Re: [musl] BUG: Calling readdir/dirfd after vfork will cause deadlock.

[Nick Peng <pymumu@gmail.com>]

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The feature I want to achieve is to close all file handles after fork to
avoid file handles being inherited to child processes.

We have a component which is used in a process that uses too much memory
(about 50BG) and uses too many file handles(about 100,000 files).
In order to avoid file handles inheriting from child processes, we
implemented a API like system,close all file handles after vfork.
We can't close all file handles by iterating over all file handles number,
because the maximum number of files limit is set very large, the
performance of this method is too poor.
And we can't use fork, fork will fail because of vm.overcommit_memory.
In order to optimize this, the method we used is to read the file handes in
/proc/self/fd directory and close the file handles one by one.

The component is based on glibc, currently this program runs for about 10
years without deadlock.
Recently, this component is used in musl-based embedded systems, and it
hangs often.

After reading the vfork manual and the guidance you gave, i understand
calling readdir after vfork is problematic. and I also learned that I can
call getent64 to achieve this
but what I want to say is that, based on glibc, after calling readdir after
vfork, there is no deadlock problem at present.

more information: I googled and found this(https://lwn.net/Articles/789023/),
probably the best solution, but it seems that the required kernel version
is too new for my program to work with.

However, I think my problem is solved anyway, thank you all.

On Sun, Jun 26, 2022 at 2:49 AM Szabolcs Nagy <nsz@port70.net> wrote:

> * Nick Peng <pymumu@gmail.com> [2022-06-25 11:40:17 +0800]:
> > Description:  After vfork, calling functions such as readdir/dirfd may
> > cause deadlock. GNU C is OK.
>
> why do you think "GNU C is OK"? is this from some real software?
>
> opendir after vfork is documented to be invalid in glibc:
>
> https://www.gnu.org/software/libc/manual/html_mono/libc.html#Low_002dlevel-Directory-Access
>
> the standard is actually much stricter than the glibc manual:
> posix conforming code must not call any libc api after vfork
> other than _exit or the exec* familiy of functions.
> (as-safe is not enough, but opendir is not even as-safe)
>
> since the example is multi-threaded even using fork would
> be invalid, but i think both musl and glibc makes that work
> (as an extension to the standard).
>
>
> >                      Also tested on x86-64 with musl, no deadlock, but
> > seems never exit, slower than GNU C.
> > Version: latest, musl-1.2.3
> > OS: debian bullseye 64bit OS. and asus router
> > CPU: raspberrypi aarch64， mips32
> > Reproduce Code:
> >
> > #include <dirent.h>
> > #include <pthread.h>
> > #include <stdint.h>
> > #include <stdio.h>
> > #include <stdlib.h>
> > #include <sys/types.h>
> > #include <sys/wait.h>
> > #include <unistd.h>
> > #include <string.h>
> >
> > pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
> > pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
> >
> > struct tlog_log *logs = NULL;
> > int do_exit = 0;
> >
> > void *test(void *arg)
> > {
> >     int i = 0;
> >
> >     for (i = 0; i < 10000000; i++) {
> >         char *b = malloc(4096);
> >         memset(b, 0, 4096);
> >         free(b);
> >     }
> >     do_exit = 1;
> >     return NULL;
> > }
> >
> > void lockfunc()
> > {
> >     char path_name[4096];
> >     DIR *dir = NULL;
> >     struct dirent *ent;
> >
> >     snprintf(path_name, sizeof(path_name), "/proc/self/fd/");
> >     dir = opendir(path_name);
> >     if (dir == NULL) {
> >         goto errout;
> >     }
> >
> >     while ((ent = readdir(dir)) != NULL) {
> >     }
> >
> >     closedir(dir);
> >
> >     return;
> > errout:
> >     if (dir) {
> >         closedir(dir);
> >     }
> >
> >     return;
> > }
> >
> > void *test_fork(void *arg)
> > {
> >     int count = 0;
> >     while (do_exit == 0) {
> >         printf("test fork count %d\n", count++);
> >         int pid = vfork();
> >         if (pid < 0) {
> >             return NULL;
> >         } else if (pid == 0) {
> >             lockfunc();
> >             _exit(0);
> >         }
> >
> >         int status;
> >         waitpid(pid, &status, 0);
> >     }
> >
> >     return NULL;
> > }
> >
> > int main(int argc, char *argv[])
> > {
> >     pthread_attr_t attr;
> >     pthread_t threads[10];
> >     pthread_t fork_test;
> >     int i;
> >     int ret;
> >
> >     pthread_attr_init(&attr);
> >
> >     ret = pthread_create(&fork_test, &attr, test_fork, NULL);
> >
> >     for (i = 0; i < 10; i++) {
> >         ret = pthread_create(&threads[i], &attr, test, NULL);
> >         if (ret != 0) {
> >             return 1;
> >         }
> >     }
> >
> >     for (i = 0; i < 10; i++) {
> >         void *retval = NULL;
> >         pthread_join(threads[i], &retval);
> >     }
> >
> >     void *retval = NULL;
> >     pthread_join(fork_test, &retval);
> >     printf("exit\n");
> >     getchar();
> >     return 0;
> > }
> >
> >
> > Log：
> > pi@raspberrypi:~/code/tinylog/test $ ./test
> > test fork count 0
> > test fork count 1   <-- lock here
> > ^C
> >
> > gdb backtrace:
> > x0000000000409524 in __lock ()
> > (gdb) bt
> > #0  0x0000000000409524 in __lock ()
> > #1  0x0000000000406278 in __libc_malloc_impl ()
>

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