Re: [PATCH] pstore: Convert buf_lock to semaphore

[Arnd Bergmann]

On Mon, Dec 3, 2018 at 12:18 PM Sebastian Andrzej Siewior
<bigeasy@xxxxxxxxxxxxx> wrote:
>
> On 2018-12-01 09:42:38 [+0100], Arnd Bergmann wrote:
> > You are right that you can't take (or release) a mutex from interrupt
> > context. However, I don't think converting a spinlock to a semaphore
> > is going to help here either.
>
> you can acquire a semaphore with a try_lock from interrupts context but
> you can't do that with a mutex. You can also a acquire a semaphore in
> one context and release in another.

Right, that is the obvious part.

> I haven't looked a general picture yet, will try to do so later today or
> tomorrow.

To speed this up, the problem I'm referring to is in
virt_efi_query_variable_info() and efi_queue_work(),
as in the original BUG_ON() that Kees quoted:

> BUG: sleeping function called from invalid context at kernel/sched/completion.c:99
> |in_atomic(): 1, irqs_disabled(): 1, pid: 2236, name: sig-xstate-bum
> |Preemption disabled at:
> |[<ffffffff99d60512>] pstore_dump+0x72/0x330
> |CPU: 26 PID: 2236 Comm: sig-xstate-bum Tainted: G      D           4.20.0-rc3 #45
> |Call Trace:
> | dump_stack+0x4f/0x6a
> | ___might_sleep.cold.91+0xd3/0xe4
> | __might_sleep+0x50/0x90
> | wait_for_completion+0x32/0x130
> | virt_efi_query_variable_info+0x14e/0x160
> | efi_query_variable_store+0x51/0x1a0
> | efivar_entry_set_safe+0xa3/0x1b0
> | efi_pstore_write+0x109/0x140
> | pstore_dump+0x11c/0x330
> | kmsg_dump+0xa4/0xd0
> | oops_exit+0x22/0x30

This will no longer happen when pstore is called from process
context with his patch, but we still get the same thing if we call
pstore from interrupt context, unless both the down_interruptible
and wait_for_completion in there are also changed to
nonblocking calls. However, once they are no longer blocking,
we don't need the outer lock to be changed from spinlock
to semaphore any more either.

        Arnd