Re: RFC: remove set_fs for m68k

[Michael Schmitz <schmitzmic@gmail.com>]

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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