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, 96 insertions, 0 deletions

diff --git a/Documentation/maintainer/messy-diffstat.rst b/Documentation/maintainer/messy-diffstat.rst
new file mode 100644
index 000000000..c015f66d7
--- /dev/null
+++ b/Documentation/maintainer/messy-diffstat.rst
@@ -0,0 +1,96 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=====================================
+Handling messy pull-request diffstats
+=====================================
+
+Subsystem maintainers routinely use ``git request-pull`` as part of the
+process of sending work upstream.  Normally, the result includes a nice
+diffstat that shows which files will be touched and how much of each will
+be changed.  Occasionally, though, a repository with a relatively
+complicated development history will yield a massive diffstat containing a
+great deal of unrelated work.  The result looks ugly and obscures what the
+pull request is actually doing.  This document describes what is happening
+and how to fix things up; it is derived from The Wisdom of Linus Torvalds,
+found in Linus1_ and Linus2_.
+
+.. _Linus1: https://lore.kernel.org/lkml/CAHk-=wg3wXH2JNxkQi+eLZkpuxqV+wPiHhw_Jf7ViH33Sw7PHA@mail.gmail.com/
+.. _Linus2: https://lore.kernel.org/lkml/CAHk-=wgXbSa8yq8Dht8at+gxb_idnJ7X5qWZQWRBN4_CUPr=eQ@mail.gmail.com/
+
+A Git development history proceeds as a series of commits.  In a simplified
+manner, mainline kernel development looks like this::
+
+  ... vM --- vN-rc1 --- vN-rc2 --- vN-rc3 --- ... --- vN-rc7 --- vN
+
+If one wants to see what has changed between two points, a command like
+this will do the job::
+
+  $ git diff --stat --summary vN-rc2..vN-rc3
+
+Here, there are two clear points in the history; Git will essentially
+"subtract" the beginning point from the end point and display the resulting
+differences.  The requested operation is unambiguous and easy enough to
+understand.
+
+When a subsystem maintainer creates a branch and commits changes to it, the
+result in the simplest case is a history that looks like::
+
+  ... vM --- vN-rc1 --- vN-rc2 --- vN-rc3 --- ... --- vN-rc7 --- vN
+                          |
+                          +-- c1 --- c2 --- ... --- cN
+
+If that maintainer now uses ``git diff`` to see what has changed between
+the mainline branch (let's call it "linus") and cN, there are still two
+clear endpoints, and the result is as expected.  So a pull request
+generated with ``git request-pull`` will also be as expected.  But now
+consider a slightly more complex development history::
+
+  ... vM --- vN-rc1 --- vN-rc2 --- vN-rc3 --- ... --- vN-rc7 --- vN
+                |         |
+                |         +-- c1 --- c2 --- ... --- cN
+                |                   /
+                +-- x1 --- x2 --- x3
+
+Our maintainer has created one branch at vN-rc1 and another at vN-rc2; the
+two were then subsequently merged into c2.  Now a pull request generated
+for cN may end up being messy indeed, and developers often end up wondering
+why.
+
+What is happening here is that there are no longer two clear end points for
+the ``git diff`` operation to use.  The development culminating in cN
+started in two different places; to generate the diffstat, ``git diff``
+ends up having pick one of them and hoping for the best.  If the diffstat
+starts at vN-rc1, it may end up including all of the changes between there
+and the second origin end point (vN-rc2), which is certainly not what our
+maintainer had in mind.  With all of that extra junk in the diffstat, it
+may be impossible to tell what actually happened in the changes leading up
+to cN.
+
+Maintainers often try to resolve this problem by, for example, rebasing the
+branch or performing another merge with the linus branch, then recreating
+the pull request.  This approach tends not to lead to joy at the receiving
+end of that pull request; rebasing and/or merging just before pushing
+upstream is a well-known way to get a grumpy response.
+
+So what is to be done?  The best response when confronted with this
+situation is to indeed to do a merge with the branch you intend your work
+to be pulled into, but to do it privately, as if it were the source of
+shame.  Create a new, throwaway branch and do the merge there::
+
+  ... vM --- vN-rc1 --- vN-rc2 --- vN-rc3 --- ... --- vN-rc7 --- vN
+                |         |                                      |
+                |         +-- c1 --- c2 --- ... --- cN           |
+                |                   /               |            |
+                +-- x1 --- x2 --- x3                +------------+-- TEMP
+
+The merge operation resolves all of the complications resulting from the
+multiple beginning points, yielding a coherent result that contains only
+the differences from the mainline branch.  Now it will be possible to
+generate a diffstat with the desired information::
+
+  $ git diff -C --stat --summary linus..TEMP
+
+Save the output from this command, then simply delete the TEMP branch;
+definitely do not expose it to the outside world.  Take the saved diffstat
+output and edit it into the messy pull request, yielding a result that
+shows what is really going on.  That request can then be sent upstream.