[ANNOUNCE] v5.15-rt17v5.15-rt17-patches

Dear RT folks!

I'm pleased to announce the v5.15-rt17 patch set.

Changes since v5.15-rt16:

  - Remove put_cpu_var() in fscache which removal was overseen in the
    last release.

  - Redo the fs/dcache/start_dir_add() related patch and its
    description.

  - Drop preempt_disable_rt(), no more users.

Known issues
     - netconsole triggers WARN.

     - The "Memory controller" (CONFIG_MEMCG) has been disabled.

     - Valentin Schneider reported a few splats on ARM64, see
          https://lkml.kernel.org/r/20210810134127.1394269-1-valentin.schneider@arm.com

The delta patch against v5.15-rt16 is appended below and can be found here:

     https://cdn.kernel.org/pub/linux/kernel/projects/rt/5.15/incr/patch-5.15-rt16-rt17.patch.xz

You can get this release via the git tree at:

    git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git v5.15-rt17

The RT patch against v5.15 can be found here:

    https://cdn.kernel.org/pub/linux/kernel/projects/rt/5.15/older/patch-5.15-rt17.patch.xz

The split quilt queue is available at:

    https://cdn.kernel.org/pub/linux/kernel/projects/rt/5.15/older/patches-5.15-rt17.tar.xz

Sebastian

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

5 files changed, 54 insertions, 108 deletions

diff --git a/patches/Add_localversion_for_-RT_release.patch b/patches/Add_localversion_for_-RT_release.patch
index 22146ab020cb..efeddd431fc4 100644
--- a/patches/Add_localversion_for_-RT_release.patch
+++ b/patches/Add_localversion_for_-RT_release.patch
@@ -15,4 +15,4 @@ Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
 --- /dev/null
 +++ b/localversion-rt
 @@ -0,0 +1 @@
-+-rt16
++-rt17
diff --git a/patches/fs_dcache__disable_preemption_on_i_dir_seqs_write_side.patch b/patches/fs_dcache__disable_preemption_on_i_dir_seqs_write_side.patch
index d603d8b410e0..e31b2b834f29 100644
--- a/patches/fs_dcache__disable_preemption_on_i_dir_seqs_write_side.patch
+++ b/patches/fs_dcache__disable_preemption_on_i_dir_seqs_write_side.patch
@@ -4,89 +4,61 @@ Date: Fri Oct 20 11:29:53 2017 +0200
 
 From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
 
-i_dir_seq is an opencoded seqcounter. Based on the code it looks like we
-could have two writers in parallel despite the fact that the d_lock is
-held. The problem is that during the write process on RT the preemption
-is still enabled and if this process is interrupted by a reader with RT
-priority then we lock up.
-To avoid that lock up I am disabling the preemption during the update.
-The rename of i_dir_seq is here to ensure to catch new write sides in
-future.
+i_dir_seq is a sequence counter with a lock which is represented by the lowest
+bit. The writer atomically updates the counter which ensures that it can be
+modified by only one writer at a time.
+The commit introducing this change claims that the lock has been integrated
+into the counter for space reasons within the inode struct. The i_dir_seq
+member is within a union which shares also a pointer. That means by using
+seqlock_t we would have a sequence counter and a lock without increasing the
+size of the data structure on 64bit and 32bit would grow by 4 bytes. With
+lockdep enabled the size would grow and on PREEMPT_RT the spinlock_t is also
+larger.
 
-Cc: stable-rt@vger.kernel.org
-Reported-by: Oleg.Karfich@wago.com
-Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
-Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
+In order to keep this construct working on PREEMPT_RT, the writer needs to
+disable preemption while obtaining the lock on the sequence counter / starting
+the write critical section. The writer acquires an otherwise unrelated
+spinlock_t which serves the same purpose on !PREEMPT_RT. With enabled
+preemption a high priority reader could preempt the writer and live lock the
+system while waiting for the locked bit to disappear.
+
+Another solution would be to have global spinlock_t which is always acquired
+by the writer. The reader would then acquire the lock if the sequence count is
+odd and by doing so force the writer out of the critical section. The global
+spinlock_t could be replaced by a hashed lock based on the address of the inode
+to lower the lock contention.
 
+For now, manually disable preemption on PREEMPT_RT to avoid live locks.
 
+Reported-by: Oleg.Karfich@wago.com
+Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
 ---
- fs/dcache.c        |   12 +++++++-----
- fs/inode.c         |    2 +-
- include/linux/fs.h |    2 +-
- 3 files changed, 9 insertions(+), 7 deletions(-)
+ fs/dcache.c |   10 +++++++++-
+ 1 file changed, 9 insertions(+), 1 deletion(-)
 ---
 --- a/fs/dcache.c
 +++ b/fs/dcache.c
-@@ -2538,9 +2538,10 @@ EXPORT_SYMBOL(d_rehash);
+@@ -2537,7 +2537,13 @@ EXPORT_SYMBOL(d_rehash);
+ 
  static inline unsigned start_dir_add(struct inode *dir)
  {
- 
-+	preempt_disable_rt();
+-
++	/*
++	 * The caller has a spinlock_t (dentry::d_lock) acquired which disables
++	 * preemption on !PREEMPT_RT. On PREEMPT_RT the lock does not disable
++	 * preemption and it has be done explicitly.
++	 */
++	if (IS_ENABLED(CONFIG_PREEMPT_RT))
++		preempt_disable();
  	for (;;) {
--		unsigned n = dir->i_dir_seq;
--		if (!(n & 1) && cmpxchg(&dir->i_dir_seq, n, n + 1) == n)
-+		unsigned n = dir->__i_dir_seq;
-+		if (!(n & 1) && cmpxchg(&dir->__i_dir_seq, n, n + 1) == n)
- 			return n;
- 		cpu_relax();
- 	}
-@@ -2548,7 +2549,8 @@ static inline unsigned start_dir_add(str
- 
+ 		unsigned n = dir->i_dir_seq;
+ 		if (!(n & 1) && cmpxchg(&dir->i_dir_seq, n, n + 1) == n)
+@@ -2549,6 +2555,8 @@ static inline unsigned start_dir_add(str
  static inline void end_dir_add(struct inode *dir, unsigned n)
  {
--	smp_store_release(&dir->i_dir_seq, n + 2);
-+	smp_store_release(&dir->__i_dir_seq, n + 2);
-+	preempt_enable_rt();
+ 	smp_store_release(&dir->i_dir_seq, n + 2);
++	if (IS_ENABLED(CONFIG_PREEMPT_RT))
++		preempt_enable();
  }
  
  static void d_wait_lookup(struct dentry *dentry)
-@@ -2584,7 +2586,7 @@ struct dentry *d_alloc_parallel(struct d
- 
- retry:
- 	rcu_read_lock();
--	seq = smp_load_acquire(&parent->d_inode->i_dir_seq);
-+	seq = smp_load_acquire(&parent->d_inode->__i_dir_seq);
- 	r_seq = read_seqbegin(&rename_lock);
- 	dentry = __d_lookup_rcu(parent, name, &d_seq);
- 	if (unlikely(dentry)) {
-@@ -2612,7 +2614,7 @@ struct dentry *d_alloc_parallel(struct d
- 	}
- 
- 	hlist_bl_lock(b);
--	if (unlikely(READ_ONCE(parent->d_inode->i_dir_seq) != seq)) {
-+	if (unlikely(READ_ONCE(parent->d_inode->__i_dir_seq) != seq)) {
- 		hlist_bl_unlock(b);
- 		rcu_read_unlock();
- 		goto retry;
---- a/fs/inode.c
-+++ b/fs/inode.c
-@@ -157,7 +157,7 @@ int inode_init_always(struct super_block
- 	inode->i_pipe = NULL;
- 	inode->i_cdev = NULL;
- 	inode->i_link = NULL;
--	inode->i_dir_seq = 0;
-+	inode->__i_dir_seq = 0;
- 	inode->i_rdev = 0;
- 	inode->dirtied_when = 0;
- 
---- a/include/linux/fs.h
-+++ b/include/linux/fs.h
-@@ -711,7 +711,7 @@ struct inode {
- 		struct pipe_inode_info	*i_pipe;
- 		struct cdev		*i_cdev;
- 		char			*i_link;
--		unsigned		i_dir_seq;
-+		unsigned		__i_dir_seq;
- 	};
- 
- 	__u32			i_generation;
diff --git a/patches/fscache-Use-only-one-fscache_object_cong_wait.patch b/patches/fscache-Use-only-one-fscache_object_cong_wait.patch
index 6a8f1b05a566..7ed718fb88fe 100644
--- a/patches/fscache-Use-only-one-fscache_object_cong_wait.patch
+++ b/patches/fscache-Use-only-one-fscache_object_cong_wait.patch
@@ -26,11 +26,12 @@ Fixes: 8b8edefa2fffb ("fscache: convert object to use workqueue instead of slow-
 Reported-by: Gregor Beck <gregor.beck@gmail.com>
 Cc: stable-rt@vger.kernel.org
 Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
+Link: https://lkml.kernel.org/r/20211029083839.xwwt7jgzru3kcpii@linutronix.de
 ---
  fs/fscache/internal.h |    1 -
  fs/fscache/main.c     |    6 ------
- fs/fscache/object.c   |   11 +++++------
- 3 files changed, 5 insertions(+), 13 deletions(-)
+ fs/fscache/object.c   |   13 +++++--------
+ 3 files changed, 5 insertions(+), 15 deletions(-)
 
 --- a/fs/fscache/internal.h
 +++ b/fs/fscache/internal.h
@@ -82,7 +83,7 @@ Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
  /*
   * enqueue an object for metadata-type processing
   */
-@@ -806,12 +808,10 @@ void fscache_enqueue_object(struct fscac
+@@ -806,16 +808,12 @@ void fscache_enqueue_object(struct fscac
  	_enter("{OBJ%x}", object->debug_id);
  
  	if (fscache_get_object(object, fscache_obj_get_queue) >= 0) {
@@ -95,8 +96,12 @@ Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
 +				wake_up(&fscache_object_cong_wait);
  		} else
  			fscache_put_object(object, fscache_obj_put_queue);
+-
+-		put_cpu_var(fscache_object_cong_wait);
+ 	}
+ }
  
-@@ -833,16 +833,15 @@ void fscache_enqueue_object(struct fscac
+@@ -833,16 +831,15 @@ void fscache_enqueue_object(struct fscac
   */
  bool fscache_object_sleep_till_congested(signed long *timeoutp)
  {
diff --git a/patches/sched-Add-preempt_disable_rt.patch b/patches/sched-Add-preempt_disable_rt.patch
deleted file mode 100644
index ab250dcb1014..000000000000
--- a/patches/sched-Add-preempt_disable_rt.patch
+++ /dev/null
@@ -1,30 +0,0 @@
-From: Thomas Gleixner <tglx@linutronix.de>
-Date: Fri, 17 Sep 2021 12:56:34 +0200
-Subject: [PATCH] sched: Add preempt_disable_rt()
-
-RT needs a few preempt_disable/enable points which are not necessary
-otherwise. Implement variants to avoid #ifdeffery.
-
-Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
-Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
----
- include/linux/preempt.h |    8 ++++++++
- 1 file changed, 8 insertions(+)
-
---- a/include/linux/preempt.h
-+++ b/include/linux/preempt.h
-@@ -283,6 +283,14 @@ do { \
- 		set_preempt_need_resched(); \
- } while (0)
- 
-+#ifdef CONFIG_PREEMPT_RT
-+# define preempt_disable_rt()		preempt_disable()
-+# define preempt_enable_rt()		preempt_enable()
-+#else
-+# define preempt_disable_rt()		barrier()
-+# define preempt_enable_rt()		barrier()
-+#endif
-+
- #ifdef CONFIG_PREEMPT_NOTIFIERS
- 
- struct preempt_notifier;
diff --git a/patches/series b/patches/series
index 6467f5571a62..04ff72f08b60 100644
--- a/patches/series
+++ b/patches/series
@@ -44,6 +44,7 @@ mm_allow_only_slub_on_preempt_rt.patch
 mm_page_alloc_use_migrate_disable_in_drain_local_pages_wq.patch
 mm_scatterlist_replace_the_preemptible_warning_in_sg_miter_stop.patch
 mm-Disable-NUMA_BALANCING_DEFAULT_ENABLED-and-TRANSP.patch
+x86-softirq-Disable-softirq-stacks-on-PREEMPT_RT.patch
 
 # KCOV (akpm)
 0001_documentation_kcov_include_types_h_in_the_example.patch
@@ -79,7 +80,6 @@ net-stats-Read-the-statistics-in-___gnet_stats_copy_.patch
 ###########################################################################
 irq_poll-Use-raise_softirq_irqoff-in-cpu_dead-notifi.patch
 smp_wake_ksoftirqd_on_preempt_rt_instead_do_softirq.patch
-x86-softirq-Disable-softirq-stacks-on-PREEMPT_RT.patch
 fs-namespace-Boost-the-mount_lock.lock-owner-instead.patch
 fscache-Use-only-one-fscache_object_cong_wait.patch
 
@@ -129,7 +129,6 @@ locking-Allow-to-include-asm-spinlock_types.h-from-l.patch
 ###########################################################################
 # preempt: Conditional variants
 ###########################################################################
-sched-Add-preempt_disable_rt.patch
 sched-Make-preempt_enable_no_resched-behave-like-pre.patch
 
 ###########################################################################