Re: RFC: remove set_fs for m68k

Michael Schmitz
2021-08-06T03:11:05Z

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Hi Christoph,

Am 01.08.2021 um 07:31 schrieb Michael Schmitz:
> Hi Christoph,
>> I'll back out the top commit of your series (the one removing set_fs())
>> and retest. Again, this may take a few days to show any results.
>
> No further format errors, kernel panics or lockups seen after almost a
> week of tests.
>
> That suggests something is wrong with your 'm68k: remove set_fs()' commit.
>
> I'll try adding a get_fc() and warn whenever FC does not match what your
> patch expects, maybe that can help to get a clearer picture.

See attached patches, to be applied on top of your RFC series - ran this
for five days now, with no further errors. Will run this a while longer
yet, but in the ordinary course of testing, I'd have seen errors by now.

Haven't seen the WARN_ON trigger once yet though, which is more than a
little odd. Heisenbug?

I am aware that this patch defeats the purpose of the 'lets lose set_fs'
patch series, and Linus was quite convincing in saying preemption
couldn't be an issue so saving DFC ought not to be necessary. If there
is anything else I can try to get to the bottom of these format errors,
please say so.

Cheers,

Michael

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From d5f7c3d4b4e4b437dfb54f9eb1ed6876dfb63744 Mon Sep 17 00:00:00 2001
From: Michael Schmitz <schmitzmic@gmail.com>
Date: Sun, 1 Aug 2021 13:06:21 +1200
Subject: [PATCH 2/2] m68k: warn in get_fc() if DFC is not USER_DATA

Trace uacess mode restore operations that mess up the SFC/DFC
registers - read out current DFC before changing modes and warn
when the current mode wasn't USER_DATA!

Signed-off-by: Michael Schmitz <schmitzmic@gmail.com>
---
arch/m68k/include/asm/processor.h | 1 +
1 file changed, 1 insertion(+)

diff --git a/arch/m68k/include/asm/processor.h b/arch/m68k/include/asm/processor.h
index 29dfa54..a8c533d 100644
--- a/arch/m68k/include/asm/processor.h
+++ b/arch/m68k/include/asm/processor.h
@@ -98,6 +98,7 @@ static inline unsigned long get_fc(void)
{
unsigned long val;
__asm__ ("movec %/dfc,%0":"=r" (val):);
+ WARN_ON_ONCE(val != USER_DATA);
return val;
}
/*
--
2.7.4

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From fcb3f2543125ca3fe269b50646f8c4782d31ba80 Mon Sep 17 00:00:00 2001
From: Michael Schmitz <schmitzmic@gmail.com>
Date: Sun, 1 Aug 2021 12:11:59 +1200
Subject: [PATCH 1/2] m68k: add get_fc() to read uaccess mode

Add back get_fc(), force_user_fc_begin() and force_user_fc_end(),
and use these to restore original DFC after uaccess operations,
instead of unconditionally setting USER_DATA.

Signed-off-by: Michael Schmitz <schmitzmic@gmail.com>
---
arch/m68k/include/asm/processor.h | 35 +++++++++++++++++++++++++++++++++++
arch/m68k/include/asm/tlbflush.h | 8 ++++++--
arch/m68k/kernel/process.c | 2 +-
arch/m68k/kernel/traps.c | 6 ++++--
arch/m68k/mm/cache.c | 4 +++-
5 files changed, 49 insertions(+), 6 deletions(-)

diff --git a/arch/m68k/include/asm/processor.h b/arch/m68k/include/asm/processor.h
index debb453..29dfa54 100644
--- a/arch/m68k/include/asm/processor.h
+++ b/arch/m68k/include/asm/processor.h
@@ -94,10 +94,45 @@ static inline void set_fc(unsigned long val)
"movec %0,%/dfc\n\t"
: /* no outputs */ : "r" (val) : "memory");
}
+static inline unsigned long get_fc(void)
+{
+ unsigned long val;
+ __asm__ ("movec %/dfc,%0":"=r" (val):);
+ return val;
+}
+/*
+ * Force the uaccess routines to be wired up for actual userspace access,
+ * overriding any possible set_fs(KERNEL_DS) still lingering around. Undone
+ * using force_uaccess_end below.
+ */
+static inline unsigned long force_user_fc_begin(void)
+{
+ unsigned long fc = get_fc();
+
+ set_fc(USER_DATA);
+ return fc;
+}
+
+static inline void force_user_fc_end(unsigned long oldfc)
+{
+ set_fc(oldfc);
+}
#else
static inline void set_fc(unsigned long val)
{
}
+static inline unsigned long get_fs(void)
+{
+ return USER_DATA;
+}
+static inline unsigned long force_user_fc_begin(void)
+{
+ return USER_DATA;
+}
+
+static inline void force_user_fc_end(unsigned long oldfc)
+{
+}
#endif /* CONFIG_CPU_HAS_ADDRESS_SPACES */

struct thread_struct {
diff --git a/arch/m68k/include/asm/tlbflush.h b/arch/m68k/include/asm/tlbflush.h
index 524b13d..7c34d14 100644
--- a/arch/m68k/include/asm/tlbflush.h
+++ b/arch/m68k/include/asm/tlbflush.h
@@ -13,12 +13,13 @@ static inline void flush_tlb_kernel_page(void *addr)
if (CPU_IS_COLDFIRE) {
mmu_write(MMUOR, MMUOR_CNL);
} else if (CPU_IS_040_OR_060) {
+ unsigned long old_fc = get_fc();
set_fc(SUPER_DATA);
__asm__ __volatile__(".chip 68040\n\t"
"pflush (%0)\n\t"
".chip 68k"
: : "a" (addr));
- set_fc(USER_DATA);
+ set_fc(old_fc);
} else if (CPU_IS_020_OR_030)
__asm__ __volatile__("pflush #4,#4,(%0)" : : "a" (addr));
}
@@ -83,8 +84,11 @@ static inline void flush_tlb_mm(struct mm_struct *mm)

static inline void flush_tlb_page(struct vm_area_struct *vma, unsigned long addr)
{
- if (vma->vm_mm == current->active_mm)
+ if (vma->vm_mm == current->active_mm) {
+ unsigned long old_fc = force_user_fc_begin();
__flush_tlb_one(addr);
+ force_user_fc_end(old_fc);
+ }
}

static inline void flush_tlb_range(struct vm_area_struct *vma,
diff --git a/arch/m68k/kernel/process.c b/arch/m68k/kernel/process.c
index fb257b7..cab012d 100644
--- a/arch/m68k/kernel/process.c
+++ b/arch/m68k/kernel/process.c
@@ -155,7 +155,7 @@ int copy_thread(unsigned long clone_flags, unsigned long usp, unsigned long arg,
* Must save the current SFC/DFC value, NOT the value when
* the parent was last descheduled - RGH 10-08-96
*/
- p->thread.fc = USER_DATA;
+ p->thread.fc = get_fc();

if (unlikely(p->flags & (PF_KTHREAD | PF_IO_WORKER))) {
/* kernel thread */
diff --git a/arch/m68k/kernel/traps.c b/arch/m68k/kernel/traps.c
index c5e0a4f..1b07329 100644
--- a/arch/m68k/kernel/traps.c
+++ b/arch/m68k/kernel/traps.c
@@ -181,6 +181,7 @@ static inline void access_error060 (struct frame *fp)
static inline unsigned long probe040(int iswrite, unsigned long addr, int wbs)
{
unsigned long mmusr;
+ unsigned long old_fc = get_fc();

set_fc(wbs);

@@ -191,7 +192,7 @@ static inline unsigned long probe040(int iswrite, unsigned long addr, int wbs)

asm volatile (".chip 68040; movec %%mmusr,%0; .chip 68k" : "=r" (mmusr));

- set_fc(USER_DATA);
+ set_fc(old_fc);

return mmusr;
}
@@ -200,6 +201,7 @@ static inline int do_040writeback1(unsigned short wbs, unsigned long wba,
unsigned long wbd)
{
int res = 0;
+ unsigned long old_fc = get_fc();

set_fc(wbs);

@@ -215,7 +217,7 @@ static inline int do_040writeback1(unsigned short wbs, unsigned long wba,
break;
}

- set_fc(USER_DATA);
+ set_fc(old_fc);

pr_debug("do_040writeback1, res=%d\n", res);

diff --git a/arch/m68k/mm/cache.c b/arch/m68k/mm/cache.c
index d26eb1a..70fd62b 100644
--- a/arch/m68k/mm/cache.c
+++ b/arch/m68k/mm/cache.c
@@ -109,14 +109,16 @@ static void __flush_icache_range(unsigned long address, unsigned long endaddr,

void flush_icache_user_range(unsigned long address, unsigned long endaddr)
{
+ unsigned long old_fc = get_fc();
__flush_icache_range(address, endaddr, USER_DATA);
}

void flush_icache_range(unsigned long address, unsigned long endaddr)
{
+ unsigned long old_fc = get_fc();
set_fc(SUPER_DATA);
__flush_icache_range(address, endaddr, SUPER_DATA);
- set_fc(USER_DATA);
+ set_fc(old_fc);
}
EXPORT_SYMBOL(flush_icache_range);

--
2.7.4