linux-kernel - Re: [PATCH] pstore: replace spinlock_t by raw_spinlock

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Message-ID: <e7b031d7-3916-4d6d-8dfd-70412c5d24b6@linux.dev>
Date: Sat, 24 Aug 2024 15:25:04 +0800
From: Wen Yang <wen.yang@...ux.dev>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: Kees Cook <kees@...nel.org>, Tony Luck <tony.luck@...el.com>,
 "Guilherme G. Piccoli" <gpiccoli@...lia.com>,
 linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] pstore: replace spinlock_t by raw_spinlock_t

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.

--
Best wishes,
Wen