[PATCH 10/12] x86/mm: Make pgd_alloc()/pgd_free() lockless

[Ingo Molnar]

The fork()/exit() code uses pgd_alloc()/pgd_free() to allocate/deallocate
the PGD, with platform specific code setting up kernel pagetables.

The x86 code uses a global pgd_list with an associated lock to update
all PGDs of all tasks in the system synchronously.

The lock is still kept to synchronize updates to all PGDs in the system,
but all users of the list have been migrated to use the task list.

So we can remove the pgd_list addition/removal from this code.

The new PGD is private while constructed, so it needs no extra
locking.

Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Waiman Long <Waiman.Long@xxxxxx>
Cc: linux-mm@xxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/x86/mm/pgtable.c | 27 +++------------------------
 1 file changed, 3 insertions(+), 24 deletions(-)

diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
index e0bf90470d70..d855010a111f 100644
--- a/arch/x86/mm/pgtable.c
+++ b/arch/x86/mm/pgtable.c
@@ -125,22 +125,6 @@ static void pgd_ctor(struct mm_struct *mm, pgd_t *pgd)
 				swapper_pg_dir + KERNEL_PGD_BOUNDARY,
 				KERNEL_PGD_PTRS);
 	}
-
-	/* list required to sync kernel mapping updates */
-	if (!SHARED_KERNEL_PMD) {
-		pgd_set_mm(pgd, mm);
-		pgd_list_add(pgd);
-	}
-}
-
-static void pgd_dtor(pgd_t *pgd)
-{
-	if (SHARED_KERNEL_PMD)
-		return;
-
-	spin_lock(&pgd_lock);
-	pgd_list_del(pgd);
-	spin_unlock(&pgd_lock);
 }
 
 /*
@@ -370,17 +354,13 @@ pgd_t *pgd_alloc(struct mm_struct *mm)
 		goto out_free_pmds;
 
 	/*
-	 * Make sure that pre-populating the pmds is atomic with
-	 * respect to anything walking the pgd_list, so that they
-	 * never see a partially populated pgd.
+	 * No locking is needed here, as the PGD is still private,
+	 * so no code walking the task list and looking at mm->pgd
+	 * will be able to see it before it's fully constructed:
 	 */
-	spin_lock(&pgd_lock);
-
 	pgd_ctor(mm, pgd);
 	pgd_prepopulate_pmd(mm, pgd, pmds);
 
-	spin_unlock(&pgd_lock);
-
 	return pgd;
 
 out_free_pmds:
@@ -451,7 +431,6 @@ void arch_pgd_init_late(struct mm_struct *mm, pgd_t *pgd)
 void pgd_free(struct mm_struct *mm, pgd_t *pgd)
 {
 	pgd_mop_up_pmds(mm, pgd);
-	pgd_dtor(pgd);
 	paravirt_pgd_free(mm, pgd);
 	_pgd_free(pgd);
 }
-- 
2.1.4

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