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, 171 insertions, 0 deletions

diff --git a/Documentation/driver-api/usb/persist.rst b/Documentation/driver-api/usb/persist.rst
new file mode 100644
index 000000000..08cafc629
--- /dev/null
+++ b/Documentation/driver-api/usb/persist.rst
@@ -0,0 +1,171 @@
+.. _usb-persist:
+
+USB device persistence during system suspend
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+:Author: Alan Stern <stern@rowland.harvard.edu>
+:Date: September 2, 2006 (Updated February 25, 2008)
+
+
+What is the problem?
+====================
+
+According to the USB specification, when a USB bus is suspended the
+bus must continue to supply suspend current (around 1-5 mA).  This
+is so that devices can maintain their internal state and hubs can
+detect connect-change events (devices being plugged in or unplugged).
+The technical term is "power session".
+
+If a USB device's power session is interrupted then the system is
+required to behave as though the device has been unplugged.  It's a
+conservative approach; in the absence of suspend current the computer
+has no way to know what has actually happened.  Perhaps the same
+device is still attached or perhaps it was removed and a different
+device plugged into the port.  The system must assume the worst.
+
+By default, Linux behaves according to the spec.  If a USB host
+controller loses power during a system suspend, then when the system
+wakes up all the devices attached to that controller are treated as
+though they had disconnected.  This is always safe and it is the
+"officially correct" thing to do.
+
+For many sorts of devices this behavior doesn't matter in the least.
+If the kernel wants to believe that your USB keyboard was unplugged
+while the system was asleep and a new keyboard was plugged in when the
+system woke up, who cares?  It'll still work the same when you type on
+it.
+
+Unfortunately problems _can_ arise, particularly with mass-storage
+devices.  The effect is exactly the same as if the device really had
+been unplugged while the system was suspended.  If you had a mounted
+filesystem on the device, you're out of luck -- everything in that
+filesystem is now inaccessible.  This is especially annoying if your
+root filesystem was located on the device, since your system will
+instantly crash.
+
+Loss of power isn't the only mechanism to worry about.  Anything that
+interrupts a power session will have the same effect.  For example,
+even though suspend current may have been maintained while the system
+was asleep, on many systems during the initial stages of wakeup the
+firmware (i.e., the BIOS) resets the motherboard's USB host
+controllers.  Result: all the power sessions are destroyed and again
+it's as though you had unplugged all the USB devices.  Yes, it's
+entirely the BIOS's fault, but that doesn't do _you_ any good unless
+you can convince the BIOS supplier to fix the problem (lots of luck!).
+
+On many systems the USB host controllers will get reset after a
+suspend-to-RAM.  On almost all systems, no suspend current is
+available during hibernation (also known as swsusp or suspend-to-disk).
+You can check the kernel log after resuming to see if either of these
+has happened; look for lines saying "root hub lost power or was reset".
+
+In practice, people are forced to unmount any filesystems on a USB
+device before suspending.  If the root filesystem is on a USB device,
+the system can't be suspended at all.  (All right, it _can_ be
+suspended -- but it will crash as soon as it wakes up, which isn't
+much better.)
+
+
+What is the solution?
+=====================
+
+The kernel includes a feature called USB-persist.  It tries to work
+around these issues by allowing the core USB device data structures to
+persist across a power-session disruption.
+
+It works like this.  If the kernel sees that a USB host controller is
+not in the expected state during resume (i.e., if the controller was
+reset or otherwise had lost power) then it applies a persistence check
+to each of the USB devices below that controller for which the
+"persist" attribute is set.  It doesn't try to resume the device; that
+can't work once the power session is gone.  Instead it issues a USB
+port reset and then re-enumerates the device.  (This is exactly the
+same thing that happens whenever a USB device is reset.)  If the
+re-enumeration shows that the device now attached to that port has the
+same descriptors as before, including the Vendor and Product IDs, then
+the kernel continues to use the same device structure.  In effect, the
+kernel treats the device as though it had merely been reset instead of
+unplugged.
+
+The same thing happens if the host controller is in the expected state
+but a USB device was unplugged and then replugged, or if a USB device
+fails to carry out a normal resume.
+
+If no device is now attached to the port, or if the descriptors are
+different from what the kernel remembers, then the treatment is what
+you would expect.  The kernel destroys the old device structure and
+behaves as though the old device had been unplugged and a new device
+plugged in.
+
+The end result is that the USB device remains available and usable.
+Filesystem mounts and memory mappings are unaffected, and the world is
+now a good and happy place.
+
+Note that the "USB-persist" feature will be applied only to those
+devices for which it is enabled.  You can enable the feature by doing
+(as root)::
+
+	echo 1 >/sys/bus/usb/devices/.../power/persist
+
+where the "..." should be filled in the with the device's ID.  Disable
+the feature by writing 0 instead of 1.  For hubs the feature is
+automatically and permanently enabled and the power/persist file
+doesn't even exist, so you only have to worry about setting it for
+devices where it really matters.
+
+
+Is this the best solution?
+==========================
+
+Perhaps not.  Arguably, keeping track of mounted filesystems and
+memory mappings across device disconnects should be handled by a
+centralized Logical Volume Manager.  Such a solution would allow you
+to plug in a USB flash device, create a persistent volume associated
+with it, unplug the flash device, plug it back in later, and still
+have the same persistent volume associated with the device.  As such
+it would be more far-reaching than USB-persist.
+
+On the other hand, writing a persistent volume manager would be a big
+job and using it would require significant input from the user.  This
+solution is much quicker and easier -- and it exists now, a giant
+point in its favor!
+
+Furthermore, the USB-persist feature applies to _all_ USB devices, not
+just mass-storage devices.  It might turn out to be equally useful for
+other device types, such as network interfaces.
+
+
+WARNING: USB-persist can be dangerous!!
+=======================================
+
+When recovering an interrupted power session the kernel does its best
+to make sure the USB device hasn't been changed; that is, the same
+device is still plugged into the port as before.  But the checks
+aren't guaranteed to be 100% accurate.
+
+If you replace one USB device with another of the same type (same
+manufacturer, same IDs, and so on) there's an excellent chance the
+kernel won't detect the change.  The serial number string and other
+descriptors are compared with the kernel's stored values, but this
+might not help since manufacturers frequently omit serial numbers
+entirely in their devices.
+
+Furthermore it's quite possible to leave a USB device exactly the same
+while changing its media.  If you replace the flash memory card in a
+USB card reader while the system is asleep, the kernel will have no
+way to know you did it.  The kernel will assume that nothing has
+happened and will continue to use the partition tables, inodes, and
+memory mappings for the old card.
+
+If the kernel gets fooled in this way, it's almost certain to cause
+data corruption and to crash your system.  You'll have no one to blame
+but yourself.
+
+For those devices with avoid_reset_quirk attribute being set, persist
+maybe fail because they may morph after reset.
+
+YOU HAVE BEEN WARNED!  USE AT YOUR OWN RISK!
+
+That having been said, most of the time there shouldn't be any trouble
+at all.  The USB-persist feature can be extremely useful.  Make the
+most of it.