[RFC PATCH] x86/delay: Do not use cpu_tss in preemptible ctxt in delay_mwaitx()

[RFC PATCH] x86/delay: Do not use cpu_tss in preemptible ctxt in delay_mwaitx()

[Borislav Petkov]

From: Borislav Petkov <bp@suse.de>

So Andy had a good idea about using a cacheline-aligned, seldomly used
per-cpu var as the MONITORX target but we can't use it in preemptible
context. The first simple idea I have is to disable preemption around us
dereffing it.

Better ideas?

Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Huang Rui <ray.huang@amd.com>
Cc: spg_linux_kernel@amd.com
---
 arch/x86/lib/delay.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/arch/x86/lib/delay.c b/arch/x86/lib/delay.c
index e912b2f6d36e..c1810afcd2ea 100644
--- a/arch/x86/lib/delay.c
+++ b/arch/x86/lib/delay.c
@@ -92,17 +92,22 @@ static void delay_tsc(unsigned long __loops)
 static void delay_mwaitx(unsigned long __loops)
 {
 	u64 start, end, delay, loops = __loops;
+	struct tss_struct *t;
+
+	/*
+	 * Use cpu_tss as a cacheline-aligned, seldomly accessed per-cpu
+	 * variable as the monitor target.
+	 */
+	preempt_disable();
+	t = this_cpu_ptr(&cpu_tss);
+	preempt_enable();
 
 	start = rdtsc_ordered();
 
 	for (;;) {
 		delay = min_t(u64, MWAITX_MAX_LOOPS, loops);
 
-		/*
-		 * Use cpu_tss as a cacheline-aligned, seldomly
-		 * accessed per-cpu variable as the monitor target.
-		 */
-		__monitorx(this_cpu_ptr(&cpu_tss), 0, 0);
+		__monitorx(t, 0, 0);
 
 		/*
 		 * AMD, like Intel, supports the EAX hint and EAX=0xf
-- 
2.3.5

Re: [RFC PATCH] x86/delay: Do not use cpu_tss in preemptible ctxt in delay_mwaitx()

[Andy Lutomirski]

On Mar 9, 2016 3:38 AM, "Borislav Petkov" <bp@alien8.de> wrote:
>
> From: Borislav Petkov <bp@suse.de>
>
> So Andy had a good idea about using a cacheline-aligned, seldomly used
> per-cpu var as the MONITORX target but we can't use it in preemptible
> context. The first simple idea I have is to disable preemption around us
> dereffing it.

What's the actual problem?  Is it the preempt warnings and, if so,
would raw_cpu_ptr fix it?

It may pay to move it into the loop, though.

--Andy

Re: [RFC PATCH] x86/delay: Do not use cpu_tss in preemptible ctxt in delay_mwaitx()

[Borislav Petkov]

On Wed, Mar 09, 2016 at 09:56:39AM -0800, Andy Lutomirski wrote:
> On Mar 9, 2016 3:38 AM, "Borislav Petkov" <bp@alien8.de> wrote:
> >
> > From: Borislav Petkov <bp@suse.de>
> >
> > So Andy had a good idea about using a cacheline-aligned, seldomly used
> > per-cpu var as the MONITORX target but we can't use it in preemptible
> > context. The first simple idea I have is to disable preemption around us
> > dereffing it.
> 
> What's the actual problem?  Is it the preempt warnings and, if so,
> would raw_cpu_ptr fix it?

Yeah, it is the warning:

[    1.565876] BUG: using smp_processor_id() in preemptible [00000000] code: udevd/312
[    1.566123] caller is delay_mwaitx+0x40/0xa0

and yes, I think so. I don't think we care about being in preemptible
context since we're going idle anyway and doesn't matter which cpu_tss
we touch.

Yeah, I'll use raw_cpu_ptr...

> It may pay to move it into the loop, though.

... and won't need to do that.

Thanks for the idea.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.

[PATCH] x86/delay: Avoid preemptible context checks in delay_mwaitx()

[Borislav Petkov]

We do use this_cpu_ptr(&cpu_tss) as a cacheline-aligned, seldomly
accessed per-cpu var as the MONITORX target in delay_mwaitx(). However,
when called in preemptible context, this_cpu_ptr -> smp_processor_id() ->
debug_smp_processor_id() fires:

  BUG: using smp_processor_id() in preemptible [00000000] code: udevd/312
  caller is delay_mwaitx+0x40/0xa0

But we don't care about that check - we only need cpu_tss as a MONITORX
target and it doesn't really matter which CPU's var we're touching as
we're going idle anyway. Fix that.

Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Huang Rui <ray.huang@amd.com>
Cc: spg_linux_kernel@amd.com
---
 arch/x86/lib/delay.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/lib/delay.c b/arch/x86/lib/delay.c
index e912b2f6d36e..2f07c291dcc8 100644
--- a/arch/x86/lib/delay.c
+++ b/arch/x86/lib/delay.c
@@ -102,7 +102,7 @@ static void delay_mwaitx(unsigned long __loops)
 		 * Use cpu_tss as a cacheline-aligned, seldomly
 		 * accessed per-cpu variable as the monitor target.
 		 */
-		__monitorx(this_cpu_ptr(&cpu_tss), 0, 0);
+		__monitorx(raw_cpu_ptr(&cpu_tss), 0, 0);
 
 		/*
 		 * AMD, like Intel, supports the EAX hint and EAX=0xf
-- 
2.3.5


-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.

[tip:x86/urgent] x86/delay: Avoid preemptible context checks in delay_mwaitx()

[tip-bot for Borislav Petkov]

Commit-ID:  84477336ec03f8061ffd6908da341e063e5d6d1f
Gitweb:     http://git.kernel.org/tip/84477336ec03f8061ffd6908da341e063e5d6d1f
Author:     Borislav Petkov <bp@alien8.de>
AuthorDate: Wed, 9 Mar 2016 21:56:22 +0100
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Thu, 10 Mar 2016 11:27:12 +0100

x86/delay: Avoid preemptible context checks in delay_mwaitx()

We do use this_cpu_ptr(&cpu_tss) as a cacheline-aligned, seldomly
accessed per-cpu var as the MONITORX target in delay_mwaitx(). However,
when called in preemptible context, this_cpu_ptr -> smp_processor_id() ->
debug_smp_processor_id() fires:

  BUG: using smp_processor_id() in preemptible [00000000] code: udevd/312
  caller is delay_mwaitx+0x40/0xa0

But we don't care about that check - we only need cpu_tss as a MONITORX
target and it doesn't really matter which CPU's var we're touching as
we're going idle anyway. Fix that.

Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Huang Rui <ray.huang@amd.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: spg_linux_kernel@amd.com
Link: http://lkml.kernel.org/r/20160309205622.GG6564@pd.tnic
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/lib/delay.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/lib/delay.c b/arch/x86/lib/delay.c
index e912b2f..2f07c29 100644
--- a/arch/x86/lib/delay.c
+++ b/arch/x86/lib/delay.c
@@ -102,7 +102,7 @@ static void delay_mwaitx(unsigned long __loops)
 		 * Use cpu_tss as a cacheline-aligned, seldomly
 		 * accessed per-cpu variable as the monitor target.
 		 */
-		__monitorx(this_cpu_ptr(&cpu_tss), 0, 0);
+		__monitorx(raw_cpu_ptr(&cpu_tss), 0, 0);
 
 		/*
 		 * AMD, like Intel, supports the EAX hint and EAX=0xf