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arm64: mm: avoid redundant READ_ONCE(*ptep) 

In set_pte_at(), we read the old pte value so that it can be passed into
checks for racy hw updates. These checks are only performed for
CONFIG_DEBUG_VM, and the value is not used otherwise.

Since we read the pte value with READ_ONCE(), the compiler cannot elide
the redundant read for !CONFIG_DEBUG_VM kernels.

Let's ameliorate matters by moving the read and the checks into a
helper, __check_racy_pte_update(), which only performs the read when the
value will be used. This also allows us to reformat the conditions for
clarity.

Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
alistair/sunxi64-5.4-dsi
Mark Rutland 2019-06-10 13:41:07 +01:00 committed by Catalin Marinas
parent 4745224b45
commit 9b60472205
1 changed files with 30 additions and 17 deletions
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47
arch/arm64/include/asm/pgtable.h
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@ -246,29 +246,42 @@ extern void __sync_icache_dcache(pte_t pteval);
*
* PTE_DIRTY || (PTE_WRITE && !PTE_RDONLY)
*/
static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
pte_t *ptep, pte_t pte)
static inline void __check_racy_pte_update(struct mm_struct *mm, pte_t *ptep,
pte_t pte)
{
pte_t old_pte;
if (!IS_ENABLED(CONFIG_DEBUG_VM))
return;
old_pte = READ_ONCE(*ptep);
if (!pte_valid(old_pte) || !pte_valid(pte))
return;
if (mm != current->active_mm && atomic_read(&mm->mm_users) <= 1)
return;
/*
* Check for potential race with hardware updates of the pte
* (ptep_set_access_flags safely changes valid ptes without going
* through an invalid entry).
*/
VM_WARN_ONCE(!pte_young(pte),
"%s: racy access flag clearing: 0x%016llx -> 0x%016llx",
__func__, pte_val(old_pte), pte_val(pte));
VM_WARN_ONCE(pte_write(old_pte) && !pte_dirty(pte),
"%s: racy dirty state clearing: 0x%016llx -> 0x%016llx",
__func__, pte_val(old_pte), pte_val(pte));
}
static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
pte_t *ptep, pte_t pte)
{
if (pte_present(pte) && pte_user_exec(pte) && !pte_special(pte))
__sync_icache_dcache(pte);
/*
* If the existing pte is valid, check for potential race with
* hardware updates of the pte (ptep_set_access_flags safely changes
* valid ptes without going through an invalid entry).
*/
old_pte = READ_ONCE(*ptep);
if (IS_ENABLED(CONFIG_DEBUG_VM) && pte_valid(old_pte) && pte_valid(pte) &&
(mm == current->active_mm || atomic_read(&mm->mm_users) > 1)) {
VM_WARN_ONCE(!pte_young(pte),
"%s: racy access flag clearing: 0x%016llx -> 0x%016llx",
__func__, pte_val(old_pte), pte_val(pte));
VM_WARN_ONCE(pte_write(old_pte) && !pte_dirty(pte),
"%s: racy dirty state clearing: 0x%016llx -> 0x%016llx",
__func__, pte_val(old_pte), pte_val(pte));
}
__check_racy_pte_update(mm, ptep, pte);
set_pte(ptep, pte);
}