tdb: test and resolultion for tdb_lockall starvation.

ccan/tdb/lock.c

@@ -485,11 +485,9 @@ int tdb_transaction_unlock(struct tdb_context *tdb, int ltype)
 	return tdb_nest_unlock(tdb, TRANSACTION_LOCK, ltype, false);
 }
 
-
-/* lock/unlock entire database.  It can only be upgradable if you have some
- * other way of guaranteeing exclusivity (ie. transaction write lock). */
-int tdb_allrecord_lock(struct tdb_context *tdb, int ltype,
-		       enum tdb_lock_flags flags, bool upgradable)
+/* Returns 0 if all done, -1 if error, 1 if ok. */
+static int tdb_allrecord_check(struct tdb_context *tdb, int ltype,
+			       enum tdb_lock_flags flags, bool upgradable)
 {
 	/* There are no locks on read-only dbs */
 	if (tdb->read_only || tdb->traverse_read) {
@@ -519,6 +517,20 @@ int tdb_allrecord_lock(struct tdb_context *tdb, int ltype,
 		tdb->ecode = TDB_ERR_LOCK;
 		return -1;
 	}
+	return 1;
+}
+
+/* lock/unlock entire database.  It can only be upgradable if you have some
+ * other way of guaranteeing exclusivity (ie. transaction write lock). */
+int tdb_allrecord_lock(struct tdb_context *tdb, int ltype,
+		       enum tdb_lock_flags flags, bool upgradable)
+{
+	switch (tdb_allrecord_check(tdb, ltype, flags, upgradable)) {
+	case -1:
+		return -1;
+	case 0:
+		return 0;
+	}
 
 	if (tdb_brlock(tdb, ltype, FREELIST_TOP, 0, flags)) {
 		if (flags & TDB_LOCK_WAIT) {
@@ -648,6 +660,85 @@ int tdb_unlockall_read(struct tdb_context *tdb)
 	return tdb_allrecord_unlock(tdb, F_RDLCK, false);
 }
 
+/* We only need to lock individual bytes, but Linux merges consecutive locks
+ * so we lock in contiguous ranges. */
+static int tdb_chainlock_gradual(struct tdb_context *tdb,
+				 size_t off, size_t len)
+{
+	int ret;
+
+	if (len <= 4) {
+		/* Single record.  Just do blocking lock. */
+		return tdb_brlock(tdb, F_WRLCK, off, len, TDB_LOCK_WAIT);
+	}
+
+	/* First we try non-blocking. */
+	ret = tdb_brlock(tdb, F_WRLCK, off, len, TDB_LOCK_NOWAIT);
+	if (ret == 0) {
+		return 0;
+	}
+
+	/* Try locking first half, then second. */
+	ret = tdb_chainlock_gradual(tdb, off, len / 2);
+	if (ret == -1)
+		return -1;
+
+	ret = tdb_chainlock_gradual(tdb, off + len / 2, len - len / 2);
+	if (ret == -1) {
+		tdb_brunlock(tdb, F_WRLCK, off, len / 2);
+		return -1;
+	}
+	return 0;
+}
+
+/* We do the locking gradually to avoid being starved by smaller locks. */
+int tdb_lockall_gradual(struct tdb_context *tdb)
+{
+	int ret;
+
+	/* This checks for other locks, nesting. */
+	ret = tdb_allrecord_check(tdb, F_WRLCK, TDB_LOCK_WAIT, false);
+	if (ret == -1 || ret == 0)
+		return ret;
+
+	/* We cover two kinds of locks:
+	 * 1) Normal chain locks.  Taken for almost all operations.
+	 * 3) Individual records locks.  Taken after normal or free
+	 *    chain locks.
+	 *
+	 * It is (1) which cause the starvation problem, so we're only
+	 * gradual for that. */
+	if (tdb_chainlock_gradual(tdb, FREELIST_TOP,
+				  tdb->header.hash_size * 4) == -1) {
+		return -1;
+	}
+
+	/* Grab individual record locks. */
+	if (tdb_brlock(tdb, F_WRLCK, lock_offset(tdb->header.hash_size), 0,
+		       TDB_LOCK_WAIT) == -1) {
+		tdb_brunlock(tdb, F_WRLCK, FREELIST_TOP,
+			     tdb->header.hash_size * 4);
+		return -1;
+	}
+
+	/* That adds up to an allrecord lock. */
+	tdb->allrecord_lock.count = 1;
+	tdb->allrecord_lock.ltype = F_WRLCK;
+	tdb->allrecord_lock.off = false;
+
+	/* Just check we don't need recovery... */
+	if (tdb_needs_recovery(tdb)) {
+		tdb_allrecord_unlock(tdb, F_WRLCK, false);
+		if (tdb_lock_and_recover(tdb) == -1) {
+			return -1;
+		}
+		/* Try again. */
+		return tdb_lockall_gradual(tdb);
+	}
+
+	return 0;
+}
+
 /* lock/unlock one hash chain. This is meant to be used to reduce
    contention - it cannot guarantee how many records will be locked */
 int tdb_chainlock(struct tdb_context *tdb, TDB_DATA key)

ccan/tdb/tdb.h

@@ -132,6 +132,7 @@ int tdb_lockall_read_nonblock(struct tdb_context *tdb);
 int tdb_unlockall_read(struct tdb_context *tdb);
 int tdb_lockall_mark(struct tdb_context *tdb);
 int tdb_lockall_unmark(struct tdb_context *tdb);
+int tdb_lockall_gradual(struct tdb_context *tdb);
 const char *tdb_name(struct tdb_context *tdb);
 int tdb_fd(struct tdb_context *tdb);
 tdb_log_func tdb_log_fn(struct tdb_context *tdb);

ccan/tdb/tools/Makefile

@@ -2,7 +2,7 @@ LDLIBS:=../../tdb.o
 CFLAGS:=-I../../.. -Wall -O3 #-g -pg
 LDFLAGS:=-L../../..
 
-default: replay_trace tdbtorture tdbdump tdbtool
+default: replay_trace tdbtorture tdbdump tdbtool starvation
 
 benchmark: replay_trace
 	@trap "rm -f /tmp/trace.$$$$" 0; for f in benchmarks/*.rz; do if runzip -k $$f -o /tmp/trace.$$$$ && echo -n "$$f": && ./replay_trace --quiet -n 5 replay.tdb /tmp/trace.$$$$ && rm /tmp/trace.$$$$; then rm -f /tmp/trace.$$$$; else exit 1; fi; done

ccan/tdb/tools/starvation.c

+/* Demonstrate starvation of tdb_lockall */
+#include <ccan/tdb/tdb.h>
+#include <err.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <time.h>
+#include <stdlib.h>
+#include <stdbool.h>
+#include <stdarg.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/time.h>
+
+static void usage(const char *extra)
+{
+	errx(1, "%s%s"
+	     "Usage: starvation [lockall|gradual] <num> <worktime-in-ms>\n"
+	     "  Each locker holds lock for between 1/2 and 1 1/2 times\n"
+	     "  worktime, then sleeps for one second.\n\n"
+	     "  Main process tries tdb_lockall or tdb_lockall_gradual.",
+	     extra ? extra : "", extra ? "\n" : "");
+}
+
+static void run_and_sleep(struct tdb_context *tdb, int parentfd, unsigned time)
+{
+	char c;
+	struct timespec hold;
+	unsigned rand, randtime;
+	TDB_DATA key;
+
+	key.dptr = (void *)&rand;
+	key.dsize = sizeof(rand);
+
+	while (read(parentfd, &c, 1) != 0) {
+		/* Lock a random key. */
+		rand = random();
+		if (tdb_chainlock(tdb, key) != 0)
+			errx(1, "chainlock failed: %s", tdb_errorstr(tdb));
+
+		/* Hold it for some variable time. */
+		randtime = time / 2 + (random() % time);
+		hold.tv_sec = randtime / 1000;
+		hold.tv_nsec = (randtime % 1000) * 1000000;
+		nanosleep(&hold, NULL);
+
+		if (tdb_chainunlock(tdb, key) != 0)
+			errx(1, "chainunlock failed: %s", tdb_errorstr(tdb));
+
+		/* Wait for a second without the lock. */
+		sleep(1);
+	}
+	exit(0);
+}
+
+static void logfn(struct tdb_context *tdb,
+		  enum tdb_debug_level level,
+		  const char *fmt, ...)
+{
+           va_list ap;
+
+           va_start(ap, fmt);
+	   vfprintf(stderr, fmt, ap);
+	   va_end(ap);
+}
+
+int main(int argc, char *argv[])
+{
+	int (*lockall)(struct tdb_context *);
+	unsigned int num, worktime, i;
+	int pfd[2];
+	struct tdb_context *tdb;
+	struct tdb_logging_context log = { logfn, NULL };
+	struct timeval start, end, duration;
+
+	if (argc != 4)
+		usage(NULL);
+
+	if (strcmp(argv[1], "lockall") == 0)
+		lockall = tdb_lockall;
+	else if (strcmp(argv[1], "gradual") == 0)
+		lockall = tdb_lockall_gradual;
+	else
+		usage("Arg1 should be 'lockall' or 'gradual'");
+
+	num = atoi(argv[2]);
+	worktime = atoi(argv[3]);
+
+	if (!num || !worktime)
+		usage("Number of threads and worktime must be non-zero");
+
+	if (pipe(pfd) != 0)
+		err(1, "Creating pipe");
+
+	tdb = tdb_open_ex("/tmp/starvation.tdb", 10000, TDB_DEFAULT,
+			  O_RDWR|O_CREAT|O_TRUNC, 0600, &log, NULL);
+	if (!tdb)
+		err(1, "Opening tdb /tmp/starvation.tdb");
+
+	for (i = 0; i < num; i++) {
+		switch (fork()) {
+		case 0:
+			close(pfd[1]);
+			fcntl(pfd[0], F_SETFL,
+			      fcntl(pfd[0], F_GETFL)|O_NONBLOCK);
+			srandom(getpid() + i);
+			if (tdb_reopen(tdb) != 0)
+				err(1, "Reopening tdb %s", tdb_name(tdb));
+
+			run_and_sleep(tdb, pfd[0], worktime);
+		case -1:
+			err(1, "forking");
+		}
+		/* Stagger the children. */
+		usleep(random() % (1000000 / num));
+	}
+
+	close(pfd[0]);
+	sleep(1);
+	gettimeofday(&start, NULL);
+	if (lockall(tdb) != 0)
+		errx(1, "lockall failed: %s", tdb_errorstr(tdb));
+	gettimeofday(&end, NULL);
+
+	duration.tv_sec = end.tv_sec - start.tv_sec;
+	duration.tv_usec = end.tv_usec - start.tv_usec;
+	if (duration.tv_usec < 0) {
+		--duration.tv_sec;
+		duration.tv_usec += 1000000;
+	}
+
+	if (tdb_unlockall(tdb) != 0)
+		errx(1, "unlockall failed: %s", tdb_errorstr(tdb));
+	tdb_close(tdb);
+	unlink("/tmp/starvation.tdb");
+
+	printf("Took %lu.%06lu seconds\n", duration.tv_sec, duration.tv_usec);
+	return 0;
+}