Revert "exec: make de_thread() freezable"

Revert commit c22397888f1e "exec: make de_thread() freezable" as
requested by Ingo Molnar:

"So there's a new regression in v4.20-rc4, my desktop produces this
lockdep splat:

[ 1772.588771] WARNING: pkexec/4633 still has locks held!
[ 1772.588773] 4.20.0-rc4-custom-00213-g93a49841322b #1 Not tainted
[ 1772.588775] ------------------------------------
[ 1772.588776] 1 lock held by pkexec/4633:
[ 1772.588778]  #0: 00000000ed85fbf8 (&sig->cred_guard_mutex){+.+.}, at: prepare_bprm_creds+0x2a/0x70
[ 1772.588786] stack backtrace:
[ 1772.588789] CPU: 7 PID: 4633 Comm: pkexec Not tainted 4.20.0-rc4-custom-00213-g93a49841322b #1
[ 1772.588792] Call Trace:
[ 1772.588800]  dump_stack+0x85/0xcb
[ 1772.588803]  flush_old_exec+0x116/0x890
[ 1772.588807]  ? load_elf_phdrs+0x72/0xb0
[ 1772.588809]  load_elf_binary+0x291/0x1620
[ 1772.588815]  ? sched_clock+0x5/0x10
[ 1772.588817]  ? search_binary_handler+0x6d/0x240
[ 1772.588820]  search_binary_handler+0x80/0x240
[ 1772.588823]  load_script+0x201/0x220
[ 1772.588825]  search_binary_handler+0x80/0x240
[ 1772.588828]  __do_execve_file.isra.32+0x7d2/0xa60
[ 1772.588832]  ? strncpy_from_user+0x40/0x180
[ 1772.588835]  __x64_sys_execve+0x34/0x40
[ 1772.588838]  do_syscall_64+0x60/0x1c0

The warning gets triggered by an ancient lockdep check in the freezer:

(gdb) list *0xffffffff812ece06
0xffffffff812ece06 is in flush_old_exec (./include/linux/freezer.h:57).
52	 * DO NOT ADD ANY NEW CALLERS OF THIS FUNCTION
53	 * If try_to_freeze causes a lockdep warning it means the caller may deadlock
54	 */
55	static inline bool try_to_freeze_unsafe(void)
56	{
57		might_sleep();
58		if (likely(!freezing(current)))
59			return false;
60		return __refrigerator(false);
61	}

I reviewed the ->cred_guard_mutex code, and the mutex is held across all
of exec() - and we always did this.

But there's this recent -rc4 commit:

> Chanho Min (1):
>       exec: make de_thread() freezable

  c22397888f1e: exec: make de_thread() freezable

I believe this commit is bogus, you cannot call try_to_freeze() from
de_thread(), because it's holding the ->cred_guard_mutex."

Reported-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

diff --git a/fs/exec.c b/fs/exec.c
index acc3a5536384cf65c6742d918cc2bd9d42305da3..fc281b738a9822a652f7d19bb60ae15acd7a7ebf 100644 (file)
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -62,7 +62,6 @@
 #include <linux/oom.h>
 #include <linux/compat.h>
 #include <linux/vmalloc.h>
-#include <linux/freezer.h>
 
 #include <linux/uaccess.h>
 #include <asm/mmu_context.h>
@@ -1084,7 +1083,7 @@ static int de_thread(struct task_struct *tsk)
        while (sig->notify_count) {
                __set_current_state(TASK_KILLABLE);
                spin_unlock_irq(lock);
-               freezable_schedule();
+               schedule();
                if (unlikely(__fatal_signal_pending(tsk)))
                        goto killed;
                spin_lock_irq(lock);
@@ -1112,7 +1111,7 @@ static int de_thread(struct task_struct *tsk)
                        __set_current_state(TASK_KILLABLE);
                        write_unlock_irq(&tasklist_lock);
                        cgroup_threadgroup_change_end(tsk);
-                       freezable_schedule();
+                       schedule();
                        if (unlikely(__fatal_signal_pending(tsk)))
                                goto killed;
                }