Merge tag 'rcu.2023.02.10a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu

Pull RCU updates from Paul McKenney:

 - Documentation updates

 - Miscellaneous fixes, perhaps most notably:

      - Throttling callback invocation based on the number of callbacks
        that are now ready to invoke instead of on the total number of
        callbacks

      - Several patches that suppress false-positive boot-time
        diagnostics, for example, due to lockdep not yet being
        initialized

      - Make expedited RCU CPU stall warnings dump stacks of any tasks
        that are blocking the stalled grace period. (Normal RCU CPU
        stall warnings have done this for many years)

      - Lazy-callback fixes to avoid delays during boot, suspend, and
        resume. (Note that lazy callbacks must be explicitly enabled, so
        this should not (yet) affect production use cases)

 - Make kfree_rcu() and friends take advantage of polled grace periods,
   thus reducing memory footprint by almost two orders of magnitude,
   admittedly on a microbenchmark

   This also begins the transition from kfree_rcu(p) to
   kfree_rcu_mightsleep(p). This transition was motivated by bugs where
   kfree_rcu(p), which can block, was typed instead of the intended
   kfree_rcu(p, rh)

 - SRCU updates, perhaps most notably fixing a bug that causes SRCU to
   fail when booted on a system with a non-zero boot CPU. This
   surprising situation actually happens for kdump kernels on the
   powerpc architecture

   This also adds an srcu_down_read() and srcu_up_read(), which act like
   srcu_read_lock() and srcu_read_unlock(), but allow an SRCU read-side
   critical section to be handed off from one task to another

 - Clean up the now-useless SRCU Kconfig option

   There are a few more commits that are not yet acked or pulled into
   maintainer trees, and these will be in a pull request for a later
   merge window

 - RCU-tasks updates, perhaps most notably these fixes:

      - A strange interaction between PID-namespace unshare and the
        RCU-tasks grace period that results in a low-probability but
        very real hang

      - A race between an RCU tasks rude grace period on a single-CPU
        system and CPU-hotplug addition of the second CPU that can
        result in a too-short grace period

      - A race between shrinking RCU tasks down to a single callback
        list and queuing a new callback to some other CPU, but where
        that queuing is delayed for more than an RCU grace period. This
        can result in that callback being stranded on the non-boot CPU

 - Torture-test updates and fixes

 - Torture-test scripting updates and fixes

 - Provide additional RCU CPU stall-warning information in kernels built
   with CONFIG_RCU_CPU_STALL_CPUTIME=y, and restore the full five-minute
   timeout limit for expedited RCU CPU stall warnings

* tag 'rcu.2023.02.10a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu: (80 commits)
  rcu/kvfree: Add kvfree_rcu_mightsleep() and kfree_rcu_mightsleep()
  kernel/notifier: Remove CONFIG_SRCU
  init: Remove "select SRCU"
  fs/quota: Remove "select SRCU"
  fs/notify: Remove "select SRCU"
  fs/btrfs: Remove "select SRCU"
  fs: Remove CONFIG_SRCU
  drivers/pci/controller: Remove "select SRCU"
  drivers/net: Remove "select SRCU"
  drivers/md: Remove "select SRCU"
  drivers/hwtracing/stm: Remove "select SRCU"
  drivers/dax: Remove "select SRCU"
  drivers/base: Remove CONFIG_SRCU
  rcu: Disable laziness if lazy-tracking says so
  rcu: Track laziness during boot and suspend
  rcu: Remove redundant call to rcu_boost_kthread_setaffinity()
  rcu: Allow up to five minutes expedited RCU CPU stall-warning timeouts
  rcu: Align the output of RCU CPU stall warning messages
  rcu: Add RCU stall diagnosis information
  sched: Add helper nr_context_switches_cpu()
  ...

1 files changed, 45 insertions, 0 deletions

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index 9b9d0bbf1d3c..74796cd7e7a9 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -215,6 +215,34 @@ srcu_read_lock_notrace(struct srcu_struct *ssp) __acquires(ssp)
 }
 
 /**
+ * srcu_down_read - register a new reader for an SRCU-protected structure.
+ * @ssp: srcu_struct in which to register the new reader.
+ *
+ * Enter a semaphore-like SRCU read-side critical section.  Note that
+ * SRCU read-side critical sections may be nested.  However, it is
+ * illegal to call anything that waits on an SRCU grace period for the
+ * same srcu_struct, whether directly or indirectly.  Please note that
+ * one way to indirectly wait on an SRCU grace period is to acquire
+ * a mutex that is held elsewhere while calling synchronize_srcu() or
+ * synchronize_srcu_expedited().  But if you want lockdep to help you
+ * keep this stuff straight, you should instead use srcu_read_lock().
+ *
+ * The semaphore-like nature of srcu_down_read() means that the matching
+ * srcu_up_read() can be invoked from some other context, for example,
+ * from some other task or from an irq handler.  However, neither
+ * srcu_down_read() nor srcu_up_read() may be invoked from an NMI handler.
+ *
+ * Calls to srcu_down_read() may be nested, similar to the manner in
+ * which calls to down_read() may be nested.
+ */
+static inline int srcu_down_read(struct srcu_struct *ssp) __acquires(ssp)
+{
+	WARN_ON_ONCE(in_nmi());
+	srcu_check_nmi_safety(ssp, false);
+	return __srcu_read_lock(ssp);
+}
+
+/**
  * srcu_read_unlock - unregister a old reader from an SRCU-protected structure.
  * @ssp: srcu_struct in which to unregister the old reader.
  * @idx: return value from corresponding srcu_read_lock().
@@ -255,6 +283,23 @@ srcu_read_unlock_notrace(struct srcu_struct *ssp, int idx) __releases(ssp)
 }
 
 /**
+ * srcu_up_read - unregister a old reader from an SRCU-protected structure.
+ * @ssp: srcu_struct in which to unregister the old reader.
+ * @idx: return value from corresponding srcu_read_lock().
+ *
+ * Exit an SRCU read-side critical section, but not necessarily from
+ * the same context as the maching srcu_down_read().
+ */
+static inline void srcu_up_read(struct srcu_struct *ssp, int idx)
+	__releases(ssp)
+{
+	WARN_ON_ONCE(idx & ~0x1);
+	WARN_ON_ONCE(in_nmi());
+	srcu_check_nmi_safety(ssp, false);
+	__srcu_read_unlock(ssp, idx);
+}
+
+/**
  * smp_mb__after_srcu_read_unlock - ensure full ordering after srcu_read_unlock
  *
  * Converts the preceding srcu_read_unlock into a two-way memory barrier.