From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [PATCH 1/2] efi: Provide a non-blocking SetVariable() operation Date: Wed, 1 Oct 2014 17:26:24 +0200 Message-ID: <20141001152624.GB2843@worktop.programming.kicks-ass.net> References: <1412165200-32141-1-git-send-email-matt@console-pimps.org> <1412165200-32141-2-git-send-email-matt@console-pimps.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <1412165200-32141-2-git-send-email-matt-HNK1S37rvNbeXh+fF434Mdi2O/JbrIOy@public.gmane.org> Sender: linux-efi-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Matt Fleming Cc: Ingo Molnar , "H. Peter Anvin" , Thomas Gleixner , linux-efi-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Ard Biesheuvel , Leif Lindholm , Matthew Garrett , Matt Fleming List-Id: linux-efi@vger.kernel.org On Wed, Oct 01, 2014 at 01:06:39PM +0100, Matt Fleming wrote: > From: Matt Fleming > > There are some circumstances that call for trying to write an EFI > variable in a non-blocking way. One such scenario is when writing pstore > data in efi_pstore_write() via the pstore_dump() kdump callback. > > Now that we have an EFI runtime spinlock we need a way of aborting if > there is contention instead of spinning, since when writing pstore data > from the kdump callback, the runtime lock may already be held by the CPU > that's running the callback if we crashed in the middle of an EFI > variable operation. > > The situation is sufficiently special that a new EFI variable operation > is warranted. > > Introduce ->set_variable_nonblocking() for this use case. It is an > optional EFI backend operation, and need only be implemented by those > backends that usually acquire locks to serialize access to EFI > variables, as is the case for virt_efi_set_variable() where we now grab > the EFI runtime spinlock. > > Cc: Peter Zijlstra > Cc: Ingo Molnar > Cc: Ard Biesheuvel > Cc: Matthew Garrett > Signed-off-by: Matt Fleming > --- > drivers/firmware/efi/runtime-wrappers.c | 19 +++++++++++++ > drivers/firmware/efi/vars.c | 47 +++++++++++++++++++++++++++++++++ > include/linux/efi.h | 6 +++++ > 3 files changed, 72 insertions(+) > > diff --git a/drivers/firmware/efi/runtime-wrappers.c b/drivers/firmware/efi/runtime-wrappers.c > index 9694cba665c4..4349206198b2 100644 > --- a/drivers/firmware/efi/runtime-wrappers.c > +++ b/drivers/firmware/efi/runtime-wrappers.c > @@ -200,6 +200,24 @@ static efi_status_t virt_efi_set_variable(efi_char16_t *name, > return status; > } > > +static efi_status_t > +virt_efi_set_variable_nonblocking(efi_char16_t *name, efi_guid_t *vendor, > + u32 attr, unsigned long data_size, > + void *data) > +{ > + unsigned long flags; > + efi_status_t status; > + > + if (!spin_trylock_irqsave(&efi_runtime_lock, flags)) > + return EFI_NOT_READY; > + > + status = efi_call_virt(set_variable, name, vendor, attr, data_size, > + data); > + spin_unlock_irqrestore(&efi_runtime_lock, flags); > + return status; > +} If you want to have this usable from NMI context, you need to convert efi_runtime_lock to a raw_spinlock_t. Also, it would probably be a good idea to have some selftest like thing that actually calls this from NMI context, right?