diff --git a/t/Makefile.am b/t/Makefile.am
index 6243123..46a2816 100644
--- a/t/Makefile.am
+++ b/t/Makefile.am
@@ -58,7 +58,8 @@ TESTS = t000-mirror-read.t \
 	t201-event-err.t \
 	t202-event-deny.t \
 	t203-event-null.t \
-	t204-event-allow.t
+	t204-event-allow.t \
+	t205-event-locking.t
 
 if ENABLE_VFSAPI
 TESTS += t500-vfs-mirror-basic.t \
diff --git a/t/t203-event-null.t b/t/t203-event-null.t
index 2d59002..8e8cf58 100755
--- a/t/t203-event-null.t
+++ b/t/t203-event-null.t
@@ -15,7 +15,7 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see http://www.gnu.org/licenses/ .
 
-test_description='projfs file operation permission denial tests
+test_description='projfs file operation permission null-denial tests
 
 Check that projfs file operation permission requests respond to
 denial responses caused by event handlers returning null.
diff --git a/t/t205-event-locking.t b/t/t205-event-locking.t
new file mode 100755
index 0000000..34c886f
--- /dev/null
+++ b/t/t205-event-locking.t
@@ -0,0 +1,44 @@
+#!/bin/sh
+#
+# Copyright (C) 2018-2019 GitHub, Inc.
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see http://www.gnu.org/licenses/ .
+
+test_description='projfs file operation locking tests
+
+Check that projfs file operation notification events are issued serially for a
+given path.
+'
+
+. ./test-lib.sh
+. "$TEST_DIRECTORY"/test-lib-event.sh
+
+rm lock >/dev/null 2>/dev/null
+projfs_start test_projfs_handlers source target --timeout 1 --lock-file lock || exit 1
+
+# wait_mount will trigger a projection, so we need to reset it to empty
+setfattr -n user.projection.empty -v 0x01 source
+
+test_expect_success 'test concurrent access does not trigger failure' '
+	projfs_run_twice ls target
+'
+
+projfs_stop || exit 1
+
+test_expect_success 'check no event error messages' '
+	test_must_be_empty test_projfs_handlers.err
+'
+
+test_done
+
diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
index 8fdfead..8217db4 100644
--- a/t/test-lib-functions.sh
+++ b/t/test-lib-functions.sh
@@ -754,6 +754,25 @@ projfs_start () {
 	trap projfs_stop EXIT
 }
 
+# Run the given command twice in parallel, wait for both to complete, and
+# return with 1 if at least one of the executions fails.
+projfs_run_twice () {
+	"$@" &
+	pidA="$!"
+	"$@" &
+	pidB="$!"
+
+	ret=0
+	if ! wait $pidA; then
+		ret=1
+	fi
+	if ! wait $pidB; then
+		ret=1
+	fi
+
+	return $ret
+}
+
 # Stop the projected filesystem command that was started by projfs_start()
 # and wait for its umount operation to be completed.
 projfs_stop () {
diff --git a/t/test_common.c b/t/test_common.c
index 62092f3..76803e6 100644
--- a/t/test_common.c
+++ b/t/test_common.c
@@ -101,9 +101,10 @@ static const struct retval vfsapi_retvals[] = {
 
 static const struct option all_long_opts[] = {
 	{ "help", no_argument, NULL, TEST_OPT_NUM_HELP },
-	{ "retval", required_argument, NULL, TEST_OPT_NUM_RETVAL},
-	{ "retval-file", required_argument, NULL, TEST_OPT_NUM_RETFILE},
-	{ "timeout", required_argument, NULL, TEST_OPT_NUM_TIMEOUT}
+	{ "retval", required_argument, NULL, TEST_OPT_NUM_RETVAL },
+	{ "retval-file", required_argument, NULL, TEST_OPT_NUM_RETFILE },
+	{ "timeout", required_argument, NULL, TEST_OPT_NUM_TIMEOUT },
+	{ "lock-file", required_argument, NULL, TEST_OPT_NUM_LOCKFILE },
 };
 
 struct opt_usage {
@@ -115,13 +116,15 @@ static const struct opt_usage all_opts_usage[] = {
 	{ NULL, 1 },
 	{ "allow|deny|null|<error>", 1 },
 	{ "<retval-file>", 1 },
-	{ "<max-seconds>", 1 }
+	{ "<max-seconds>", 1 },
+	{ "<lock-file>", 1 },
 };
 
 /* option values */
 static int optval_retval;
 static const char *optval_retfile;
 static long int optval_timeout;
+static const char *optval_lockfile;
 
 static unsigned int opt_set_flags = TEST_OPT_NONE;
 
@@ -351,6 +354,11 @@ void test_parse_opts(int argc, char *const argv[], unsigned int opt_flags,
 			opt_set_flags |= TEST_OPT_TIMEOUT;
 			break;
 
+		case TEST_OPT_NUM_LOCKFILE:
+			optval_lockfile = optarg;
+			opt_set_flags |= TEST_OPT_LOCKFILE;
+			break;
+
 		case '?':
 			if (optopt > 0)
 				test_exit_error(argv[0],
@@ -442,6 +450,12 @@ unsigned int test_get_opts(unsigned int opt_flags, ...)
 					*l = optval_timeout;
 				break;
 
+			case TEST_OPT_LOCKFILE:
+				s = va_arg(ap, const char**);
+				if (ret_flag != TEST_OPT_NONE)
+					*s = optval_lockfile;
+				break;
+
 			default:
 				errx(EXIT_FAILURE,
 				     "unknown option flag: %u", opt_flag);
diff --git a/t/test_common.h b/t/test_common.h
index 19382ff..f7028b2 100644
--- a/t/test_common.h
+++ b/t/test_common.h
@@ -30,11 +30,13 @@
 #define TEST_OPT_NUM_RETVAL	1
 #define TEST_OPT_NUM_RETFILE	2
 #define TEST_OPT_NUM_TIMEOUT	3
+#define TEST_OPT_NUM_LOCKFILE	4
 
 #define TEST_OPT_HELP		(0x0001 << TEST_OPT_NUM_HELP)
 #define TEST_OPT_RETVAL		(0x0001 << TEST_OPT_NUM_RETVAL)
 #define TEST_OPT_RETFILE	(0x0001 << TEST_OPT_NUM_RETFILE)
 #define TEST_OPT_TIMEOUT	(0x0001 << TEST_OPT_NUM_TIMEOUT)
+#define TEST_OPT_LOCKFILE	(0x0001 << TEST_OPT_NUM_LOCKFILE)
 
 #define TEST_OPT_NONE		0x0000
 #define TEST_OPT_VFSAPI		0x8000		// not a command-line option
diff --git a/t/test_projfs_handlers.c b/t/test_projfs_handlers.c
index 60937d2..ad6ebee 100644
--- a/t/test_projfs_handlers.c
+++ b/t/test_projfs_handlers.c
@@ -23,6 +23,7 @@
 #include <inttypes.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <unistd.h>
 
 #include "test_common.h"
 
@@ -30,11 +31,13 @@ static int test_handle_event(struct projfs_event *event, const char *desc,
 			     int proj, int perm)
 {
 	unsigned int opt_flags, ret_flags;
-	const char *retfile;
-	int ret;
+	const char *retfile, *lockfile = NULL;
+	int ret, timeout = 0, fd = 0, res;
 
-	opt_flags = test_get_opts((TEST_OPT_RETVAL | TEST_OPT_RETFILE),
-				  &ret, &ret_flags, &retfile);
+	opt_flags = test_get_opts((TEST_OPT_RETVAL | TEST_OPT_RETFILE |
+				   TEST_OPT_TIMEOUT | TEST_OPT_LOCKFILE),
+				  &ret, &ret_flags, &retfile, &timeout,
+				  &lockfile);
 
 	if ((opt_flags & TEST_OPT_RETFILE) == TEST_OPT_NONE ||
 	    (ret_flags & TEST_FILE_EXIST) != TEST_FILE_NONE) {
@@ -57,9 +60,27 @@ static int test_handle_event(struct projfs_event *event, const char *desc,
 		// TODO: hydrate file/dir based on projection list
 	}
 
+	if (lockfile) {
+		fd = open(lockfile, (O_CREAT | O_EXCL | O_RDWR), 0600);
+		if (fd == -1 && errno == EEXIST)
+			return -EEXIST;
+		else if (fd == -1)
+			return -EINVAL;
+	}
+
+	if (timeout)
+		sleep(timeout);
+
+	if (lockfile) {
+		close(fd);
+		res = unlink(lockfile);
+		if (res == -1)
+			return -EINVAL;
+	}
+
 	if ((ret_flags & TEST_VAL_SET) == TEST_VAL_UNSET)
 		ret = perm ? PROJFS_ALLOW : 0;
-	else if(!perm && ret > 0)
+	else if (!perm && ret > 0)
 		ret = 0;
 
 	return ret;
@@ -87,7 +108,8 @@ int main(int argc, char *const argv[])
 	struct projfs_handlers handlers = { 0 };
 
 	test_parse_mount_opts(argc, argv,
-			      (TEST_OPT_RETVAL | TEST_OPT_RETFILE),
+			      (TEST_OPT_RETVAL | TEST_OPT_RETFILE |
+			       TEST_OPT_TIMEOUT | TEST_OPT_LOCKFILE),
 			      &lower_path, &mount_path);
 
 	handlers.handle_proj_event = &test_proj_event;