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, 191 insertions, 0 deletions

diff --git a/Documentation/gpu/vgaarbiter.rst b/Documentation/gpu/vgaarbiter.rst
new file mode 100644
index 000000000..d1e953712
--- /dev/null
+++ b/Documentation/gpu/vgaarbiter.rst
@@ -0,0 +1,191 @@
+===========
+VGA Arbiter
+===========
+
+Graphic devices are accessed through ranges in I/O or memory space. While most
+modern devices allow relocation of such ranges, some "Legacy" VGA devices
+implemented on PCI will typically have the same "hard-decoded" addresses as
+they did on ISA. For more details see "PCI Bus Binding to IEEE Std 1275-1994
+Standard for Boot (Initialization Configuration) Firmware Revision 2.1"
+Section 7, Legacy Devices.
+
+The Resource Access Control (RAC) module inside the X server [0] existed for
+the legacy VGA arbitration task (besides other bus management tasks) when more
+than one legacy device co-exist on the same machine. But the problem happens
+when these devices are trying to be accessed by different userspace clients
+(e.g. two servers in parallel). Their address assignments conflict. Moreover,
+ideally, being a userspace application, it is not the role of the X server to
+control bus resources. Therefore an arbitration scheme outside of the X server
+is needed to control the sharing of these resources. This document introduces
+the operation of the VGA arbiter implemented for the Linux kernel.
+
+vgaarb kernel/userspace ABI
+---------------------------
+
+The vgaarb is a module of the Linux Kernel. When it is initially loaded, it
+scans all PCI devices and adds the VGA ones inside the arbitration. The
+arbiter then enables/disables the decoding on different devices of the VGA
+legacy instructions. Devices which do not want/need to use the arbiter may
+explicitly tell it by calling vga_set_legacy_decoding().
+
+The kernel exports a char device interface (/dev/vga_arbiter) to the clients,
+which has the following semantics:
+
+open
+        Opens a user instance of the arbiter. By default, it's attached to the
+        default VGA device of the system.
+
+close
+        Close a user instance. Release locks made by the user
+
+read
+        Return a string indicating the status of the target like:
+
+        "<card_ID>,decodes=<io_state>,owns=<io_state>,locks=<io_state> (ic,mc)"
+
+        An IO state string is of the form {io,mem,io+mem,none}, mc and
+        ic are respectively mem and io lock counts (for debugging/
+        diagnostic only). "decodes" indicate what the card currently
+        decodes, "owns" indicates what is currently enabled on it, and
+        "locks" indicates what is locked by this card. If the card is
+        unplugged, we get "invalid" then for card_ID and an -ENODEV
+        error is returned for any command until a new card is targeted.
+
+
+write
+        Write a command to the arbiter. List of commands:
+
+        target <card_ID>
+                switch target to card <card_ID> (see below)
+        lock <io_state>
+                acquires locks on target ("none" is an invalid io_state)
+        trylock <io_state>
+                non-blocking acquire locks on target (returns EBUSY if
+                unsuccessful)
+        unlock <io_state>
+                release locks on target
+        unlock all
+                release all locks on target held by this user (not implemented
+                yet)
+        decodes <io_state>
+                set the legacy decoding attributes for the card
+
+        poll
+                event if something changes on any card (not just the target)
+
+        card_ID is of the form "PCI:domain:bus:dev.fn". It can be set to "default"
+        to go back to the system default card (TODO: not implemented yet). Currently,
+        only PCI is supported as a prefix, but the userland API may support other bus
+        types in the future, even if the current kernel implementation doesn't.
+
+Note about locks:
+
+The driver keeps track of which user has which locks on which card. It
+supports stacking, like the kernel one. This complexifies the implementation
+a bit, but makes the arbiter more tolerant to user space problems and able
+to properly cleanup in all cases when a process dies.
+Currently, a max of 16 cards can have locks simultaneously issued from
+user space for a given user (file descriptor instance) of the arbiter.
+
+In the case of devices hot-{un,}plugged, there is a hook - pci_notify() - to
+notify them being added/removed in the system and automatically added/removed
+in the arbiter.
+
+There is also an in-kernel API of the arbiter in case DRM, vgacon, or other
+drivers want to use it.
+
+In-kernel interface
+-------------------
+
+.. kernel-doc:: include/linux/vgaarb.h
+   :internal:
+
+.. kernel-doc:: drivers/pci/vgaarb.c
+   :export:
+
+libpciaccess
+------------
+
+To use the vga arbiter char device, an API was implemented inside the
+libpciaccess library. One field was added to struct pci_device (each device
+on the system)::
+
+    /* the type of resource decoded by the device */
+    int vgaarb_rsrc;
+
+Besides it, in pci_system were added::
+
+    int vgaarb_fd;
+    int vga_count;
+    struct pci_device *vga_target;
+    struct pci_device *vga_default_dev;
+
+The vga_count is used to track how many cards are being arbitrated, so for
+instance, if there is only one card, then it can completely escape arbitration.
+
+These functions below acquire VGA resources for the given card and mark those
+resources as locked. If the resources requested are "normal" (and not legacy)
+resources, the arbiter will first check whether the card is doing legacy
+decoding for that type of resource. If yes, the lock is "converted" into a
+legacy resource lock. The arbiter will first look for all VGA cards that
+might conflict and disable their IOs and/or Memory access, including VGA
+forwarding on P2P bridges if necessary, so that the requested resources can
+be used. Then, the card is marked as locking these resources and the IO and/or
+Memory access is enabled on the card (including VGA forwarding on parent
+P2P bridges if any). In the case of vga_arb_lock(), the function will block
+if some conflicting card is already locking one of the required resources (or
+any resource on a different bus segment, since P2P bridges don't differentiate
+VGA memory and IO afaik). If the card already owns the resources, the function
+succeeds.  vga_arb_trylock() will return (-EBUSY) instead of blocking. Nested
+calls are supported (a per-resource counter is maintained).
+
+Set the target device of this client. ::
+
+    int  pci_device_vgaarb_set_target   (struct pci_device *dev);
+
+For instance, in x86 if two devices on the same bus want to lock different
+resources, both will succeed (lock). If devices are in different buses and
+trying to lock different resources, only the first who tried succeeds. ::
+
+    int  pci_device_vgaarb_lock         (void);
+    int  pci_device_vgaarb_trylock      (void);
+
+Unlock resources of device. ::
+
+    int  pci_device_vgaarb_unlock       (void);
+
+Indicates to the arbiter if the card decodes legacy VGA IOs, legacy VGA
+Memory, both, or none. All cards default to both, the card driver (fbdev for
+example) should tell the arbiter if it has disabled legacy decoding, so the
+card can be left out of the arbitration process (and can be safe to take
+interrupts at any time. ::
+
+    int  pci_device_vgaarb_decodes      (int new_vgaarb_rsrc);
+
+Connects to the arbiter device, allocates the struct ::
+
+    int  pci_device_vgaarb_init         (void);
+
+Close the connection ::
+
+    void pci_device_vgaarb_fini         (void);
+
+xf86VGAArbiter (X server implementation)
+----------------------------------------
+
+X server basically wraps all the functions that touch VGA registers somehow.
+
+References
+----------
+
+Benjamin Herrenschmidt (IBM?) started this work when he discussed such design
+with the Xorg community in 2005 [1, 2]. In the end of 2007, Paulo Zanoni and
+Tiago Vignatti (both of C3SL/Federal University of Paraná) proceeded his work
+enhancing the kernel code to adapt as a kernel module and also did the
+implementation of the user space side [3]. Now (2009) Tiago Vignatti and Dave
+Airlie finally put this work in shape and queued to Jesse Barnes' PCI tree.
+
+0) https://cgit.freedesktop.org/xorg/xserver/commit/?id=4b42448a2388d40f257774fbffdccaea87bd0347
+1) https://lists.freedesktop.org/archives/xorg/2005-March/006663.html
+2) https://lists.freedesktop.org/archives/xorg/2005-March/006745.html
+3) https://lists.freedesktop.org/archives/xorg/2007-October/029507.html