tunables: Clean up hooks to get and set tunables

The TUNABLE_SET_VALUE and family of macros (and my later attempt to
add a TUNABLE_GET) never quite went together very well because the
overall interface was not clearly defined.  This patch is an attempt
to do just that.

This patch consolidates the API to two simple sets of macros,
TUNABLE_GET* and TUNABLE_SET*.  If TUNABLE_NAMESPACE is defined,
TUNABLE_GET takes just the tunable name, type and a (optionally NULL)
callback function to get the value of the tunable.  The callback
function, if non-NULL, is called if the tunable was externally set
(i.e. via GLIBC_TUNABLES or any future mechanism).  For example:

    val = TUNABLE_GET (check, int32_t, check_callback)

returns the value of the glibc.malloc.check tunable (assuming
TUNABLE_NAMESPACE is set to malloc) as an int32_t into VAL after
calling check_callback.

Likewise, TUNABLE_SET can be used to set the value of the tunable,
although this is currently possible only in the dynamic linker before
it relocates itself.  For example:

  TUNABLE_SET (check, int32_t, 2)

will set glibc.malloc.check to 2.  Of course, this is not possible
since we set (or read) glibc.malloc.check long after it is relocated.

To access or set a tunable outside of TUNABLE_NAMESPACE, use the
TUNABLE_GET_FULL and TUNABLE_SET_FULL macros, which have the following
prototype:

  TUNABLE_GET_FULL (glibc, tune, hwcap_mask, uint64_t, NULL)
  TUNABLE_SET_FULL (glibc, tune, hwcap_mask, uint64_t, 0xffff)

In future the tunable list may get split into mutable and immutable
tunables where mutable tunables can be modified by the library and
userspace after relocation as well and TUNABLE_SET will be more useful
than it currently is.  However whenever we actually do that split, we
will have to ensure that the mutable tunables are protected with
locks.

	* elf/Versions (__tunable_set_val): Rename to __tunable_get_val.
	* elf/dl-tunables.c: Likewise.
	(do_tunable_update_val): New function.
	(__tunable_set_val): New function.
	(__tunable_get_val): Call CB only if the tunable was externally
	initialized.
	(tunables_strtoul): Replace strval with initialized.
	* elf/dl-tunables.h (strval): Replace with a bool initialized.
	(TUNABLE_ENUM_NAME, TUNABLE_ENUM_NAME1): Adjust names to
	prevent collision.
	(__tunable_set_val): New function.
	(TUNABLE_GET, TUNABLE_GET_FULL): New macros.
	(TUNABLE_SET, TUNABLE_SET_FULL): Likewise.
	(TUNABLE_SET_VAL): Remove.
	(TUNABLE_SET_VAL_WITH_CALLBACK): Likewise.
	* README.tunables: Document the new macros.
	* malloc/arena.c (ptmalloc_init): Adjust.

1 files changed, 58 insertions, 11 deletions

diff --git a/README.tunables b/README.tunables
index 0e9b0d7..3967679 100644
--- a/README.tunables
+++ b/README.tunables
@@ -20,14 +20,12 @@ namespace by overriding the TOP_NAMESPACE macro for that tunable.  Downstream
 implementations are discouraged from using the 'glibc' top namespace for
 tunables they don't already have consensus to push upstream.
 
-There are two steps to adding a tunable:
+There are three steps to adding a tunable:
 
-1. Add a tunable ID:
+1. Add a tunable to the list and fully specify its properties:
 
-Modules that wish to use the tunables interface must define the
-TUNABLE_NAMESPACE macro.  Following this, for each tunable you want to
-add, make an entry in elf/dl-tunables.list.  The format of the file is as
-follows:
+For each tunable you want to add, make an entry in elf/dl-tunables.list.  The
+format of the file is as follows:
 
 TOP_NAMESPACE {
   NAMESPACE1 {
@@ -69,11 +67,60 @@ The list of allowed attributes are:
 				     non-AT_SECURE subprocesses.
 			NONE: Read all the time.
 
-2. Call either the TUNABLE_SET_VALUE and pass into it the tunable name and a
-   pointer to the variable that should be set with the tunable value.
-   If additional work needs to be done after setting the value, use the
-   TUNABLE_SET_VALUE_WITH_CALLBACK instead and additionally pass a pointer to
-   the function that should be called if the tunable value has been set.
+2. Use TUNABLE_GET/TUNABLE_SET to get and set tunables.
+
+3. OPTIONAL: If tunables in a namespace are being used multiple times within a
+   specific module, set the TUNABLE_NAMESPACE macro to reduce the amount of
+   typing.
+
+GETTING AND SETTING TUNABLES
+----------------------------
+
+When the TUNABLE_NAMESPACE macro is defined, one may get tunables in that
+module using the TUNABLE_GET macro as follows:
+
+  val = TUNABLE_GET (check, int32_t, TUNABLE_CALLBACK (check_callback))
+
+where 'check' is the tunable name, 'int32_t' is the C type of the tunable and
+'check_callback' is the function to call if the tunable got initialized to a
+non-default value.  The macro returns the value as type 'int32_t'.
+
+The callback function should be defined as follows:
+
+  void
+  TUNABLE_CALLBACK (check_callback) (int32_t *valp)
+  {
+  ...
+  }
+
+where it can expect the tunable value to be passed in VALP.
+
+Tunables in the module can be updated using:
+
+  TUNABLE_SET (check, int32_t, val)
+
+where 'check' is the tunable name, 'int32_t' is the C type of the tunable and
+'val' is a value of same type.
+
+To get and set tunables in a different namespace from that module, use the full
+form of the macros as follows:
+
+  val = TUNABLE_GET_FULL (glibc, tune, hwcap_mask, uint64_t, NULL)
+
+  TUNABLE_SET_FULL (glibc, tune, hwcap_mask, uint64_t, val)
+
+where 'glibc' is the top namespace, 'tune' is the tunable namespace and the
+remaining arguments are the same as the short form macros.
+
+When TUNABLE_NAMESPACE is not defined in a module, TUNABLE_GET is equivalent to
+TUNABLE_GET_FULL, so you will need to provide full namespace information for
+both macros.  Likewise for TUNABLE_SET and TUNABLE_SET_FULL.
+
+** IMPORTANT NOTE **
+
+The tunable list is set as read-only after the dynamic linker relocates itself,
+so setting tunable values must be limited only to tunables within the dynamic
+linker, that too before relocation.
 
 FUTURE WORK
 -----------