| Message ID | e1664cf686034204b8dd5dc1d2bf18e4058b00fd.1641659630.git.luto@kernel.org (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm, sched: Rework lazy mm handling | expand |
Le 08/01/2022 à 17:43, Andy Lutomirski a écrit : > powerpc did the following on some, but not all, paths through > switch_mm_irqs_off(): > > /* > * Only need the full barrier when switching between processes. > * Barrier when switching from kernel to userspace is not > * required here, given that it is implied by mmdrop(). Barrier > * when switching from userspace to kernel is not needed after > * store to rq->curr. > */ > if (likely(!(atomic_read(&next->membarrier_state) & > (MEMBARRIER_STATE_PRIVATE_EXPEDITED | > MEMBARRIER_STATE_GLOBAL_EXPEDITED)) || !prev)) > return; > > This is puzzling: if !prev, then one might expect that we are switching > from kernel to user, not user to kernel, which is inconsistent with the > comment. But this is all nonsense, because the one and only caller would > never have prev == NULL and would, in fact, OOPS if prev == NULL. > > In any event, this code is unnecessary, since the new generic > membarrier_finish_switch_mm() provides the same barrier without arch help. I can't find this function membarrier_finish_switch_mm(), neither in Linus tree, nor in linux-next tree. > > arch/powerpc/include/asm/membarrier.h remains as an empty header, > because a later patch in this series will add code to it. > > Cc: Michael Ellerman <mpe@ellerman.id.au> > Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> > Cc: Paul Mackerras <paulus@samba.org> > Cc: linuxppc-dev@lists.ozlabs.org > Cc: Nicholas Piggin <npiggin@gmail.com> > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> > Cc: Peter Zijlstra <peterz@infradead.org> > Signed-off-by: Andy Lutomirski <luto@kernel.org> > --- > arch/powerpc/include/asm/membarrier.h | 24 ------------------------ > arch/powerpc/mm/mmu_context.c | 1 - > 2 files changed, 25 deletions(-) > > diff --git a/arch/powerpc/include/asm/membarrier.h b/arch/powerpc/include/asm/membarrier.h > index de7f79157918..b90766e95bd1 100644 > --- a/arch/powerpc/include/asm/membarrier.h > +++ b/arch/powerpc/include/asm/membarrier.h > @@ -1,28 +1,4 @@ > #ifndef _ASM_POWERPC_MEMBARRIER_H > #define _ASM_POWERPC_MEMBARRIER_H > > -static inline void membarrier_arch_switch_mm(struct mm_struct *prev, > - struct mm_struct *next, > - struct task_struct *tsk) > -{ > - /* > - * Only need the full barrier when switching between processes. > - * Barrier when switching from kernel to userspace is not > - * required here, given that it is implied by mmdrop(). Barrier > - * when switching from userspace to kernel is not needed after > - * store to rq->curr. > - */ > - if (IS_ENABLED(CONFIG_SMP) && > - likely(!(atomic_read(&next->membarrier_state) & > - (MEMBARRIER_STATE_PRIVATE_EXPEDITED | > - MEMBARRIER_STATE_GLOBAL_EXPEDITED)) || !prev)) > - return; > - > - /* > - * The membarrier system call requires a full memory barrier > - * after storing to rq->curr, before going back to user-space. > - */ > - smp_mb(); > -} > - > #endif /* _ASM_POWERPC_MEMBARRIER_H */ > diff --git a/arch/powerpc/mm/mmu_context.c b/arch/powerpc/mm/mmu_context.c > index 74246536b832..5f2daa6b0497 100644 > --- a/arch/powerpc/mm/mmu_context.c > +++ b/arch/powerpc/mm/mmu_context.c > @@ -84,7 +84,6 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next, > asm volatile ("dssall"); > > if (!new_on_cpu) > - membarrier_arch_switch_mm(prev, next, tsk); Are you sure that's what you want ? It now means you have: if (!new_on_cpu) switch_mmu_context(prev, next, tsk); > > /* > * The actual HW switching method differs between the various
----- On Jan 8, 2022, at 11:43 AM, Andy Lutomirski luto@kernel.org wrote: > powerpc did the following on some, but not all, paths through > switch_mm_irqs_off(): > > /* > * Only need the full barrier when switching between processes. > * Barrier when switching from kernel to userspace is not > * required here, given that it is implied by mmdrop(). Barrier > * when switching from userspace to kernel is not needed after > * store to rq->curr. > */ > if (likely(!(atomic_read(&next->membarrier_state) & > (MEMBARRIER_STATE_PRIVATE_EXPEDITED | > MEMBARRIER_STATE_GLOBAL_EXPEDITED)) || !prev)) > return; > > This is puzzling: if !prev, then one might expect that we are switching > from kernel to user, not user to kernel, which is inconsistent with the > comment. But this is all nonsense, because the one and only caller would > never have prev == NULL and would, in fact, OOPS if prev == NULL. > > In any event, this code is unnecessary, since the new generic > membarrier_finish_switch_mm() provides the same barrier without arch help. > > arch/powerpc/include/asm/membarrier.h remains as an empty header, > because a later patch in this series will add code to it. My disagreement with "membarrier: Make the post-switch-mm barrier explicit" may affect this patch significantly, or even make it irrelevant. Thanks, Mathieu > > Cc: Michael Ellerman <mpe@ellerman.id.au> > Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> > Cc: Paul Mackerras <paulus@samba.org> > Cc: linuxppc-dev@lists.ozlabs.org > Cc: Nicholas Piggin <npiggin@gmail.com> > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> > Cc: Peter Zijlstra <peterz@infradead.org> > Signed-off-by: Andy Lutomirski <luto@kernel.org> > --- > arch/powerpc/include/asm/membarrier.h | 24 ------------------------ > arch/powerpc/mm/mmu_context.c | 1 - > 2 files changed, 25 deletions(-) > > diff --git a/arch/powerpc/include/asm/membarrier.h > b/arch/powerpc/include/asm/membarrier.h > index de7f79157918..b90766e95bd1 100644 > --- a/arch/powerpc/include/asm/membarrier.h > +++ b/arch/powerpc/include/asm/membarrier.h > @@ -1,28 +1,4 @@ > #ifndef _ASM_POWERPC_MEMBARRIER_H > #define _ASM_POWERPC_MEMBARRIER_H > > -static inline void membarrier_arch_switch_mm(struct mm_struct *prev, > - struct mm_struct *next, > - struct task_struct *tsk) > -{ > - /* > - * Only need the full barrier when switching between processes. > - * Barrier when switching from kernel to userspace is not > - * required here, given that it is implied by mmdrop(). Barrier > - * when switching from userspace to kernel is not needed after > - * store to rq->curr. > - */ > - if (IS_ENABLED(CONFIG_SMP) && > - likely(!(atomic_read(&next->membarrier_state) & > - (MEMBARRIER_STATE_PRIVATE_EXPEDITED | > - MEMBARRIER_STATE_GLOBAL_EXPEDITED)) || !prev)) > - return; > - > - /* > - * The membarrier system call requires a full memory barrier > - * after storing to rq->curr, before going back to user-space. > - */ > - smp_mb(); > -} > - > #endif /* _ASM_POWERPC_MEMBARRIER_H */ > diff --git a/arch/powerpc/mm/mmu_context.c b/arch/powerpc/mm/mmu_context.c > index 74246536b832..5f2daa6b0497 100644 > --- a/arch/powerpc/mm/mmu_context.c > +++ b/arch/powerpc/mm/mmu_context.c > @@ -84,7 +84,6 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct > mm_struct *next, > asm volatile ("dssall"); > > if (!new_on_cpu) > - membarrier_arch_switch_mm(prev, next, tsk); > > /* > * The actual HW switching method differs between the various > -- > 2.33.1
diff --git a/arch/powerpc/include/asm/membarrier.h b/arch/powerpc/include/asm/membarrier.h index de7f79157918..b90766e95bd1 100644 --- a/arch/powerpc/include/asm/membarrier.h +++ b/arch/powerpc/include/asm/membarrier.h @@ -1,28 +1,4 @@ #ifndef _ASM_POWERPC_MEMBARRIER_H #define _ASM_POWERPC_MEMBARRIER_H -static inline void membarrier_arch_switch_mm(struct mm_struct *prev, - struct mm_struct *next, - struct task_struct *tsk) -{ - /* - * Only need the full barrier when switching between processes. - * Barrier when switching from kernel to userspace is not - * required here, given that it is implied by mmdrop(). Barrier - * when switching from userspace to kernel is not needed after - * store to rq->curr. - */ - if (IS_ENABLED(CONFIG_SMP) && - likely(!(atomic_read(&next->membarrier_state) & - (MEMBARRIER_STATE_PRIVATE_EXPEDITED | - MEMBARRIER_STATE_GLOBAL_EXPEDITED)) || !prev)) - return; - - /* - * The membarrier system call requires a full memory barrier - * after storing to rq->curr, before going back to user-space. - */ - smp_mb(); -} - #endif /* _ASM_POWERPC_MEMBARRIER_H */ diff --git a/arch/powerpc/mm/mmu_context.c b/arch/powerpc/mm/mmu_context.c index 74246536b832..5f2daa6b0497 100644 --- a/arch/powerpc/mm/mmu_context.c +++ b/arch/powerpc/mm/mmu_context.c @@ -84,7 +84,6 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next, asm volatile ("dssall"); if (!new_on_cpu) - membarrier_arch_switch_mm(prev, next, tsk); /* * The actual HW switching method differs between the various
powerpc did the following on some, but not all, paths through switch_mm_irqs_off(): /* * Only need the full barrier when switching between processes. * Barrier when switching from kernel to userspace is not * required here, given that it is implied by mmdrop(). Barrier * when switching from userspace to kernel is not needed after * store to rq->curr. */ if (likely(!(atomic_read(&next->membarrier_state) & (MEMBARRIER_STATE_PRIVATE_EXPEDITED | MEMBARRIER_STATE_GLOBAL_EXPEDITED)) || !prev)) return; This is puzzling: if !prev, then one might expect that we are switching from kernel to user, not user to kernel, which is inconsistent with the comment. But this is all nonsense, because the one and only caller would never have prev == NULL and would, in fact, OOPS if prev == NULL. In any event, this code is unnecessary, since the new generic membarrier_finish_switch_mm() provides the same barrier without arch help. arch/powerpc/include/asm/membarrier.h remains as an empty header, because a later patch in this series will add code to it. Cc: Michael Ellerman <mpe@ellerman.id.au> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> Cc: Paul Mackerras <paulus@samba.org> Cc: linuxppc-dev@lists.ozlabs.org Cc: Nicholas Piggin <npiggin@gmail.com> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> Cc: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Andy Lutomirski <luto@kernel.org> --- arch/powerpc/include/asm/membarrier.h | 24 ------------------------ arch/powerpc/mm/mmu_context.c | 1 - 2 files changed, 25 deletions(-)