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

* Linus Torvalds <torvalds@linux-foundation.org> 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@kernel.org>

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;
}
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