[PATCH] Revert "exec: make de_thread() freezable (was: Re: Linux 4.20-rc4)

[Ingo Molnar]

* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> The patch stats this week look a little bit more normal than last tim,
> probably simply because it's also a normal-sized rc4 rather than the
> unusually small rc3.

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.

Also, I'm 3 times grumpy:

 #1: I think this commit was never tested with lockdep turned on, as I 
     think splat this should trigger 100% of the time with the ELF 
     binfmt loader.

 #2: Less than 4 days passed between the commit on Nov 19 and it being 
     upstream by Nov 23. *Please* give it more testing if you change 
     something as central as fs/exec.c ...

 #3  Also, please also proof-read changelogs before committing them:

     >>  The casue is that de_thread() sleeps in TASK_UNINTERRUPTIBLE waiting for
     >>  [...]
     >>
     >>  In our machine, it causes freeze timeout as bellows.

     That's *three* typos in a single commit:

        s/casue/cause
        s/In our/On our
        s/bellows/below

     ...

Grump! :-)

Note that I haven't tested the revert yet, but the code and the breakage 
looks pretty obvious. (I'll boot the revert, will follow up if that 
didn't solve the problem.)

Thanks,

	Ingo

Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>

This reverts commit c22397888f1eed98cd59f0a88f2a5f6925f80e15.
---
 fs/exec.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index acc3a5536384..fc281b738a98 100644
--- 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;
 		}