_SOMETHING_ELSE where SOMETHING_ELSE is something
518  * appropriately descriptive. Get rid of this function and instead have
519  * ProcessBarrierSomethingElse. Most likely, that function should live in
520  * the file pertaining to that subsystem, rather than here.
521  */
522 }
523 
524 /*
525  * CheckProcSignal - check to see if a particular reason has been
526  * signaled, and clear the signal flag. Should be called after receiving
527  * SIGUSR1.
528  */
529 static bool
531 {
532  volatile ProcSignalSlot *slot = MyProcSignalSlot;
533 
534  if (slot != NULL)
535  {
536  /* Careful here --- don't clear flag if we haven't seen it set */
537  if (slot->pss_signalFlags[reason])
538  {
539  slot->pss_signalFlags[reason] = false;
540  return true;
541  }
542  }
543 
544  return false;
545 }
546 
547 /*
548  * procsignal_sigusr1_handler - handle SIGUSR1 signal.
549  */
550 void
552 {
553  int save_errno = errno;
554 
557 
560 
563 
566 
569 
572 
575 
578 
581 
584 
587 
588  SetLatch(MyLatch);
589 
591 
592  errno = save_errno;
593 }
uint64 EmitProcSignalBarrier(ProcSignalBarrierType type)
Definition: procsignal.c:321
void RecoveryConflictInterrupt(ProcSignalReason reason)
Definition: postgres.c:2896
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Definition: globals.c:40
#define PG_UINT64_MAX
Definition: c.h:461
static void HandleProcSignalBarrierInterrupt(void)
Definition: procsignal.c:436
#define WL_TIMEOUT
Definition: latch.h:127
#define DatumGetInt32(X)
Definition: postgres.h:472
#define SIGUSR1
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Definition: procsignal.c:75
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Definition: procsignal.c:452
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Definition: c.h:978
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Definition: procsignal.c:512
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Definition: elog.h:26
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Definition: latch.c:457
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Definition: procsignal.c:63
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Definition: latch.c:540
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Definition: atomics.h:438
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Definition: latch.c:365
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Definition: procsignal.c:76
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Definition: procsignal.c:202
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Definition: walsender.c:3003
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Definition: procsignal.c:250
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Definition: atomics.h:415
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Definition: procsignal.c:64
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Definition: procsignal.c:102
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Definition: shmem.c:515
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Definition: backendid.h:23
uintptr_t Datum
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Definition: backendid.h:21
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Definition: procsignal.c:90
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Definition: procsignal.c:530
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Definition: atomics.h:145
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ProcSignalBarrierType
Definition: procsignal.h:49
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ProcSignalReason
Definition: procsignal.h:30
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Definition: procsignal.c:62
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Definition: procsignal.c:116
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Definition: procsignal.c:91
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Definition: procsignal.c:84
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Definition: procsignal.c:386
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