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mm, locking/barriers: Clarify tlb_flush_pending() barriers

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Better document the ordering around tlb_flush_pending().

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra 2017-08-11 16:04:50 +02:00 committed by Ingo Molnar
parent 10c9850cb2
commit 0e709703af
1 changed files with 45 additions and 33 deletions
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78
include/linux/mm_types.h
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@ -526,30 +526,6 @@ extern void tlb_gather_mmu(struct mmu_gather *tlb, struct mm_struct *mm,
extern void tlb_finish_mmu(struct mmu_gather *tlb,
unsigned long start, unsigned long end);
/*
* Memory barriers to keep this state in sync are graciously provided by
* the page table locks, outside of which no page table modifications happen.
* The barriers are used to ensure the order between tlb_flush_pending updates,
* which happen while the lock is not taken, and the PTE updates, which happen
* while the lock is taken, are serialized.
*/
static inline bool mm_tlb_flush_pending(struct mm_struct *mm)
{
/*
* Must be called with PTL held; such that our PTL acquire will have
* observed the store from set_tlb_flush_pending().
*/
return atomic_read(&mm->tlb_flush_pending) > 0;
}
/*
* Returns true if there are two above TLB batching threads in parallel.
*/
static inline bool mm_tlb_flush_nested(struct mm_struct *mm)
{
return atomic_read(&mm->tlb_flush_pending) > 1;
}
static inline void init_tlb_flush_pending(struct mm_struct *mm)
{
atomic_set(&mm->tlb_flush_pending, 0);
@ -558,7 +534,6 @@ static inline void init_tlb_flush_pending(struct mm_struct *mm)
static inline void inc_tlb_flush_pending(struct mm_struct *mm)
{
atomic_inc(&mm->tlb_flush_pending);
/*
* The only time this value is relevant is when there are indeed pages
* to flush. And we'll only flush pages after changing them, which
@ -580,24 +555,61 @@ static inline void inc_tlb_flush_pending(struct mm_struct *mm)
* flush_tlb_range();
* atomic_dec(&mm->tlb_flush_pending);
*
* So the =true store is constrained by the PTL unlock, and the =false
* store is constrained by the TLB invalidate.
* Where the increment if constrained by the PTL unlock, it thus
* ensures that the increment is visible if the PTE modification is
* visible. After all, if there is no PTE modification, nobody cares
* about TLB flushes either.
*
* This very much relies on users (mm_tlb_flush_pending() and
* mm_tlb_flush_nested()) only caring about _specific_ PTEs (and
* therefore specific PTLs), because with SPLIT_PTE_PTLOCKS and RCpc
* locks (PPC) the unlock of one doesn't order against the lock of
* another PTL.
*
* The decrement is ordered by the flush_tlb_range(), such that
* mm_tlb_flush_pending() will not return false unless all flushes have
* completed.
*/
}
/* Clearing is done after a TLB flush, which also provides a barrier. */
static inline void dec_tlb_flush_pending(struct mm_struct *mm)
{
/*
* Guarantee that the tlb_flush_pending does not not leak into the
* critical section, since we must order the PTE change and changes to
* the pending TLB flush indication. We could have relied on TLB flush
* as a memory barrier, but this behavior is not clearly documented.
* See inc_tlb_flush_pending().
*
* This cannot be smp_mb__before_atomic() because smp_mb() simply does
* not order against TLB invalidate completion, which is what we need.
*
* Therefore we must rely on tlb_flush_*() to guarantee order.
*/
smp_mb__before_atomic();
atomic_dec(&mm->tlb_flush_pending);
}
static inline bool mm_tlb_flush_pending(struct mm_struct *mm)
{
/*
* Must be called after having acquired the PTL; orders against that
* PTLs release and therefore ensures that if we observe the modified
* PTE we must also observe the increment from inc_tlb_flush_pending().
*
* That is, it only guarantees to return true if there is a flush
* pending for _this_ PTL.
*/
return atomic_read(&mm->tlb_flush_pending);
}
static inline bool mm_tlb_flush_nested(struct mm_struct *mm)
{
/*
* Similar to mm_tlb_flush_pending(), we must have acquired the PTL
* for which there is a TLB flush pending in order to guarantee
* we've seen both that PTE modification and the increment.
*
* (no requirement on actually still holding the PTL, that is irrelevant)
*/
return atomic_read(&mm->tlb_flush_pending) > 1;
}
struct vm_fault;
struct vm_special_mapping {