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Date:   Thu, 26 Mar 2020 19:10:54 -0700
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Subject: [PATCH v2 06/10] mmap locking API: convert nested write lock sites
From:   Michel Lespinasse <walken@google.com>
To:     Andrew Morton <akpm@linux-foundation.org>,
        linux-mm <linux-mm@kvack.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Laurent Dufour <ldufour@linux.ibm.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Matthew Wilcox <willy@infradead.org>,
        Liam Howlett <Liam.Howlett@oracle.com>,
        Jerome Glisse <jglisse@redhat.com>,
        Davidlohr Bueso <dave@stgolabs.net>,
        David Rientjes <rientjes@google.com>,
        Hugh Dickins <hughd@google.com>, Ying Han <yinghan@google.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Markus Elfring <Markus.Elfring@web.de>,
        Michel Lespinasse <walken@google.com>
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Sender: linux-kernel-owner@vger.kernel.org
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Add API for nested write locks and convert the few call sites doing that.

Signed-off-by: Michel Lespinasse <walken@google.com>
---
 arch/um/include/asm/mmu_context.h |  5 +++--
 include/linux/mmap_lock.h         | 11 +++++++++++
 kernel/fork.c                     |  4 ++--
 3 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/arch/um/include/asm/mmu_context.h b/arch/um/include/asm/mmu_context.h
index 62262c5c7785..cc15173f7518 100644
--- a/arch/um/include/asm/mmu_context.h
+++ b/arch/um/include/asm/mmu_context.h
@@ -8,6 +8,7 @@
 
 #include <linux/sched.h>
 #include <linux/mm_types.h>
+#include <linux/mmap_lock.h>
 
 #include <asm/mmu.h>
 
@@ -47,9 +48,9 @@ static inline void activate_mm(struct mm_struct *old, struct mm_struct *new)
 	 * when the new ->mm is used for the first time.
 	 */
 	__switch_mm(&new->context.id);
-	down_write_nested(&new->mmap_sem, 1);
+	mmap_write_lock_nested(new, 1);
 	uml_setup_stubs(new);
-	mmap_write_unlock(new);
+	mmap_write_unlock_nested(new);
 }
 
 static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next, 
diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
index cffd25afe92b..40a972a26857 100644
--- a/include/linux/mmap_lock.h
+++ b/include/linux/mmap_lock.h
@@ -11,6 +11,11 @@ static inline void mmap_write_lock(struct mm_struct *mm)
 	down_write(&mm->mmap_sem);
 }
 
+static inline void mmap_write_lock_nested(struct mm_struct *mm, int subclass)
+{
+	down_write_nested(&mm->mmap_sem, subclass);
+}
+
 static inline int mmap_write_lock_killable(struct mm_struct *mm)
 {
 	return down_write_killable(&mm->mmap_sem);
@@ -26,6 +31,12 @@ static inline void mmap_write_unlock(struct mm_struct *mm)
 	up_write(&mm->mmap_sem);
 }
 
+/* Pairs with mmap_write_lock_nested() */
+static inline void mmap_write_unlock_nested(struct mm_struct *mm)
+{
+	up_write(&mm->mmap_sem);
+}
+
 static inline void mmap_downgrade_write_lock(struct mm_struct *mm)
 {
 	downgrade_write(&mm->mmap_sem);
diff --git a/kernel/fork.c b/kernel/fork.c
index c321910d46e8..3460308b2213 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -497,7 +497,7 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
 	/*
 	 * Not linked in yet - no deadlock potential:
 	 */
-	down_write_nested(&mm->mmap_sem, SINGLE_DEPTH_NESTING);
+	mmap_write_lock_nested(mm, SINGLE_DEPTH_NESTING);
 
 	/* No ordering required: file already has been exposed. */
 	RCU_INIT_POINTER(mm->exe_file, get_mm_exe_file(oldmm));
@@ -612,7 +612,7 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
 	/* a new mm has just been created */
 	retval = arch_dup_mmap(oldmm, mm);
 out:
-	mmap_write_unlock(mm);
+	mmap_write_unlock_nested(mm);
 	flush_tlb_mm(oldmm);
 	mmap_write_unlock(oldmm);
 	dup_userfaultfd_complete(&uf);
-- 
2.26.0.rc2.310.g2932bb562d-goog