Re: [PATCH] pstore: replace spinlock_t by raw_spinlock_t
From: Kees Cook
Date: Mon Aug 26 2024 - 16:33:51 EST
On Sat, Aug 24, 2024 at 03:25:04PM +0800, Wen Yang wrote:
>
>
> On 2024/8/20 01:45, Greg KH wrote:
> > On Mon, Aug 19, 2024 at 10:59:45PM +0800, Wen Yang wrote:
> > > pstore_dump() is called when both preemption and local IRQ are disabled,
> > > and a spinlock is obtained, which is problematic for the RT kernel because
> > > in this configuration, spinlocks are sleep locks.
> > >
> > > Replace the spinlock_t with raw_spinlock_t to avoid sleeping in atomic context.
> >
> > This feels odd, is it only an out-of-tree RT thing? Or does this affect
> > in-kernel code as well? What prevents any normal spinlock from sleeping
> > in your system configuration as well?
> >
>
> Thank you for your comment.
>
> If we enable PREEMPT_RT, it will also affect in-kernel code, such as in the
> following scenario:
>
> echo b > /proc/sysrq-trigger
> - sysrq_handle_reboot
> - emergency_restart
> - kmsg_dump
> - pstore_dump
> Obtained psinfo->buf_lock, if there is a lot of error log in the kernel, it
> will last for a long time
>
> If the system unexpectedly crashes at this time:
> - panic()
> - kmsg_dump
> - pstore_dump
> Attempting to obtain psinfo->buf_lock while disabling interrupts and
> preemption, but since this lock is already held by the above process, it
> will result in illegal sleep.
Reading Documentation/locking/locktypes.rst seems to suggest pstore does
want the raw version. I'm surprised there aren't many more cases where
this is a problem. :P
--
Kees Cook