linux-kernel - RE: [PATCH V4 0/3] Use efi_rts_wq to invoke EFI Runtime Services

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Date:   Sun, 27 May 2018 05:32:52 +0000
From:   "Prakhya, Sai Praneeth" <sai.praneeth.prakhya@...el.com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
CC:     "linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        Lee Chun-Yi <jlee@...e.com>, Borislav Petkov <bp@...en8.de>,
        "Luck, Tony" <tony.luck@...el.com>,
        "Will Deacon" <will.deacon@....com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Mark Rutland" <mark.rutland@....com>,
        Bhupesh Sharma <bhsharma@...hat.com>,
        "Naresh Bhat" <naresh.bhat@...aro.org>,
        "Neri, Ricardo" <ricardo.neri@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Subject: RE: [PATCH V4 0/3] Use efi_rts_wq to invoke EFI Runtime Services

> > Assume some user requested to execute some non-blocking variant of
> > efi_rts and the kernel hasn't called efi_call_virt() yet, but was
> > scheduled out. IOW, even though user requests for non-blocking efi call, we
> might still block. Am I right?
> >
> 
> No, that is the whole point. These functions may be called from atomic context,
> which is why they trylock() and give up rather than block on the semaphore if a rt
> services call is already in progress. E.g.,
> 
> /*
>  * efivar_entry_set_nonblocking - call set_variable_nonblocking()
>  *
>  * This function is guaranteed to not block and is suitable for calling
>  * from crash/panic handlers.
>  *
>  * Crucially, this function will not block if it cannot acquire
>  * efivars_lock. Instead, it returns -EBUSY.
>  */
>

One more question again, if we are sure that non-blocking variants will
_always_ be called in atomic context, then, we got it covered. Because, in
set_variable() and query_variable_info() (both blocking and non-blocking) we check
for in_atomic() and if so, we don't use efi_rts_wq (please refer to patch 3).

If you think, there might be a probability of calling non-blocking efi_rts out of atomic
context, then, sure! Let's make them never use efi_rts_wq.

Regards,
Sai