There are various problems and short-comings with the current
static_key interface:
- static_key_{true,false}() read like a branch depending on the key
value, instead of the actual likely/unlikely branch depending on
init value.
- static_key_{true,false}() are, as stated above, tied to the
static_key init values STATIC_KEY_INIT_{TRUE,FALSE}.
- we're limited to the 2 (out of 4) possible options that compile to
a default NOP because that's what our arch_static_branch() assembly
emits.
So provide a new static_key interface:
DEFINE_STATIC_KEY_TRUE(name);
DEFINE_STATIC_KEY_FALSE(name);
Which define a key of different types with an initial true/false
value.
Then allow:
static_branch_likely()
static_branch_unlikely()
to take a key of either type and emit the right instruction for the
case.
This means adding a second arch_static_branch_jump() assembly helper
which emits a JMP per default.
In order to determine the right instruction for the right state,
encode the branch type in the LSB of jump_entry::key.
This is the final step in removing the naming confusion that has led to
a stream of avoidable bugs such as:
a833581e37 ("x86, perf: Fix static_key bug in load_mm_cr4()")
... but it also allows new static key combinations that will give us
performance enhancements in the subsequent patches.
Tested-by: Rabin Vincent <rabin@rab.in> # arm
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> # ppc
Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com> # s390
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
hcall tracepoints add quite a few instructions to our hcall path:
plpar_hcall:
mr r2,r2
mfcr r0
stw r0,8(r1)
b 164 <---- start
ld r12,0(r2)
std r12,32(r1)
cmpdi r12,0
beq 164 <---- end
...
We have an unconditional branch that gets noped out during boot and
a load/compare/branch. We also store the tracepoint value to the
stack for the hcall_exit path to use.
By using jump labels we can simplify this to just a single nop that
gets replaced with a branch when the tracepoint is enabled:
plpar_hcall:
mr r2,r2
mfcr r0
stw r0,8(r1)
nop <----
...
If jump labels are not enabled, we fall back to the old method.
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Fengguang Wu, Oleg Nesterov and Peter Zijlstra tracked down
a kernel crash to a GCC bug: GCC miscompiles certain 'asm goto'
constructs, as outlined here:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58670
Implement a workaround suggested by Jakub Jelinek.
Reported-and-tested-by: Fengguang Wu <fengguang.wu@intel.com>
Reported-by: Oleg Nesterov <oleg@redhat.com>
Reported-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Suggested-by: Jakub Jelinek <jakub@redhat.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: <stable@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
So here's a boot tested patch on top of Jason's series that does
all the cleanups I talked about and turns jump labels into a
more intuitive to use facility. It should also address the
various misconceptions and confusions that surround jump labels.
Typical usage scenarios:
#include <linux/static_key.h>
struct static_key key = STATIC_KEY_INIT_TRUE;
if (static_key_false(&key))
do unlikely code
else
do likely code
Or:
if (static_key_true(&key))
do likely code
else
do unlikely code
The static key is modified via:
static_key_slow_inc(&key);
...
static_key_slow_dec(&key);
The 'slow' prefix makes it abundantly clear that this is an
expensive operation.
I've updated all in-kernel code to use this everywhere. Note
that I (intentionally) have not pushed through the rename
blindly through to the lowest levels: the actual jump-label
patching arch facility should be named like that, so we want to
decouple jump labels from the static-key facility a bit.
On non-jump-label enabled architectures static keys default to
likely()/unlikely() branches.
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Jason Baron <jbaron@redhat.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: a.p.zijlstra@chello.nl
Cc: mathieu.desnoyers@efficios.com
Cc: davem@davemloft.net
Cc: ddaney.cavm@gmail.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/20120222085809.GA26397@elte.hu
Signed-off-by: Ingo Molnar <mingo@elte.hu>
I hit an oops at boot on the first instruction of timer_cpu_notify:
NIP [c000000000722f88] .timer_cpu_notify+0x0/0x388
The code should look like:
c000000000722f78: eb e9 00 30 ld r31,48(r9)
c000000000722f7c: 2f bf 00 00 cmpdi cr7,r31,0
c000000000722f80: 40 9e ff 44 bne+ cr7,c000000000722ec4
c000000000722f84: 4b ff ff 74 b c000000000722ef8
c000000000722f88 <.timer_cpu_notify>:
c000000000722f88: 7c 08 02 a6 mflr r0
c000000000722f8c: 2f a4 00 07 cmpdi cr7,r4,7
c000000000722f90: fb c1 ff f0 std r30,-16(r1)
c000000000722f94: fb 61 ff d8 std r27,-40(r1)
But the oops output shows:
eb61ffd8 eb81ffe0 eba1ffe8 ebc1fff0 7c0803a6 ebe1fff8 4e800020
00000000 ebe90030 c0000000 00ad0a28 00000000 2fa40007 fbc1fff0 fb61ffd8
So we scribbled over our instructions with c000000000ad0a28, which
is an address inside the jump_table ELF section.
It turns out the jump_table section is only aligned to 8 bytes but
we are aligning our entries within the section to 16 bytes. This
means our entries are offset from the table:
c000000000acd4a8 <__start___jump_table>:
...
c000000000ad0a10: c0 00 00 00 lfs f0,0(0)
c000000000ad0a14: 00 70 cd 5c .long 0x70cd5c
c000000000ad0a18: c0 00 00 00 lfs f0,0(0)
c000000000ad0a1c: 00 70 cd 90 .long 0x70cd90
c000000000ad0a20: c0 00 00 00 lfs f0,0(0)
c000000000ad0a24: 00 ac a4 20 .long 0xaca420
And the jump table sort code gets very confused and writes into the
wrong spot. Remove the alignment, and also remove the padding since
we it saves some space and we shouldn't need it.
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
This patch adds support for the new "jump label" feature.
Unlike x86 and sparc we just merrily patch the code with no locks etc,
as far as I know this is safe, but I'm not really sure what the x86/sparc
code is protecting against so maybe it's not.
I also don't see any reason for us to implement the poke_early() routine,
even though sparc does.
[BenH: Updated the patch to upstream generic changes]
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
