grep: work around LSan threading race with barrier

Git fork

There's a race with LSan when spawning threads and one of the threads
calls die(). We worked around one such problem with index-pack in the
previous commit, but it exists in git-grep, too. You can see it with:

make SANITIZE=leak THREAD_BARRIER_PTHREAD=YesOnLinux
cd t
./t0003-attributes.sh --stress

which fails pretty quickly with:

==git==4096424==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 32 byte(s) in 1 object(s) allocated from:
#0 0x7f906de14556 in realloc ../../../../src/libsanitizer/lsan/lsan_interceptors.cpp:98
#1 0x7f906dc9d2c1 in __pthread_getattr_np nptl/pthread_getattr_np.c:180
#2 0x7f906de2500d in __sanitizer::GetThreadStackTopAndBottom(bool, unsigned long*, unsigned long*) ../../../../src/libsanitizer/sanitizer_common/sanitizer_linux_libcdep.cpp:150
#3 0x7f906de25187 in __sanitizer::GetThreadStackAndTls(bool, unsigned long*, unsigned long*, unsigned long*, unsigned long*) ../../../../src/libsanitizer/sanitizer_common/sanitizer_linux_libcdep.cpp:614
#4 0x7f906de17d18 in __lsan::ThreadStart(unsigned int, unsigned long long, __sanitizer::ThreadType) ../../../../src/libsanitizer/lsan/lsan_posix.cpp:53
#5 0x7f906de143a9 in ThreadStartFunc<false> ../../../../src/libsanitizer/lsan/lsan_interceptors.cpp:431
#6 0x7f906dc9bf51 in start_thread nptl/pthread_create.c:447
#7 0x7f906dd1a677 in __clone3 ../sysdeps/unix/sysv/linux/x86_64/clone3.S:78

As with the previous commit, we can fix this by inserting a barrier that
makes sure all threads have finished their setup before continuing. But
there's one twist in this case: the thread which calls die() is not one
of the worker threads, but the main thread itself!

So we need the main thread to wait in the barrier, too, until all
threads have gotten to it. And thus we initialize the barrier for
num_threads+1, to account for all of the worker threads plus the main
one.

If we then test as above, t0003 should run indefinitely.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

authored by

Jeff King and committed by

Junio C Hamano 1 year ago 7a8d9efc 526c0a85

1 changed file

expand all

unified split

builtin

grep.c

builtin/grep.c

··· 101 101 /* Signalled when we are finished with everything. */ 102 102 static pthread_cond_t cond_result; 103 103 104 + /* Synchronize the start of all threads */ 105 + static maybe_thread_barrier_t start_barrier; 106 + 104 107 static int skip_first_line; 105 108 106 109 static void add_work(struct grep_opt *opt, struct grep_source *gs) ··· 198 201 int hit = 0; 199 202 struct grep_opt *opt = arg; 200 203 204 + maybe_thread_barrier_wait(&start_barrier); 205 + 201 206 while (1) { 202 207 struct work_item *w = get_work(); 203 208 if (!w) ··· 229 234 pthread_cond_init(&cond_add, NULL); 230 235 pthread_cond_init(&cond_write, NULL); 231 236 pthread_cond_init(&cond_result, NULL); 237 + maybe_thread_barrier_init(&start_barrier, NULL, num_threads + 1); 232 238 grep_use_locks = 1; 233 239 enable_obj_read_lock(); 234 240 ··· 248 254 die(_("grep: failed to create thread: %s"), 249 255 strerror(err)); 250 256 } 257 + maybe_thread_barrier_wait(&start_barrier); 251 258 } 252 259 253 260 static int wait_all(void) ··· 284 291 pthread_cond_destroy(&cond_add); 285 292 pthread_cond_destroy(&cond_write); 286 293 pthread_cond_destroy(&cond_result); 294 + maybe_thread_barrier_destroy(&start_barrier); 287 295 grep_use_locks = 0; 288 296 disable_obj_read_lock(); 289 297