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procsignal.c
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1 /*-------------------------------------------------------------------------
2  *
3  * procsignal.c
4  * Routines for interprocess signalling
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 signalling 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 signalling. (We do support
44  * signalling 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 {
76  ProcSignalSlot psh_slot[FLEXIBLE_ARRAY_MEMBER];
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
172  * and 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
177  * be 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  uint64 flagbit = UINT64CONST(1) << (uint64) 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
330  * is 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
332  * to 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 function
353  * must already have current state, since the caller is responsible for
354  * making sure that the relevant state is entirely visible before calling
355  * this function in the first place. We still have to wake them up -
356  * because we can't distinguish between such backends and older backends
357  * that need to update state - but they won't actually need to change
358  * 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  kill(pid, SIGUSR1);
367  }
368 
369  return generation;
370 }
371 
372 /*
373  * WaitForProcSignalBarrier - wait until it is guaranteed that all changes
374  * requested by a specific call to EmitProcSignalBarrier() have taken effect.
375  *
376  * We expect that the barrier will normally be absorbed very quickly by other
377  * backends, so we start by waiting just 1/8 of a second and then back off
378  * by a factor of two every time we time out, to a maximum wait time of
379  * 1 second.
380  */
381 void
382 WaitForProcSignalBarrier(uint64 generation)
383 {
384  long timeout = 125L;
385 
386  for (int i = NumProcSignalSlots - 1; i >= 0; i--)
387  {
388  volatile ProcSignalSlot *slot = &ProcSignal->psh_slot[i];
389  uint64 oldval;
390 
391  oldval = pg_atomic_read_u64(&slot->pss_barrierGeneration);
392  while (oldval < generation)
393  {
394  int events;
395 
397 
398  events =
403 
404  oldval = pg_atomic_read_u64(&slot->pss_barrierGeneration);
405  if (events & WL_TIMEOUT)
406  timeout = Min(timeout * 2, 1000L);
407  }
408  }
409 
410  /*
411  * The caller is probably calling this function because it wants to
412  * read the shared state or perform further writes to shared state once
413  * all backends are known to have absorbed the barrier. However, the
414  * read of pss_barrierGeneration was performed unlocked; insert a memory
415  * barrier to separate it from whatever follows.
416  */
418 }
419 
420 /*
421  * Perform global barrier related interrupt checking.
422  *
423  * Any backend that participates in ProcSignal signalling must arrange to
424  * call this function periodically. It is called from CHECK_FOR_INTERRUPTS(),
425  * which is enough for normal backends, but not necessarily for all types of
426  * background processes.
427  */
428 void
430 {
431  uint64 generation;
432  uint32 flags;
433 
434  /* Exit quickly if there's no work to do. */
436  return;
437  ProcSignalBarrierPending = false;
438 
439  /*
440  * Read the current barrier generation, and then get the flags that
441  * are set for this backend. Note that pg_atomic_exchange_u32 is a full
442  * barrier, so we're guaranteed that the read of the barrier generation
443  * happens before we atomically extract the flags, and that any subsequent
444  * state changes happen afterward.
445  */
446  generation = pg_atomic_read_u64(&ProcSignal->psh_barrierGeneration);
447  flags = pg_atomic_exchange_u32(&MyProcSignalSlot->pss_barrierCheckMask, 0);
448 
449  /*
450  * Process each type of barrier. It's important that nothing we call from
451  * here throws an error, because pss_barrierCheckMask has already been
452  * cleared. If we jumped out of here before processing all barrier types,
453  * then we'd forget about the need to do so later.
454  *
455  * NB: It ought to be OK to call the barrier-processing functions
456  * unconditionally, but it's more efficient to call only the ones that
457  * might need us to do something based on the flags.
458  */
461 
462  /*
463  * State changes related to all types of barriers that might have been
464  * emitted have now been handled, so we can update our notion of the
465  * generation to the one we observed before beginning the updates. If
466  * things have changed further, it'll get fixed up when this function is
467  * next called.
468  */
469  pg_atomic_write_u64(&MyProcSignalSlot->pss_barrierGeneration, generation);
470 }
471 
472 static void
474 {
475  /*
476  * XXX. This is just a placeholder until the first real user of this
477  * machinery gets committed. Rename PROCSIGNAL_BARRIER_PLACEHOLDER to
478  * PROCSIGNAL_BARRIER_SOMETHING_ELSE where SOMETHING_ELSE is something
479  * appropriately descriptive. Get rid of this function and instead have
480  * ProcessBarrierSomethingElse. Most likely, that function should live
481  * in the file pertaining to that subsystem, rather than here.
482  */
483 }
484 
485 /*
486  * CheckProcSignal - check to see if a particular reason has been
487  * signaled, and clear the signal flag. Should be called after receiving
488  * SIGUSR1.
489  */
490 static bool
492 {
493  volatile ProcSignalSlot *slot = MyProcSignalSlot;
494 
495  if (slot != NULL)
496  {
497  /* Careful here --- don't clear flag if we haven't seen it set */
498  if (slot->pss_signalFlags[reason])
499  {
500  slot->pss_signalFlags[reason] = false;
501  return true;
502  }
503  }
504 
505  return false;
506 }
507 
508 /*
509  * CheckProcSignalBarrier - check for new barriers we need to absorb
510  */
511 static bool
513 {
514  volatile ProcSignalSlot *slot = MyProcSignalSlot;
515 
516  if (slot != NULL)
517  {
518  uint64 mygen;
519  uint64 curgen;
520 
522  curgen = pg_atomic_read_u64(&ProcSignal->psh_barrierGeneration);
523  return (mygen != curgen);
524  }
525 
526  return false;
527 }
528 
529 /*
530  * procsignal_sigusr1_handler - handle SIGUSR1 signal.
531  */
532 void
534 {
535  int save_errno = errno;
536 
539 
542 
545 
548 
551 
554 
557 
560 
563 
566 
568  {
569  InterruptPending = true;
571  }
572 
573  SetLatch(MyLatch);
574 
576 
577  errno = save_errno;
578 }
uint64 EmitProcSignalBarrier(ProcSignalBarrierType type)
Definition: procsignal.c:321
void RecoveryConflictInterrupt(ProcSignalReason reason)
Definition: postgres.c:2872
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Definition: globals.c:40
#define PG_UINT64_MAX
Definition: c.h:446
#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:429
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Definition: procsignal.c:473
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Definition: elog.h:26
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Definition: procsignal.c:63
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Definition: atomics.h:438
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Definition: latch.c:344
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Definition: procsignal.c:76
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Definition: procsignal.c:202
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Definition: procsignal.c:250
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Definition: atomics.h:415
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Definition: shmem.c:392
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Definition: procsignal.c:64
unsigned int uint32
Definition: c.h:359
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Definition: procsignal.c:102
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Definition: shmem.c:515
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Definition: backendid.h:23
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Definition: postgres.h:367
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Definition: shmem.c:498
int BackendId
Definition: backendid.h:21
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Definition: procsignal.c:90
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Definition: procsignal.c:491
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Definition: atomics.h:145
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Definition: procsignal.c:512
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ProcSignalBarrierType
Definition: procsignal.h:48
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ProcSignalReason
Definition: procsignal.h:30
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Definition: procsignal.c:116
static volatile ProcSignalSlot * MyProcSignalSlot
Definition: procsignal.c:91
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Definition: procsignal.c:84
struct Latch * MyLatch
Definition: globals.c:54
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Definition: atomics.h:372
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Definition: parallel.c:957
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Definition: procsignal.c:382
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