Re: [PATCH v2 1/4] scsi: sd: Let sd_shutdown() fail future I/O

[James Bottomley <jejb@linux.ibm.com>]

On Mon, 2023-04-17 at 16:06 -0700, Bart Van Assche wrote:
> System shutdown happens as follows (see e.g. the systemd source file
> src/shutdown/shutdown.c):
> * sync() is called.
> * reboot(RB_AUTOBOOT/RB_HALT_SYSTEM/RB_POWER_OFF) is called.
> * If the reboot() system call returns, log an error message.
> 
> The reboot() system call causes the kernel to call kernel_restart(),
> kernel_halt() or kernel_power_off(). Each of these functions calls
> device_shutdown(). device_shutdown() calls sd_shutdown(). After
> sd_shutdown() has been called the .shutdown() callback of the LLD
> will be called. Hence, I/O submitted after sd_shutdown() will hang or
> may even cause a kernel crash.
> 
> Let sd_shutdown() fail future I/O such that LLD .shutdown() callbacks
> can be simplified.

What is the actual reason for this?  What is it you think might be
submitting I/O after the system gets into this state?  Current
sd_shutdown is constructed on the premise that it's the last thing that
ever happens to the device before reboot/power off which is why it
flushes the cache if necessary and stops the device if required, but
for most standard devices neither is required because we don't expect
Linux to go down with pending items in the block queue and for a write
through disk cache anything that's completed on the block queue is
safely durable on the device.

James