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, 144 insertions, 0 deletions

diff --git a/Documentation/locking/preempt-locking.rst b/Documentation/locking/preempt-locking.rst
new file mode 100644
index 000000000..dce336134
--- /dev/null
+++ b/Documentation/locking/preempt-locking.rst
@@ -0,0 +1,144 @@
+===========================================================================
+Proper Locking Under a Preemptible Kernel: Keeping Kernel Code Preempt-Safe
+===========================================================================
+
+:Author: Robert Love <rml@tech9.net>
+
+
+Introduction
+============
+
+
+A preemptible kernel creates new locking issues.  The issues are the same as
+those under SMP: concurrency and reentrancy.  Thankfully, the Linux preemptible
+kernel model leverages existing SMP locking mechanisms.  Thus, the kernel
+requires explicit additional locking for very few additional situations.
+
+This document is for all kernel hackers.  Developing code in the kernel
+requires protecting these situations.
+ 
+
+RULE #1: Per-CPU data structures need explicit protection
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+
+Two similar problems arise. An example code snippet::
+
+	struct this_needs_locking tux[NR_CPUS];
+	tux[smp_processor_id()] = some_value;
+	/* task is preempted here... */
+	something = tux[smp_processor_id()];
+
+First, since the data is per-CPU, it may not have explicit SMP locking, but
+require it otherwise.  Second, when a preempted task is finally rescheduled,
+the previous value of smp_processor_id may not equal the current.  You must
+protect these situations by disabling preemption around them.
+
+You can also use put_cpu() and get_cpu(), which will disable preemption.
+
+
+RULE #2: CPU state must be protected.
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+
+Under preemption, the state of the CPU must be protected.  This is arch-
+dependent, but includes CPU structures and state not preserved over a context
+switch.  For example, on x86, entering and exiting FPU mode is now a critical
+section that must occur while preemption is disabled.  Think what would happen
+if the kernel is executing a floating-point instruction and is then preempted.
+Remember, the kernel does not save FPU state except for user tasks.  Therefore,
+upon preemption, the FPU registers will be sold to the lowest bidder.  Thus,
+preemption must be disabled around such regions.
+
+Note, some FPU functions are already explicitly preempt safe.  For example,
+kernel_fpu_begin and kernel_fpu_end will disable and enable preemption.
+
+
+RULE #3: Lock acquire and release must be performed by same task
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+
+A lock acquired in one task must be released by the same task.  This
+means you can't do oddball things like acquire a lock and go off to
+play while another task releases it.  If you want to do something
+like this, acquire and release the task in the same code path and
+have the caller wait on an event by the other task.
+
+
+Solution
+========
+
+
+Data protection under preemption is achieved by disabling preemption for the
+duration of the critical region.
+
+::
+
+  preempt_enable()		decrement the preempt counter
+  preempt_disable()		increment the preempt counter
+  preempt_enable_no_resched()	decrement, but do not immediately preempt
+  preempt_check_resched()	if needed, reschedule
+  preempt_count()		return the preempt counter
+
+The functions are nestable.  In other words, you can call preempt_disable
+n-times in a code path, and preemption will not be reenabled until the n-th
+call to preempt_enable.  The preempt statements define to nothing if
+preemption is not enabled.
+
+Note that you do not need to explicitly prevent preemption if you are holding
+any locks or interrupts are disabled, since preemption is implicitly disabled
+in those cases.
+
+But keep in mind that 'irqs disabled' is a fundamentally unsafe way of
+disabling preemption - any cond_resched() or cond_resched_lock() might trigger
+a reschedule if the preempt count is 0. A simple printk() might trigger a
+reschedule. So use this implicit preemption-disabling property only if you
+know that the affected codepath does not do any of this. Best policy is to use
+this only for small, atomic code that you wrote and which calls no complex
+functions.
+
+Example::
+
+	cpucache_t *cc; /* this is per-CPU */
+	preempt_disable();
+	cc = cc_data(searchp);
+	if (cc && cc->avail) {
+		__free_block(searchp, cc_entry(cc), cc->avail);
+		cc->avail = 0;
+	}
+	preempt_enable();
+	return 0;
+
+Notice how the preemption statements must encompass every reference of the
+critical variables.  Another example::
+
+	int buf[NR_CPUS];
+	set_cpu_val(buf);
+	if (buf[smp_processor_id()] == -1) printf(KERN_INFO "wee!\n");
+	spin_lock(&buf_lock);
+	/* ... */
+
+This code is not preempt-safe, but see how easily we can fix it by simply
+moving the spin_lock up two lines.
+
+
+Preventing preemption using interrupt disabling
+===============================================
+
+
+It is possible to prevent a preemption event using local_irq_disable and
+local_irq_save.  Note, when doing so, you must be very careful to not cause
+an event that would set need_resched and result in a preemption check.  When
+in doubt, rely on locking or explicit preemption disabling.
+
+Note in 2.5 interrupt disabling is now only per-CPU (e.g. local).
+
+An additional concern is proper usage of local_irq_disable and local_irq_save.
+These may be used to protect from preemption, however, on exit, if preemption
+may be enabled, a test to see if preemption is required should be done.  If
+these are called from the spin_lock and read/write lock macros, the right thing
+is done.  They may also be called within a spin-lock protected region, however,
+if they are ever called outside of this context, a test for preemption should
+be made. Do note that calls from interrupt context or bottom half/ tasklets
+are also protected by preemption locks and so may use the versions which do
+not check preemption.