Re: [PATCH] pstore: Convert buf_lock to semaphore

[Arnd Bergmann]

On Fri, Nov 30, 2018 at 11:48 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> Instead of running with interrupts disabled, use a semaphore. This should
> make it easier for backends that may need to sleep (e.g. EFI) when
> performing a write:
>
> |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
> ...
>
> Reported-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> Fixes: 21b3ddd39fee ("efi: Don't use spinlocks for efi vars")
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>

Hmm, I've actually been working on a patch set recently to deprecate
all semaphores from the kernel and replace them with something
else as much as possible.

Why can't this be a mutex instead?

      Arnd