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SubjectRe: [PATCH] Revert "exec: make de_thread() freezable (was: Re: Linux 4.20-rc4)
Date
On Monday, December 3, 2018 8:47:00 AM CET Ingo Molnar wrote:
>
> * 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:

Well, sorry about that.

> #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.

It has been there in my local builds/tests, so I must have overlooked the
splat.

> #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 ...

It has been under review for quite a bit more time though and I had hoped
that there would be some reports from linux-next coverage if there were
problems with it, but nothig has been reported till now.

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

I do that as a rule and I tend to fix up such things in changelogs.

Not sure why I didn't do that this time. Because of travel perhaps.

> >> 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.)

I can queue up a revert unless anyone beats me to that.

Thanks,
Rafael

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