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Subjectfsnotify_mark_srcu wtf?
We've got a report where a fanotify daemon that implements permission checks
screws up and doesn't send a reply. This then causes widespread hangs due to
fsnotify_mark_srcu read side lock being held and thus causing synchronize_srcu()
called from e.g. inotify_release()-> fsnotify_destroy_group()->
fsnotify_mark_destroy_list() to block.

Below program demonstrates the issue. It should output a single line:

close(inotify_fd): success

Instead it outputs nothing, which means that close(inotify_fd) got blocked by
the waiting permission event.

Wouldn't making the srcu per-group fix this? Would that be too expensive?

Thanks,
Miklos
---

#include <err.h>
#include <fcntl.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <unistd.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/fanotify.h>
#include <sys/inotify.h>

int main(void)
{
int fanotify_fd, inotify_fd;
int ret, pid1, pid2;
const char *testfile = "t";

ret = unlink(testfile);
if (ret == -1 && errno != ENOENT)
err(1, "unlink()");

ret = mknod(testfile, S_IFREG | 0666, 0);
if (ret == -1)
err(1, "mknod()");

fanotify_fd = fanotify_init(FAN_CLASS_PRE_CONTENT, O_RDONLY);
if (fanotify_fd == -1)
err(1, "fanotify_init()");

ret = fanotify_mark(fanotify_fd, FAN_MARK_ADD, FAN_OPEN_PERM, AT_FDCWD, testfile);
if (ret == -1)
err(1, "fanotify_mark()");

pid1 = fork();
if (pid1 == -1)
err(1, "fork()");

if (pid1 == 0) {
close(fanotify_fd);
ret = open(testfile, O_RDONLY);
if (ret == -1)
err(1, "open()");
fprintf(stderr, "something went wrong: open succeeded\n");
exit(1);
}
sleep(1);

pid2 = fork();
if (pid2 == -1)
err(1, "fork()");

if (pid2 == 0) {
close(fanotify_fd);
inotify_fd = inotify_init();
if (inotify_fd == -1)
err(1, "inotify_init()");
close(inotify_fd);
fprintf(stderr, "close(inotify_fd): success\n");
exit(0);
}

sleep(1);
kill(pid1, SIGKILL);
kill(pid2, SIGKILL);

return 0;
}

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