Re: [PATCH for 4.16 v7 02/11] powerpc: membarrier: Skip memory barrier in switch_mm()

[Christophe Leroy]

Le 29/01/2018 à 21:20, Mathieu Desnoyers a écrit :
> Allow PowerPC to skip the full memory barrier in switch_mm(), and
> only issue the barrier when scheduling into a task belonging to a
> process that has registered to use expedited private.
>
> Threads targeting the same VM but which belong to different thread
> groups is a tricky case. It has a few consequences:
>
> It turns out that we cannot rely on get_nr_threads(p) to count the
> number of threads using a VM. We can use
> (atomic_read(&mm->mm_users) == 1 && get_nr_threads(p) == 1)
> instead to skip the synchronize_sched() for cases where the VM only has
> a single user, and that user only has a single thread.
>
> It also turns out that we cannot use for_each_thread() to set
> thread flags in all threads using a VM, as it only iterates on the
> thread group.
>
> Therefore, test the membarrier state variable directly rather than
> relying on thread flags. This means
> membarrier_register_private_expedited() needs to set the
> MEMBARRIER_STATE_PRIVATE_EXPEDITED flag, issue synchronize_sched(), and
> only then set MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY which allows
> private expedited membarrier commands to succeed.
> membarrier_arch_switch_mm() now tests for the
> MEMBARRIER_STATE_PRIVATE_EXPEDITED flag.

Looking at switch_mm_irqs_off(), I found it more complex than expected and found that this patch is 
the reason for that complexity.

Before the patch (ie in kernel 4.14), we have:

00000000 <switch_mm_irqs_off>:
   0:	81 24 01 c8 	lwz     r9,456(r4)
   4:	71 29 00 01 	andi.   r9,r9,1
   8:	40 82 00 1c 	bne     24 <switch_mm_irqs_off+0x24>
   c:	39 24 01 c8 	addi    r9,r4,456
  10:	39 40 00 01 	li      r10,1
  14:	7d 00 48 28 	lwarx   r8,0,r9
  18:	7d 08 53 78 	or      r8,r8,r10
  1c:	7d 00 49 2d 	stwcx.  r8,0,r9
  20:	40 c2 ff f4 	bne-    14 <switch_mm_irqs_off+0x14>
  24:	7c 04 18 40 	cmplw   r4,r3
  28:	81 24 00 24 	lwz     r9,36(r4)
  2c:	91 25 04 4c 	stw     r9,1100(r5)
  30:	4d 82 00 20 	beqlr
  34:	48 00 00 00 	b       34 <switch_mm_irqs_off+0x34>
			34: R_PPC_REL24	switch_mmu_context


After the patch (ie in 5.13-rc6), that now is:

00000000 <switch_mm_irqs_off>:
   0:	81 24 02 18 	lwz     r9,536(r4)
   4:	71 29 00 01 	andi.   r9,r9,1
   8:	41 82 00 24 	beq     2c <switch_mm_irqs_off+0x2c>
   c:	7c 04 18 40 	cmplw   r4,r3
  10:	81 24 00 24 	lwz     r9,36(r4)
  14:	91 25 04 d0 	stw     r9,1232(r5)
  18:	4d 82 00 20 	beqlr
  1c:	81 24 00 28 	lwz     r9,40(r4)
  20:	71 29 00 0a 	andi.   r9,r9,10
  24:	40 82 00 34 	bne     58 <switch_mm_irqs_off+0x58>
  28:	48 00 00 00 	b       28 <switch_mm_irqs_off+0x28>
			28: R_PPC_REL24	switch_mmu_context
  2c:	39 24 02 18 	addi    r9,r4,536
  30:	39 40 00 01 	li      r10,1
  34:	7d 00 48 28 	lwarx   r8,0,r9
  38:	7d 08 53 78 	or      r8,r8,r10
  3c:	7d 00 49 2d 	stwcx.  r8,0,r9
  40:	40 a2 ff f4 	bne     34 <switch_mm_irqs_off+0x34>
  44:	7c 04 18 40 	cmplw   r4,r3
  48:	81 24 00 24 	lwz     r9,36(r4)
  4c:	91 25 04 d0 	stw     r9,1232(r5)
  50:	4d 82 00 20 	beqlr
  54:	48 00 00 00 	b       54 <switch_mm_irqs_off+0x54>
			54: R_PPC_REL24	switch_mmu_context
  58:	2c 03 00 00 	cmpwi   r3,0
  5c:	41 82 ff cc 	beq     28 <switch_mm_irqs_off+0x28>
  60:	48 00 00 00 	b       60 <switch_mm_irqs_off+0x60>
			60: R_PPC_REL24	switch_mmu_context


Especially, the comparison of 'prev' to 0 is pointless as both cases end up with just branching to 
'switch_mmu_context'

I don't understand all that complexity to just replace a simple 'smp_mb__after_unlock_lock()'.

#define smp_mb__after_unlock_lock()	smp_mb()
#define smp_mb()	barrier()
# define barrier() __asm__ __volatile__("": : :"memory")


Am I missing some subtility ?

Thanks
Christophe