Re: [tip:x86/urgent] x86/mm: Add barriers and document switch_mm() -vs-flush synchronization

Re: [tip:x86/urgent] x86/mm: Add barriers and document switch_mm() -vs-flush synchronization

[Peter Zijlstra]

On Mon, Jan 11, 2016 at 03:42:40AM -0800, tip-bot for Andy Lutomirski wrote:
> --- a/arch/x86/include/asm/mmu_context.h
> +++ b/arch/x86/include/asm/mmu_context.h
> @@ -116,8 +116,34 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
>  #endif
>  		cpumask_set_cpu(cpu, mm_cpumask(next));
>  
> -		/* Re-load page tables */
> +		/*
> +		 * Re-load page tables.
> +		 *
> +		 * This logic has an ordering constraint:
> +		 *
> +		 *  CPU 0: Write to a PTE for 'next'
> +		 *  CPU 0: load bit 1 in mm_cpumask.  if nonzero, send IPI.
> +		 *  CPU 1: set bit 1 in next's mm_cpumask
> +		 *  CPU 1: load from the PTE that CPU 0 writes (implicit)
> +		 *
> +		 * We need to prevent an outcome in which CPU 1 observes
> +		 * the new PTE value and CPU 0 observes bit 1 clear in
> +		 * mm_cpumask.  (If that occurs, then the IPI will never
> +		 * be sent, and CPU 0's TLB will contain a stale entry.)
> +		 *
> +		 * The bad outcome can occur if either CPU's load is
> +		 * reordered before that CPU's store, so both CPUs much

s/much/must/ ?

> +		 * execute full barriers to prevent this from happening.
> +		 *
> +		 * Thus, switch_mm needs a full barrier between the
> +		 * store to mm_cpumask and any operation that could load
> +		 * from next->pgd.  This barrier synchronizes with
> +		 * remote TLB flushers.  Fortunately, load_cr3 is
> +		 * serializing and thus acts as a full barrier.
> +		 *
> +		 */
>  		load_cr3(next->pgd);
> +
>  		trace_tlb_flush(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL);
>  
>  		/* Stop flush ipis for the previous mm */
> @@ -156,10 +182,15 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
>  			 * schedule, protecting us from simultaneous changes.
>  			 */
>  			cpumask_set_cpu(cpu, mm_cpumask(next));
> +
>  			/*
>  			 * We were in lazy tlb mode and leave_mm disabled
>  			 * tlb flush IPI delivery. We must reload CR3
>  			 * to make sure to use no freed page tables.
> +			 *
> +			 * As above, this is a barrier that forces
> +			 * TLB repopulation to be ordered after the
> +			 * store to mm_cpumask.

somewhat confused by this comment, cpumask_set_cpu() is a LOCK BTS, that
is already fully ordered.

>  			 */
>  			load_cr3(next->pgd);
>  			trace_tlb_flush(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL);
> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> index 8ddb5d0..8f4cc3d 100644


> --- a/arch/x86/mm/tlb.c
> +++ b/arch/x86/mm/tlb.c

> @@ -188,17 +191,29 @@ void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start,

>  	if (!current->mm) {
>  		leave_mm(smp_processor_id());
> +
> +		/* Synchronize with switch_mm. */
> +		smp_mb();
> +
>  		goto out;
>  	}

> +		} else {
>  			leave_mm(smp_processor_id());
> +
> +			/* Synchronize with switch_mm. */
> +			smp_mb();
> +		}
>  	}

The alternative is making leave_mm() unconditionally imply a full
barrier. I've not looked at other sites using it though.

Re: [tip:x86/urgent] x86/mm: Add barriers and document switch_mm() -vs-flush synchronization

[Andy Lutomirski]

On Mon, Jan 11, 2016 at 10:25 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Mon, Jan 11, 2016 at 03:42:40AM -0800, tip-bot for Andy Lutomirski wrote:
>> --- a/arch/x86/include/asm/mmu_context.h
>> +++ b/arch/x86/include/asm/mmu_context.h
>> @@ -116,8 +116,34 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
>>  #endif
>>               cpumask_set_cpu(cpu, mm_cpumask(next));
>>
>> -             /* Re-load page tables */
>> +             /*
>> +              * Re-load page tables.
>> +              *
>> +              * This logic has an ordering constraint:
>> +              *
>> +              *  CPU 0: Write to a PTE for 'next'
>> +              *  CPU 0: load bit 1 in mm_cpumask.  if nonzero, send IPI.
>> +              *  CPU 1: set bit 1 in next's mm_cpumask
>> +              *  CPU 1: load from the PTE that CPU 0 writes (implicit)
>> +              *
>> +              * We need to prevent an outcome in which CPU 1 observes
>> +              * the new PTE value and CPU 0 observes bit 1 clear in
>> +              * mm_cpumask.  (If that occurs, then the IPI will never
>> +              * be sent, and CPU 0's TLB will contain a stale entry.)
>> +              *
>> +              * The bad outcome can occur if either CPU's load is
>> +              * reordered before that CPU's store, so both CPUs much
>
> s/much/must/ ?

Indeed.  Is this worth a follow-up patch?

>
>> +              * execute full barriers to prevent this from happening.
>> +              *
>> +              * Thus, switch_mm needs a full barrier between the
>> +              * store to mm_cpumask and any operation that could load
>> +              * from next->pgd.  This barrier synchronizes with
>> +              * remote TLB flushers.  Fortunately, load_cr3 is
>> +              * serializing and thus acts as a full barrier.
>> +              *
>> +              */
>>               load_cr3(next->pgd);
>> +
>>               trace_tlb_flush(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL);
>>
>>               /* Stop flush ipis for the previous mm */
>> @@ -156,10 +182,15 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
>>                        * schedule, protecting us from simultaneous changes.
>>                        */
>>                       cpumask_set_cpu(cpu, mm_cpumask(next));
>> +
>>                       /*
>>                        * We were in lazy tlb mode and leave_mm disabled
>>                        * tlb flush IPI delivery. We must reload CR3
>>                        * to make sure to use no freed page tables.
>> +                      *
>> +                      * As above, this is a barrier that forces
>> +                      * TLB repopulation to be ordered after the
>> +                      * store to mm_cpumask.
>
> somewhat confused by this comment, cpumask_set_cpu() is a LOCK BTS, that
> is already fully ordered.

There are more than enough barriers here.  v1 had cpumask_set_cpu;
smp_mb__after_atomic, which is more portable and generates identical
code.  I don't have a real preference for which barrier we should
consider to the important one.

>
>>                        */
>>                       load_cr3(next->pgd);
>>                       trace_tlb_flush(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL);
>> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
>> index 8ddb5d0..8f4cc3d 100644
>
>
>> --- a/arch/x86/mm/tlb.c
>> +++ b/arch/x86/mm/tlb.c
>
>> @@ -188,17 +191,29 @@ void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start,
>
>>       if (!current->mm) {
>>               leave_mm(smp_processor_id());
>> +
>> +             /* Synchronize with switch_mm. */
>> +             smp_mb();
>> +
>>               goto out;
>>       }
>
>> +             } else {
>>                       leave_mm(smp_processor_id());
>> +
>> +                     /* Synchronize with switch_mm. */
>> +                     smp_mb();
>> +             }
>>       }
>
> The alternative is making leave_mm() unconditionally imply a full
> barrier. I've not looked at other sites using it though.

For a quick fix, I preferred the more self-contained change.

--Andy

Re: [tip:x86/urgent] x86/mm: Add barriers and document switch_mm() -vs-flush synchronization

[Peter Zijlstra]

On Mon, Jan 11, 2016 at 01:50:24PM -0800, Andy Lutomirski wrote:
> On Mon, Jan 11, 2016 at 10:25 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> > On Mon, Jan 11, 2016 at 03:42:40AM -0800, tip-bot for Andy Lutomirski wrote:

> >> +              * The bad outcome can occur if either CPU's load is
> >> +              * reordered before that CPU's store, so both CPUs much
> >
> > s/much/must/ ?
> 
> Indeed.  Is this worth a follow-up patch?

Dunno, I didn't even spot the typo the first time I read it.. :-)

Re: [tip:x86/urgent] x86/mm: Add barriers and document switch_mm() -vs-flush synchronization

[Ingo Molnar]

* Andy Lutomirski <luto@amacapital.net> wrote:

> On Mon, Jan 11, 2016 at 10:25 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> > On Mon, Jan 11, 2016 at 03:42:40AM -0800, tip-bot for Andy Lutomirski wrote:
> >> --- a/arch/x86/include/asm/mmu_context.h
> >> +++ b/arch/x86/include/asm/mmu_context.h
> >> @@ -116,8 +116,34 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
> >>  #endif
> >>               cpumask_set_cpu(cpu, mm_cpumask(next));
> >>
> >> -             /* Re-load page tables */
> >> +             /*
> >> +              * Re-load page tables.
> >> +              *
> >> +              * This logic has an ordering constraint:
> >> +              *
> >> +              *  CPU 0: Write to a PTE for 'next'
> >> +              *  CPU 0: load bit 1 in mm_cpumask.  if nonzero, send IPI.
> >> +              *  CPU 1: set bit 1 in next's mm_cpumask
> >> +              *  CPU 1: load from the PTE that CPU 0 writes (implicit)
> >> +              *
> >> +              * We need to prevent an outcome in which CPU 1 observes
> >> +              * the new PTE value and CPU 0 observes bit 1 clear in
> >> +              * mm_cpumask.  (If that occurs, then the IPI will never
> >> +              * be sent, and CPU 0's TLB will contain a stale entry.)
> >> +              *
> >> +              * The bad outcome can occur if either CPU's load is
> >> +              * reordered before that CPU's store, so both CPUs much
> >
> > s/much/must/ ?
> 
> Indeed.  Is this worth a follow-up patch?

Absolutely! Any typos in code noticed by humans are worth fixing, especially when 
it's comments around tricky code. Could be done together with improving this part 
of the comments:

> > +
> >                       /*
> >                        * We were in lazy tlb mode and leave_mm disabled
> >                        * tlb flush IPI delivery. We must reload CR3
> >                        * to make sure to use no freed page tables.
> > +                      *
> > +                      * As above, this is a barrier that forces
> > +                      * TLB repopulation to be ordered after the
> > +                      * store to mm_cpumask.
>
> somewhat confused by this comment, cpumask_set_cpu() is a LOCK BTS, that is 
> already fully ordered.

... as pretty much any barriers related comment that confuses Peter needs to be 
improved, by definition. ;-)

Thanks,

	Ingo

[PATCH] x86/mm: Improve switch_mm barrier comments

[Andy Lutomirski]

My previous comments were still a bit confusing and there was a
typo.  Fix it up.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Fixes: 71b3c126e611 ("x86/mm: Add barriers and document switch_mm()-vs-flush synchronization")
Cc: stable@vger.kernel.org
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/include/asm/mmu_context.h | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index 1edc9cd198b8..e08d27369fce 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -132,14 +132,14 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 		 * be sent, and CPU 0's TLB will contain a stale entry.)
 		 *
 		 * The bad outcome can occur if either CPU's load is
-		 * reordered before that CPU's store, so both CPUs much
+		 * reordered before that CPU's store, so both CPUs must
 		 * execute full barriers to prevent this from happening.
 		 *
 		 * Thus, switch_mm needs a full barrier between the
 		 * store to mm_cpumask and any operation that could load
 		 * from next->pgd.  This barrier synchronizes with
-		 * remote TLB flushers.  Fortunately, load_cr3 is
-		 * serializing and thus acts as a full barrier.
+		 * remote TLB flushers.  Fortunately, cpumask_set_cpu is
+		 * (on x86) a full barrier, and load_cr3 is serializing.
 		 *
 		 */
 		load_cr3(next->pgd);
@@ -188,9 +188,10 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 			 * tlb flush IPI delivery. We must reload CR3
 			 * to make sure to use no freed page tables.
 			 *
-			 * As above, this is a barrier that forces
-			 * TLB repopulation to be ordered after the
-			 * store to mm_cpumask.
+			 * As above, either of cpumask_set_cpu and
+			 * load_cr3 is a sufficient barrier to force TLB
+			 * repopulation to be ordered after the store to
+			 * mm_cpumask.
 			 */
 			load_cr3(next->pgd);
 			trace_tlb_flush(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL);
-- 
2.5.0

[PATCH] x86/mm: Improve switch_mm barrier comments

[Andy Lutomirski]

My previous comments were still a bit confusing and there was a
typo.  Fix it up.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Fixes: 71b3c126e611 ("x86/mm: Add barriers and document switch_mm()-vs-flush synchronization")
Cc: stable@vger.kernel.org
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/include/asm/mmu_context.h | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index 1edc9cd198b8..e08d27369fce 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -132,14 +132,14 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 		 * be sent, and CPU 0's TLB will contain a stale entry.)
 		 *
 		 * The bad outcome can occur if either CPU's load is
-		 * reordered before that CPU's store, so both CPUs much
+		 * reordered before that CPU's store, so both CPUs must
 		 * execute full barriers to prevent this from happening.
 		 *
 		 * Thus, switch_mm needs a full barrier between the
 		 * store to mm_cpumask and any operation that could load
 		 * from next->pgd.  This barrier synchronizes with
-		 * remote TLB flushers.  Fortunately, load_cr3 is
-		 * serializing and thus acts as a full barrier.
+		 * remote TLB flushers.  Fortunately, cpumask_set_cpu is
+		 * (on x86) a full barrier, and load_cr3 is serializing.
 		 *
 		 */
 		load_cr3(next->pgd);
@@ -188,9 +188,10 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 			 * tlb flush IPI delivery. We must reload CR3
 			 * to make sure to use no freed page tables.
 			 *
-			 * As above, this is a barrier that forces
-			 * TLB repopulation to be ordered after the
-			 * store to mm_cpumask.
+			 * As above, either of cpumask_set_cpu and
+			 * load_cr3 is a sufficient barrier to force TLB
+			 * repopulation to be ordered after the store to
+			 * mm_cpumask.
 			 */
 			load_cr3(next->pgd);
 			trace_tlb_flush(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL);
-- 
2.5.0

[PATCH v2] x86/mm: Improve switch_mm barrier comments

[Andy Lutomirski]

My previous comments were still a bit confusing and there was a
typo.  Fix it up.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Fixes: 71b3c126e611 ("x86/mm: Add barriers and document switch_mm()-vs-flush synchronization")
Cc: stable@vger.kernel.org
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---

Changes from v1: Totally different.

arch/x86/include/asm/mmu_context.h | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index 1edc9cd198b8..4fcae1e066f3 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -132,14 +132,16 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 		 * be sent, and CPU 0's TLB will contain a stale entry.)
 		 *
 		 * The bad outcome can occur if either CPU's load is
-		 * reordered before that CPU's store, so both CPUs much
+		 * reordered before that CPU's store, so both CPUs must
 		 * execute full barriers to prevent this from happening.
 		 *
 		 * Thus, switch_mm needs a full barrier between the
 		 * store to mm_cpumask and any operation that could load
-		 * from next->pgd.  This barrier synchronizes with
-		 * remote TLB flushers.  Fortunately, load_cr3 is
-		 * serializing and thus acts as a full barrier.
+		 * from next->pgd.  TLB fills are special and can happen
+		 * due to instruction fetches or for no reason at all,
+		 * and neither LOCK nor MFENCE orders them.
+		 * Fortunately, load_cr3 is serializing and gives the
+		 * ordering guarantee we need.
 		 *
 		 */
 		load_cr3(next->pgd);
@@ -188,9 +190,8 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 			 * tlb flush IPI delivery. We must reload CR3
 			 * to make sure to use no freed page tables.
 			 *
-			 * As above, this is a barrier that forces
-			 * TLB repopulation to be ordered after the
-			 * store to mm_cpumask.
+			 * As above, load_cr3 is serializing and orders TLB
+			 * fills with respect to the mm_cpumask write.
 			 */
 			load_cr3(next->pgd);
 			trace_tlb_flush(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL);
-- 
2.5.0

[PATCH v2] x86/mm: Improve switch_mm barrier comments

[Andy Lutomirski]

My previous comments were still a bit confusing and there was a
typo.  Fix it up.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Fixes: 71b3c126e611 ("x86/mm: Add barriers and document switch_mm()-vs-flush synchronization")
Cc: stable@vger.kernel.org
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---

Changes from v1: Totally different.

arch/x86/include/asm/mmu_context.h | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index 1edc9cd198b8..4fcae1e066f3 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -132,14 +132,16 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 		 * be sent, and CPU 0's TLB will contain a stale entry.)
 		 *
 		 * The bad outcome can occur if either CPU's load is
-		 * reordered before that CPU's store, so both CPUs much
+		 * reordered before that CPU's store, so both CPUs must
 		 * execute full barriers to prevent this from happening.
 		 *
 		 * Thus, switch_mm needs a full barrier between the
 		 * store to mm_cpumask and any operation that could load
-		 * from next->pgd.  This barrier synchronizes with
-		 * remote TLB flushers.  Fortunately, load_cr3 is
-		 * serializing and thus acts as a full barrier.
+		 * from next->pgd.  TLB fills are special and can happen
+		 * due to instruction fetches or for no reason at all,
+		 * and neither LOCK nor MFENCE orders them.
+		 * Fortunately, load_cr3 is serializing and gives the
+		 * ordering guarantee we need.
 		 *
 		 */
 		load_cr3(next->pgd);
@@ -188,9 +190,8 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 			 * tlb flush IPI delivery. We must reload CR3
 			 * to make sure to use no freed page tables.
 			 *
-			 * As above, this is a barrier that forces
-			 * TLB repopulation to be ordered after the
-			 * store to mm_cpumask.
+			 * As above, load_cr3 is serializing and orders TLB
+			 * fills with respect to the mm_cpumask write.
 			 */
 			load_cr3(next->pgd);
 			trace_tlb_flush(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL);
-- 
2.5.0

[tip:x86/urgent] x86/mm: Improve switch_mm() barrier comments

[tip-bot for Andy Lutomirski]

Commit-ID:  4eaffdd5a5fe6ff9f95e1ab4de1ac904d5e0fa8b
Gitweb:     http://git.kernel.org/tip/4eaffdd5a5fe6ff9f95e1ab4de1ac904d5e0fa8b
Author:     Andy Lutomirski <luto@kernel.org>
AuthorDate: Tue, 12 Jan 2016 12:47:40 -0800
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Wed, 13 Jan 2016 10:42:49 +0100

x86/mm: Improve switch_mm() barrier comments

My previous comments were still a bit confusing and there was a
typo. Fix it up.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Fixes: 71b3c126e611 ("x86/mm: Add barriers and document switch_mm()-vs-flush synchronization")
Link: http://lkml.kernel.org/r/0a0b43cdcdd241c5faaaecfbcc91a155ddedc9a1.1452631609.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/include/asm/mmu_context.h | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index 1edc9cd..bfd9b2a 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -132,14 +132,16 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 		 * be sent, and CPU 0's TLB will contain a stale entry.)
 		 *
 		 * The bad outcome can occur if either CPU's load is
-		 * reordered before that CPU's store, so both CPUs much
+		 * reordered before that CPU's store, so both CPUs must
 		 * execute full barriers to prevent this from happening.
 		 *
 		 * Thus, switch_mm needs a full barrier between the
 		 * store to mm_cpumask and any operation that could load
-		 * from next->pgd.  This barrier synchronizes with
-		 * remote TLB flushers.  Fortunately, load_cr3 is
-		 * serializing and thus acts as a full barrier.
+		 * from next->pgd.  TLB fills are special and can happen
+		 * due to instruction fetches or for no reason at all,
+		 * and neither LOCK nor MFENCE orders them.
+		 * Fortunately, load_cr3() is serializing and gives the
+		 * ordering guarantee we need.
 		 *
 		 */
 		load_cr3(next->pgd);
@@ -188,9 +190,8 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 			 * tlb flush IPI delivery. We must reload CR3
 			 * to make sure to use no freed page tables.
 			 *
-			 * As above, this is a barrier that forces
-			 * TLB repopulation to be ordered after the
-			 * store to mm_cpumask.
+			 * As above, load_cr3() is serializing and orders TLB
+			 * fills with respect to the mm_cpumask write.
 			 */
 			load_cr3(next->pgd);
 			trace_tlb_flush(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL);