Use Linux interrupt management functions

The use of custom code makes debugging difficult and there does not
appear any reason for it given that the native Linux functions work on
ARM, so lets get rid of it.

This resolves potential issues where Linux's memory allocation functions
fail to warn that we are in an interrupt context. Memory allocations in
such contexts are unsafe and do not happen on any other DTrace platform,
so it is a very good thing to ensure that we are warned when this
happens.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Use Linux Locking Infrastructure

The custom locking infrastructure in dtrace4linux was designed to
operate in an interrupt context and consequently, many locks seem to have
been modified to incorporate a change in locking semantics where their
use creates an interrupt context on entering a lock and leaves it on
exiting a lock. This has multiple problems.

1. The new locking semantics clobber the interrupt masks whenever
locking fails to be done in reverse order. For instance,
dtrace_module_unloaded() locks variables in dtrace4linux this way:

dmutex_enter(&dtrace_provider_lock);
dmutex_enter(&mod_lock);
dmutex_enter(&dtrace_lock);

It then unlocks them this way:

dmutex_exit(&dtrace_provider_lock);
dmutex_exit(&mod_lock);
dmutex_exit(&dtrace_lock);

Since we have multiple locks with the new semantics,
dtrace_provider_lock will be the one that stores the original interrupt
mask while the others store interrupt masks that have interrupts
disabled. Since unlocking is not done in reverse, we exit this function
with interrupts disabled and consequently, bad things can happen. This
is not the only example of this problem and while the lock ordering
should be fixed, it does explain some problems specific to dtrace4linux
that do not occur on other dtrace ports.

2. The mutexes were replaced with semaphores, but we have no stack on
which to store multiple interrupt contexts, so they get clobbered then.

3. Other DTrace platforms, such as Illumos, expect these locks to have
mutex semantics that are unsafe inside interrupt contexts. There is no
apparent benefit to being different, but plenty of issues stem from
being different. The change in semantics risks of plenty of things that
should work in routines known to work fine on other platforms can and
will fail in non-obvious ways.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Use Linux infrastructure for dtrace_xcall()

Comments suggest that the rationale for using custom code for
dtrace_xcall() was that it on_each_cpu did not interact well with the
custom locking. Since that code needs to be replaced with more
conventional Linux locks, the rationale for a custom xcall
implementation is gone. Furthermore, it is not clear that this interacts
well with on_each_cpu(), so lets drop it in favor of wrapping
on_each_cpu(). This avoids having to worry about strange interactions.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Remove lock_teardown

The locking and xcall changes make this unnecessary. It triggers a
warning on preemptible kernels, so lets get rid of it:

[53134.451543] BUG: using smp_processor_id() in preemptible [00000000] code: dtrace/19160
[53134.451559] caller is dtrace_ecb_destroy+0x523/0x6d0 [dtracedrv]
[53134.451563] CPU: 3 PID: 19160 Comm: dtrace Tainted: P           O 3.12.6 #3
[53134.451564] Hardware name: Supermicro X9SRA/X9SRA-3/X9SRA/X9SRA-3, BIOS 3.0a 08/08/2013
[53134.451566]  0000000000000000 ffff880ec889bdb0 ffffffff813ce370 0000000000000003
[53134.451572]  ffff880ec889bdc8 ffffffff8120ce93 ffff880ed1daff60 ffff880ec889be20
[53134.451576]  ffffffffa1156b83 00000001c889bde8 ffffffffa117808d ffff880ec889bdf8
[53134.451579] Call Trace:
[53134.451586]  [<ffffffff813ce370>] dump_stack+0x4e/0x82
[53134.451592]  [<ffffffff8120ce93>] debug_smp_processor_id+0xd3/0xf0
[53134.451601]  [<ffffffffa1156b83>] dtrace_ecb_destroy+0x523/0x6d0 [dtracedrv]
[53134.451616]  [<ffffffffa117808d>] ? dtrace_xcall+0x2d/0x30 [dtracedrv]
[53134.451626]  [<ffffffffa115c8c7>] dtrace_state_destroy+0x167/0x860 [dtracedrv]
[53134.451636]  [<ffffffffa115e16c>] dtrace_close+0x8c/0x150 [dtracedrv]
[53134.451645]  [<ffffffffa11637f2>] dtracedrv_release+0x12/0x20 [dtracedrv]
[53134.451652]  [<ffffffff8114631a>] __fput+0xea/0x2c0
[53134.451655]  [<ffffffff81146529>] ____fput+0x9/0x10
[53134.451660]  [<ffffffff81063c8c>] task_work_run+0x9c/0xd0
[53134.451665]  [<ffffffff81002a0a>] do_notify_resume+0x8a/0xa0
[53134.451669]  [<ffffffff813dafea>] int_signal+0x12/0x17

Signed-off-by: Richard Yao <ryao@gentoo.org>
Convert dtrace_stack_mutex into a spin-lock

It is unsafe to use mutex functions inside an interrupt context.

Signed-off-by: Richard Yao <ryao@gentoo.org>
HACK: Disable preemption when getting time stamps

This creates a flood of warnings to dmesg on systems with preemptible
kernels. These warnings are not of huge importance, so lets disable them
until a different mechanism is available for our use. I believe that
doing this is safe in >99.9% of cases.

Signed-off-by: Richard Yao <ryao@gentoo.org>