diff mbox series

[v3,3/3] mm: introduce mmap_lock_speculate_{try_begin|retry}

Message ID 20241122174416.1367052-3-surenb@google.com (mailing list archive)
State New
Headers show
Series [v3,1/3] seqlock: add raw_seqcount_try_begin | expand

Commit Message

Suren Baghdasaryan Nov. 22, 2024, 5:44 p.m. UTC 
Add helper functions to speculatively perform operations without
read-locking mmap_lock, expecting that mmap_lock will not be
write-locked and mm is not modified from under us.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
---
Changes since v2 [1]
- Added SOB, per Liam Howlett

[1] https://lore.kernel.org/all/20241121162826.987947-3-surenb@google.com/

 include/linux/mmap_lock.h | 33 +++++++++++++++++++++++++++++++--
 1 file changed, 31 insertions(+), 2 deletions(-)

Comments

Wei Yang Nov. 25, 2024, 12:58 a.m. UTC | #1 
On Fri, Nov 22, 2024 at 09:44:16AM -0800, Suren Baghdasaryan wrote:
>Add helper functions to speculatively perform operations without
>read-locking mmap_lock, expecting that mmap_lock will not be
>write-locked and mm is not modified from under us.
>
>Suggested-by: Peter Zijlstra <peterz@infradead.org>
>Signed-off-by: Suren Baghdasaryan <surenb@google.com>
>Reviewed-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
>---
>Changes since v2 [1]
>- Added SOB, per Liam Howlett
>
>[1] https://lore.kernel.org/all/20241121162826.987947-3-surenb@google.com/
>
> include/linux/mmap_lock.h | 33 +++++++++++++++++++++++++++++++--
> 1 file changed, 31 insertions(+), 2 deletions(-)
>
>diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
>index 9715326f5a85..8ac3041df053 100644
>--- a/include/linux/mmap_lock.h
>+++ b/include/linux/mmap_lock.h
>@@ -71,6 +71,7 @@ static inline void mmap_assert_write_locked(const struct mm_struct *mm)
> }
> 
> #ifdef CONFIG_PER_VMA_LOCK
>+
> static inline void mm_lock_seqcount_init(struct mm_struct *mm)
> {
> 	seqcount_init(&mm->mm_lock_seq);
>@@ -87,11 +88,39 @@ static inline void mm_lock_seqcount_end(struct mm_struct *mm)
> 	do_raw_write_seqcount_end(&mm->mm_lock_seq);
> }
> 
>-#else
>+static inline bool mmap_lock_speculate_try_begin(struct mm_struct *mm, unsigned int *seq)
>+{
>+	/*
>+	 * Since mmap_lock is a sleeping lock, and waiting for it to become
>+	 * unlocked is more or less equivalent with taking it ourselves, don't
>+	 * bother with the speculative path if mmap_lock is already write-locked
>+	 * and take the slow path, which takes the lock.
>+	 */
>+	return raw_seqcount_try_begin(&mm->mm_lock_seq, *seq);
>+}
>+
>+static inline bool mmap_lock_speculate_retry(struct mm_struct *mm, unsigned int seq)
>+{
>+	return do_read_seqcount_retry(&mm->mm_lock_seq, seq);

Just curious why we don't use read_seqcount_retry().

Looks this is the only user outside seqlock.h.

>+}
>+
>+#else /* CONFIG_PER_VMA_LOCK */
>+
> static inline void mm_lock_seqcount_init(struct mm_struct *mm) {}
> static inline void mm_lock_seqcount_begin(struct mm_struct *mm) {}
> static inline void mm_lock_seqcount_end(struct mm_struct *mm) {}
>-#endif
>+
>+static inline bool mmap_lock_speculate_try_begin(struct mm_struct *mm, unsigned int *seq)
>+{
>+	return false;
>+}
>+
>+static inline bool mmap_lock_speculate_retry(struct mm_struct *mm, unsigned int seq)
>+{
>+	return true;
>+}
>+
>+#endif /* CONFIG_PER_VMA_LOCK */
> 
> static inline void mmap_init_lock(struct mm_struct *mm)
> {
>-- 
>2.47.0.371.ga323438b13-goog
Suren Baghdasaryan Nov. 25, 2024, 4:18 p.m. UTC | #2 
On Sun, Nov 24, 2024 at 4:58 PM Wei Yang <richard.weiyang@gmail.com> wrote:
>
> On Fri, Nov 22, 2024 at 09:44:16AM -0800, Suren Baghdasaryan wrote:
> >Add helper functions to speculatively perform operations without
> >read-locking mmap_lock, expecting that mmap_lock will not be
> >write-locked and mm is not modified from under us.
> >
> >Suggested-by: Peter Zijlstra <peterz@infradead.org>
> >Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> >Reviewed-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
> >---
> >Changes since v2 [1]
> >- Added SOB, per Liam Howlett
> >
> >[1] https://lore.kernel.org/all/20241121162826.987947-3-surenb@google.com/
> >
> > include/linux/mmap_lock.h | 33 +++++++++++++++++++++++++++++++--
> > 1 file changed, 31 insertions(+), 2 deletions(-)
> >
> >diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
> >index 9715326f5a85..8ac3041df053 100644
> >--- a/include/linux/mmap_lock.h
> >+++ b/include/linux/mmap_lock.h
> >@@ -71,6 +71,7 @@ static inline void mmap_assert_write_locked(const struct mm_struct *mm)
> > }
> >
> > #ifdef CONFIG_PER_VMA_LOCK
> >+
> > static inline void mm_lock_seqcount_init(struct mm_struct *mm)
> > {
> >       seqcount_init(&mm->mm_lock_seq);
> >@@ -87,11 +88,39 @@ static inline void mm_lock_seqcount_end(struct mm_struct *mm)
> >       do_raw_write_seqcount_end(&mm->mm_lock_seq);
> > }
> >
> >-#else
> >+static inline bool mmap_lock_speculate_try_begin(struct mm_struct *mm, unsigned int *seq)
> >+{
> >+      /*
> >+       * Since mmap_lock is a sleeping lock, and waiting for it to become
> >+       * unlocked is more or less equivalent with taking it ourselves, don't
> >+       * bother with the speculative path if mmap_lock is already write-locked
> >+       * and take the slow path, which takes the lock.
> >+       */
> >+      return raw_seqcount_try_begin(&mm->mm_lock_seq, *seq);
> >+}
> >+
> >+static inline bool mmap_lock_speculate_retry(struct mm_struct *mm, unsigned int seq)
> >+{
> >+      return do_read_seqcount_retry(&mm->mm_lock_seq, seq);
>
> Just curious why we don't use read_seqcount_retry().
>
> Looks this is the only user outside seqlock.h.

Ah, good eye! read_seqcount_retry() would be better.

Peter, do you want me to post a new patchset or you can patch it when
picking it up?

>
> >+}
> >+
> >+#else /* CONFIG_PER_VMA_LOCK */
> >+
> > static inline void mm_lock_seqcount_init(struct mm_struct *mm) {}
> > static inline void mm_lock_seqcount_begin(struct mm_struct *mm) {}
> > static inline void mm_lock_seqcount_end(struct mm_struct *mm) {}
> >-#endif
> >+
> >+static inline bool mmap_lock_speculate_try_begin(struct mm_struct *mm, unsigned int *seq)
> >+{
> >+      return false;
> >+}
> >+
> >+static inline bool mmap_lock_speculate_retry(struct mm_struct *mm, unsigned int seq)
> >+{
> >+      return true;
> >+}
> >+
> >+#endif /* CONFIG_PER_VMA_LOCK */
> >
> > static inline void mmap_init_lock(struct mm_struct *mm)
> > {
> >--
> >2.47.0.371.ga323438b13-goog
>
> --
> Wei Yang
> Help you, Help me
Peter Zijlstra Nov. 25, 2024, 4:53 p.m. UTC | #3 
On Mon, Nov 25, 2024 at 08:18:38AM -0800, Suren Baghdasaryan wrote:

> > >+static inline bool mmap_lock_speculate_retry(struct mm_struct *mm, unsigned int seq)
> > >+{
> > >+      return do_read_seqcount_retry(&mm->mm_lock_seq, seq);
> >
> > Just curious why we don't use read_seqcount_retry().
> >
> > Looks this is the only user outside seqlock.h.
> 
> Ah, good eye! read_seqcount_retry() would be better.
> 
> Peter, do you want me to post a new patchset or you can patch it when
> picking it up?

Fixed up my local copy, thanks!
diff mbox series

Patch

diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
index 9715326f5a85..8ac3041df053 100644
--- a/include/linux/mmap_lock.h
+++ b/include/linux/mmap_lock.h
@@ -71,6 +71,7 @@  static inline void mmap_assert_write_locked(const struct mm_struct *mm)
 }
 
 #ifdef CONFIG_PER_VMA_LOCK
+
 static inline void mm_lock_seqcount_init(struct mm_struct *mm)
 {
 	seqcount_init(&mm->mm_lock_seq);
@@ -87,11 +88,39 @@  static inline void mm_lock_seqcount_end(struct mm_struct *mm)
 	do_raw_write_seqcount_end(&mm->mm_lock_seq);
 }
 
-#else
+static inline bool mmap_lock_speculate_try_begin(struct mm_struct *mm, unsigned int *seq)
+{
+	/*
+	 * Since mmap_lock is a sleeping lock, and waiting for it to become
+	 * unlocked is more or less equivalent with taking it ourselves, don't
+	 * bother with the speculative path if mmap_lock is already write-locked
+	 * and take the slow path, which takes the lock.
+	 */
+	return raw_seqcount_try_begin(&mm->mm_lock_seq, *seq);
+}
+
+static inline bool mmap_lock_speculate_retry(struct mm_struct *mm, unsigned int seq)
+{
+	return do_read_seqcount_retry(&mm->mm_lock_seq, seq);
+}
+
+#else /* CONFIG_PER_VMA_LOCK */
+
 static inline void mm_lock_seqcount_init(struct mm_struct *mm) {}
 static inline void mm_lock_seqcount_begin(struct mm_struct *mm) {}
 static inline void mm_lock_seqcount_end(struct mm_struct *mm) {}
-#endif
+
+static inline bool mmap_lock_speculate_try_begin(struct mm_struct *mm, unsigned int *seq)
+{
+	return false;
+}
+
+static inline bool mmap_lock_speculate_retry(struct mm_struct *mm, unsigned int seq)
+{
+	return true;
+}
+
+#endif /* CONFIG_PER_VMA_LOCK */
 
 static inline void mmap_init_lock(struct mm_struct *mm)
 {