[RFC,2/8] thread_info: allow custom in-task thread_info
diff mbox

Message ID 1473947349-14521-3-git-send-email-mark.rutland@arm.com
State New
Headers show

Commit Message

Mark Rutland Sept. 15, 2016, 1:49 p.m. UTC
Currently, task_struct is defined in <linux/sched.h>, which (indirectly)
pulls in a number of low-level arch headers such as <asm/preempt.h>
through a number of other headers. Thus, code and structures in these
headers can't rely on the definition of task_struct. Some of these
headers are necessary for the definition of task_struct, so moving
task_struct into its own header is insufficient tio avoid circular
includes.

With CONFIG_THREAD_INFO_IN_TASK, arch code needs to implement its own
get_current() for the generic get_thread_info(). To avoid header
dependency issues, this relies on thread_info being the first member of
task_struct. This can be used by low-level arch code, as it doesn't
depend on the definition of task_struct.

For architectures without preempt-safe this_cpu ops, some data required
by low-level arch code (e.g. preempt_count) has to be stored per-thread,
and for the reasons above, we cannot place this in task_struct (or
thread_struct, since we cannot know its offset from within task_struct).
The only practical location for these is thread_info.

This patch allows architectures with CONFIG_ARCH_HAS_OWN_THREAD_INFO to
define their own thread_info, avoiding the problems described above.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: linux-kernel@vger.kernel.org
---
 include/linux/thread_info.h | 3 ++-
 init/Kconfig                | 3 +++
 2 files changed, 5 insertions(+), 1 deletion(-)

Comments

Andy Lutomirski Sept. 15, 2016, 6:37 p.m. UTC | #1
On Thu, Sep 15, 2016 at 6:49 AM, Mark Rutland <mark.rutland@arm.com> wrote:
> Currently, task_struct is defined in <linux/sched.h>, which (indirectly)
> pulls in a number of low-level arch headers such as <asm/preempt.h>
> through a number of other headers. Thus, code and structures in these
> headers can't rely on the definition of task_struct. Some of these
> headers are necessary for the definition of task_struct, so moving
> task_struct into its own header is insufficient tio avoid circular
> includes.

The flippant answer is to fix the headers, but I tried that myself and
gave up :(

But how about this slightly less duplicative alternative:

struct thread_info {
#ifdef arch_thread_info
  struct arch_thread_info arch_ti;
#endif
};
Mark Rutland Sept. 16, 2016, 10:33 a.m. UTC | #2
On Thu, Sep 15, 2016 at 11:37:47AM -0700, Andy Lutomirski wrote:
> On Thu, Sep 15, 2016 at 6:49 AM, Mark Rutland <mark.rutland@arm.com> wrote:
> > Currently, task_struct is defined in <linux/sched.h>, which (indirectly)
> > pulls in a number of low-level arch headers such as <asm/preempt.h>
> > through a number of other headers. Thus, code and structures in these
> > headers can't rely on the definition of task_struct. Some of these
> > headers are necessary for the definition of task_struct, so moving
> > task_struct into its own header is insufficient tio avoid circular
> > includes.
> 
> The flippant answer is to fix the headers, but I tried that myself and
> gave up :(

Agreed; likewise (though I gave up quicker, I suspect). :(

Longer-term I'd still hope that we can do this.

> But how about this slightly less duplicative alternative:
> 
> struct thread_info {
> #ifdef arch_thread_info
>   struct arch_thread_info arch_ti;
> #endif
> };

I'm happy to have an arch_thread_info.

Just to check, what do you mean to happen with the flags field? Should
that always be in the generic thread_info? e.g.

struct thread_info {
	u32 flags;
#ifdef arch_thread_info
	struct arch_thread_info arch_ti;
#endif
};

Thanks,
Mark,
Andy Lutomirski Sept. 16, 2016, 3:11 p.m. UTC | #3
> On Thu, Sep 15, 2016 at 11:37:47AM -0700, Andy Lutomirski wrote:
> > On Thu, Sep 15, 2016 at 6:49 AM, Mark Rutland <mark.rutland@arm.com> wrote:
> > > Currently, task_struct is defined in <linux/sched.h>, which (indirectly)
> > > pulls in a number of low-level arch headers such as <asm/preempt.h>
> > > through a number of other headers. Thus, code and structures in these
> > > headers can't rely on the definition of task_struct. Some of these
> > > headers are necessary for the definition of task_struct, so moving
> > > task_struct into its own header is insufficient tio avoid circular
> > > includes.
> >
> > The flippant answer is to fix the headers, but I tried that myself and
> > gave up :(
>
> Agreed; likewise (though I gave up quicker, I suspect). :(
>
> Longer-term I'd still hope that we can do this.
>
> > But how about this slightly less duplicative alternative:
> >
> > struct thread_info {
> > #ifdef arch_thread_info
> >   struct arch_thread_info arch_ti;
> > #endif
> > };
>
> I'm happy to have an arch_thread_info.
>
> Just to check, what do you mean to happen with the flags field? Should
> that always be in the generic thread_info? e.g.
>
> struct thread_info {
>         u32 flags;
> #ifdef arch_thread_info
>         struct arch_thread_info arch_ti;
> #endif
> };

Exactly.  Possibly with a comment that using thread_struct should be
preferred and that arch_thread_info should be used only if some header
file requires access via current_thread_info() or task_thread_info().

--Andy

>
> Thanks,
> Mark,
Mark Rutland Sept. 21, 2016, 10:28 a.m. UTC | #4
Hi Andy,

On Fri, Sep 16, 2016 at 08:11:14AM -0700, Andy Lutomirski wrote:
> > On Thu, Sep 15, 2016 at 11:37:47AM -0700, Andy Lutomirski wrote:
> > Just to check, what do you mean to happen with the flags field? Should
> > that always be in the generic thread_info? e.g.
> >
> > struct thread_info {
> >         u32 flags;
> > #ifdef arch_thread_info
> >         struct arch_thread_info arch_ti;
> > #endif
> > };
> 
> Exactly.  Possibly with a comment that using thread_struct should be
> preferred and that arch_thread_info should be used only if some header
> file requires access via current_thread_info() or task_thread_info().

While fixing up these patches, I realised that I'm somewhat concerned by 
flags becoming a u32 (where it was previously an unsigned long for
arm64).

The generic {test,set,*}_ti_thread_flag() helpers use the usual bitops,
which perform accesses of sizeof(unsigned long) at a time, and for arm64
these need to be naturally-aligned.

We happen to get that alignment from subsequent fields in task_struct
and/or thread_info, and for arm64 we don't seem to have a problem with
tearing, but it feels somewhat fragile, and leaves me uneasy.

Looking at the git log, it seems that x86 also use unsigned long until
commit affa219b60a11b32 ("x86: change thread_info's flag field back to
32 bits"), where if I'm reading correctly, this was done to get rid of
unnecessary padding. With THREAD_INFO_IN_STACK, thread_info::flags is
immediately followed by a long on x86, so we save no padding.

Given all that, can we make the generic thread_info::flags an unsigned
long, matching what the thread flag helpers implicitly assume?

Thanks,
Mark.
Andy Lutomirski Sept. 22, 2016, 10:23 p.m. UTC | #5
On Sep 21, 2016 12:28 AM, "Mark Rutland" <mark.rutland@arm.com> wrote:
>
> Hi Andy,
>
> On Fri, Sep 16, 2016 at 08:11:14AM -0700, Andy Lutomirski wrote:
> > > On Thu, Sep 15, 2016 at 11:37:47AM -0700, Andy Lutomirski wrote:
> > > Just to check, what do you mean to happen with the flags field? Should
> > > that always be in the generic thread_info? e.g.
> > >
> > > struct thread_info {
> > >         u32 flags;
> > > #ifdef arch_thread_info
> > >         struct arch_thread_info arch_ti;
> > > #endif
> > > };
> >
> > Exactly.  Possibly with a comment that using thread_struct should be
> > preferred and that arch_thread_info should be used only if some header
> > file requires access via current_thread_info() or task_thread_info().
>
> While fixing up these patches, I realised that I'm somewhat concerned by
> flags becoming a u32 (where it was previously an unsigned long for
> arm64).
>
> The generic {test,set,*}_ti_thread_flag() helpers use the usual bitops,
> which perform accesses of sizeof(unsigned long) at a time, and for arm64
> these need to be naturally-aligned.
>
> We happen to get that alignment from subsequent fields in task_struct
> and/or thread_info, and for arm64 we don't seem to have a problem with
> tearing, but it feels somewhat fragile, and leaves me uneasy.
>
> Looking at the git log, it seems that x86 also use unsigned long until
> commit affa219b60a11b32 ("x86: change thread_info's flag field back to
> 32 bits"), where if I'm reading correctly, this was done to get rid of
> unnecessary padding. With THREAD_INFO_IN_STACK, thread_info::flags is
> immediately followed by a long on x86, so we save no padding.
>
> Given all that, can we make the generic thread_info::flags an unsigned
> long, matching what the thread flag helpers implicitly assume?
>

Yes.  Want to send the patch or should I?

--Andy
Mark Rutland Sept. 23, 2016, 5:31 p.m. UTC | #6
On Thu, Sep 22, 2016 at 03:23:59PM -0700, Andy Lutomirski wrote:
> On Sep 21, 2016 12:28 AM, "Mark Rutland" <mark.rutland@arm.com> wrote:

> > Given all that, can we make the generic thread_info::flags an unsigned
> > long, matching what the thread flag helpers implicitly assume?
> 
> Yes.  Want to send the patch or should I?

I've sent a patch out to LKML [1], though if that doesn't look right I'm
more than happy for you to send a correct one. ;)

Thanks,
Mark.

[1] http://lkml.kernel.org/r/1474651447-30447-1-git-send-email-mark.rutland@arm.com

Patch
diff mbox

diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
index d9622f7..7984f87 100644
--- a/include/linux/thread_info.h
+++ b/include/linux/thread_info.h
@@ -13,7 +13,8 @@ 
 struct timespec;
 struct compat_timespec;
 
-#ifdef CONFIG_THREAD_INFO_IN_TASK
+#if defined(CONFIG_THREAD_INFO_IN_TASK) && \
+    !defined(CONFIG_ARCH_HAS_OWN_THREAD_INFO)
 struct thread_info {
 	u32			flags;		/* low level flags */
 };
diff --git a/init/Kconfig b/init/Kconfig
index 3b9a47f..f812098 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -26,6 +26,9 @@  config IRQ_WORK
 config BUILDTIME_EXTABLE_SORT
 	bool
 
+config ARCH_HAS_OWN_THREAD_INFO
+	bool
+
 config THREAD_INFO_IN_TASK
 	bool
 	help