Merge tag 'trace-v7.3-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-tracegrafted

Pull tracing fixes from Steven Rostedt:

 - Fix error output of boot instance creation failure

   Currently if a boot instance creation fails, instead of printing out
   the name of the instance that failed, it prints "(null)". That is
   because it prints "cur_str" that had already been processed by
   strsep(). Print the saved name instead.

   While at it, print the error code of the failure.

 - Fix use-after-free for same named historgrams

   Histograms can be named so that they can be used in multiple events.
   But if the named histogram has a variable attached, the second event
   that uses the named histogram which duplicates it and needs to free
   the original after duplication leaves the old variable in place and
   still visible. If another histogram uses than variable, it will use
   the stale one which will try to reference the freed duplicate
   histogram and crash the kernel.

   Free the duplicate variables along with the duplicated histogram
   data.

 - Check return value of kthread_run() in event self test

   The events self tests uses a kthread for testing but does not check
   if it succeeded in creating a kthread. If the kthread creation were
   to fail, the code will still try to call kthread_stop() on the error
   returned.

 - Fix race between reading trace_pipe and updating subbuffer size

   If a user is reading the trace_pipe file at the same time they update
   the ring buffer sub-buffer size, can cause the trace_pipe read to
   read stale data. Add trace_access_lock() around updating the ring
   buffer sub-buffer size.

 - Fix eventfs_inode on failure path in creation of the events directory

   In the creation of the "events" directory, if after allocating the
   eventfs_inode a failure is detected, it calls cleanup_ei() which
   calls free_ei(). The free_ei() will test if eventfs_inode being freed
   has no children. It is a bug if it does. But on the failure case of
   the creation of the "events" directory, the children lists have not
   yet been initialized and the free will trigger a warning because
   list_empty() on an uninitialized list returns false.

   Move the initialization into init_ei() where it makes more sense and
   makes sure that a created eventfs_inode has its lists initialized
   upon creation.

 - Check return value of kthread_run() in ftrace direct sample code

   The sample code that shows how to use the ftrace direct calls does
   not test the return of kthread_run() to see if it succeeds. Return a
   failure if the kthread_run() doesn't succeed.

 - Clear user events state on fork in case of alloc failure

   On fork, the child gets a pointer to the parent's user events state.
   It makes a copy of it then updates the child's pointer to it. But if
   the allocation fails, the duplication function leaves the child with
   a pointer to its parent's descriptor. When the child cleans up its
   data, it will free the parent's descriptor while the parent is still
   using it.

   In the duplication function, set the child's user_event_mm to NULL
   before testing if the allocation succeeded, and when it exits it will
   not free the parent's descriptor.

 - Fix retry exhaustion in simple ring buffer reader swap

   simple_ring_buffer_swap_reader_page() starts with retry set to 8 and
   post-decrements it only after a failed link replacement. On the final
   attempt, a successful replacement leaves retry at zero, while a
   failed replacement leaves it at -1.

   But the check for success expects the retry value to be non-zero and
   exits with an error on zero. This is the opposite result. Fix it.

 - Fail nicely when the remote swap_reader_page() returns an error

   Currently, if the swap_reader_page() of a remote buffer fails, it
   triggers a WARN_ON_ONCE() and continues normally. Instead, have it
   exit with an error and a pr_warn() print instead of a full WARNING.

* tag 'trace-v7.3-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace:
  ring-buffer: Stop remote reader update when page swap fails
  tracing: Fix retry exhaustion in simple ring buffer reader swap
  tracing/user_events: Clear copied tracing state before fork duplication
  samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-multi-modify
  samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-modify
  eventfs: Initialize ei->children and ei->list in init_ei()
  tracing: Fix use-after-free in trace_pipe read on sub-buffer order change
  tracing: Fix crash passing ERR_PTR to kthread_stop()
  tracing: Fix use-after-free with same-name named triggers
  tracing: Fix logged instance name on creation failure

1 files changed, 152 insertions, 0 deletions

diff --git a/Documentation/RCU/UP.rst b/Documentation/RCU/UP.rst
new file mode 100644
index 000000000..4060d7a2f
--- /dev/null
+++ b/Documentation/RCU/UP.rst
@@ -0,0 +1,152 @@
+.. _up_doc:
+
+RCU on Uniprocessor Systems
+===========================
+
+A common misconception is that, on UP systems, the call_rcu() primitive
+may immediately invoke its function.  The basis of this misconception
+is that since there is only one CPU, it should not be necessary to
+wait for anything else to get done, since there are no other CPUs for
+anything else to be happening on.  Although this approach will *sort of*
+work a surprising amount of the time, it is a very bad idea in general.
+This document presents three examples that demonstrate exactly how bad
+an idea this is.
+
+Example 1: softirq Suicide
+--------------------------
+
+Suppose that an RCU-based algorithm scans a linked list containing
+elements A, B, and C in process context, and can delete elements from
+this same list in softirq context.  Suppose that the process-context scan
+is referencing element B when it is interrupted by softirq processing,
+which deletes element B, and then invokes call_rcu() to free element B
+after a grace period.
+
+Now, if call_rcu() were to directly invoke its arguments, then upon return
+from softirq, the list scan would find itself referencing a newly freed
+element B.  This situation can greatly decrease the life expectancy of
+your kernel.
+
+This same problem can occur if call_rcu() is invoked from a hardware
+interrupt handler.
+
+Example 2: Function-Call Fatality
+---------------------------------
+
+Of course, one could avert the suicide described in the preceding example
+by having call_rcu() directly invoke its arguments only if it was called
+from process context.  However, this can fail in a similar manner.
+
+Suppose that an RCU-based algorithm again scans a linked list containing
+elements A, B, and C in process context, but that it invokes a function
+on each element as it is scanned.  Suppose further that this function
+deletes element B from the list, then passes it to call_rcu() for deferred
+freeing.  This may be a bit unconventional, but it is perfectly legal
+RCU usage, since call_rcu() must wait for a grace period to elapse.
+Therefore, in this case, allowing call_rcu() to immediately invoke
+its arguments would cause it to fail to make the fundamental guarantee
+underlying RCU, namely that call_rcu() defers invoking its arguments until
+all RCU read-side critical sections currently executing have completed.
+
+Quick Quiz #1:
+	Why is it *not* legal to invoke synchronize_rcu() in this case?
+
+:ref:`Answers to Quick Quiz <answer_quick_quiz_up>`
+
+Example 3: Death by Deadlock
+----------------------------
+
+Suppose that call_rcu() is invoked while holding a lock, and that the
+callback function must acquire this same lock.  In this case, if
+call_rcu() were to directly invoke the callback, the result would
+be self-deadlock *even if* this invocation occurred from a later
+call_rcu() invocation a full grace period later.
+
+In some cases, it would possible to restructure to code so that
+the call_rcu() is delayed until after the lock is released.  However,
+there are cases where this can be quite ugly:
+
+1.	If a number of items need to be passed to call_rcu() within
+	the same critical section, then the code would need to create
+	a list of them, then traverse the list once the lock was
+	released.
+
+2.	In some cases, the lock will be held across some kernel API,
+	so that delaying the call_rcu() until the lock is released
+	requires that the data item be passed up via a common API.
+	It is far better to guarantee that callbacks are invoked
+	with no locks held than to have to modify such APIs to allow
+	arbitrary data items to be passed back up through them.
+
+If call_rcu() directly invokes the callback, painful locking restrictions
+or API changes would be required.
+
+Quick Quiz #2:
+	What locking restriction must RCU callbacks respect?
+
+:ref:`Answers to Quick Quiz <answer_quick_quiz_up>`
+
+It is important to note that userspace RCU implementations *do*
+permit call_rcu() to directly invoke callbacks, but only if a full
+grace period has elapsed since those callbacks were queued.  This is
+the case because some userspace environments are extremely constrained.
+Nevertheless, people writing userspace RCU implementations are strongly
+encouraged to avoid invoking callbacks from call_rcu(), thus obtaining
+the deadlock-avoidance benefits called out above.
+
+Summary
+-------
+
+Permitting call_rcu() to immediately invoke its arguments breaks RCU,
+even on a UP system.  So do not do it!  Even on a UP system, the RCU
+infrastructure *must* respect grace periods, and *must* invoke callbacks
+from a known environment in which no locks are held.
+
+Note that it *is* safe for synchronize_rcu() to return immediately on
+UP systems, including PREEMPT SMP builds running on UP systems.
+
+Quick Quiz #3:
+	Why can't synchronize_rcu() return immediately on UP systems running
+	preemptible RCU?
+
+.. _answer_quick_quiz_up:
+
+Answer to Quick Quiz #1:
+	Why is it *not* legal to invoke synchronize_rcu() in this case?
+
+	Because the calling function is scanning an RCU-protected linked
+	list, and is therefore within an RCU read-side critical section.
+	Therefore, the called function has been invoked within an RCU
+	read-side critical section, and is not permitted to block.
+
+Answer to Quick Quiz #2:
+	What locking restriction must RCU callbacks respect?
+
+	Any lock that is acquired within an RCU callback must be acquired
+	elsewhere using an _bh variant of the spinlock primitive.
+	For example, if "mylock" is acquired by an RCU callback, then
+	a process-context acquisition of this lock must use something
+	like spin_lock_bh() to acquire the lock.  Please note that
+	it is also OK to use _irq variants of spinlocks, for example,
+	spin_lock_irqsave().
+
+	If the process-context code were to simply use spin_lock(),
+	then, since RCU callbacks can be invoked from softirq context,
+	the callback might be called from a softirq that interrupted
+	the process-context critical section.  This would result in
+	self-deadlock.
+
+	This restriction might seem gratuitous, since very few RCU
+	callbacks acquire locks directly.  However, a great many RCU
+	callbacks do acquire locks *indirectly*, for example, via
+	the kfree() primitive.
+
+Answer to Quick Quiz #3:
+	Why can't synchronize_rcu() return immediately on UP systems
+	running preemptible RCU?
+
+	Because some other task might have been preempted in the middle
+	of an RCU read-side critical section.  If synchronize_rcu()
+	simply immediately returned, it would prematurely signal the
+	end of the grace period, which would come as a nasty shock to
+	that other thread when it started running again.