linux-kernel - Re: [PATCH] pstore: Convert buf

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Date:   Fri, 30 Nov 2018 23:51:48 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Kees Cook <keescook@...omium.org>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        linux-efi <linux-efi@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Anton Vorontsov <anton@...msg.org>,
        Colin Cross <ccross@...roid.com>,
        Tony Luck <tony.luck@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] pstore: Convert buf_lock to semaphore

On Fri, Nov 30, 2018 at 11:48 PM Kees Cook <keescook@...omium.org> 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@...utronix.de>
> Fixes: 21b3ddd39fee ("efi: Don't use spinlocks for efi vars")
> Signed-off-by: Kees Cook <keescook@...omium.org>

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