Re: [PATCH 0/3] ring-buffer: less locking and only disable preemption

Mathieu Desnoyers
2008-10-04T16:34:17Z

* Steven Rostedt (rostedt@goodmis.org) wrote:
>
> [ Added Arjan to CC regarding the last statements ]
>
> On Sat, 4 Oct 2008, Ingo Molnar wrote:
> >
> > * Steven Rostedt <rostedt@goodmis.org> wrote:
> > > These patches need to be put through the ringer. Could you add them to
> > > your ring-buffer branch, so we can test them out before putting them
> > > into your master branch.
> >
> > hey, in fact your latest iteration already tested out so well on a wide
> > range of boxes that I've merged it all into tip/tracing/core already.
>
> Great to hear!
>
> >
> > I'll reuse tip/tracing/ring-buffer for these latest 3 patches (merge it
> > up to tip/tracing/core and add these three patches) but it's a delta,
> > i.e. the whole ring-buffer approach is ready for prime time i think.
> >
> > Hm, do we do deallocation of the buffers already when we switch tracers?
>
> Not yet, but that is one of the trivial changes. I spent too much time
> on these more complex changes to get around to it.
>
> >
> > > The following patches bring the ring buffer closer to a lockless
> > > solution. They move the locking only to the actual moving the
> > > tail/write pointer from one page to the next. Interrupts are now
> > > enabled during most of the writes.
> >
> > very nice direction!
>
> Thanks!
>
> >
> > > A lot of the locking protection is still within the ftrace
> > > infrastructure. The last patch takes some of that away.
> > >
> > > The function tracer cannot be reentrant just due to the nature that it
> > > traces everything, and can cause recursion issues.
> >
> > Correct, and that's by far the yuckiest aspect of it. And there's
> > another aspect: NMIs. We've still got the tip/tracing/nmisafe angle with
> > these commits:
> >
> > d979781: ftrace: mark lapic_wd_event() notrace
> > c2c27ba: ftrace: ignore functions that cannot be kprobe-ed
> > 431e946: ftrace: do not trace NMI contexts
> > 1eda930: x86, tracing, nmisafe: fix threadinfo_ -> TI_ rename fallout
> > 84c2ca9: sched: sched_clock() improvement: use in_nmi()
> > 0d84b78: x86 NMI-safe INT3 and Page Fault
> > a04464b: x86_64 page fault NMI-safe
> > b335389: Change avr32 active count bit
> > a581cbd: Change Alpha active count bit
> > eca0999: Stringify support commas
> >
> > but I'm not yet fully convinced about the NMI angle, the practical cross
> > section to random low level x86 code is wider than any sched_clock()
> > impact for example. We might be best off avoiding it: force-disable the
> > NMI watchdog when we trace?
>
> Since we still have the locking in the ring buffer, it is still not NMI
> safe. But once we remove all locking, then the tracer is fine.
>
> BUT!
>
> The dynamic function tracer is another issue. The problem with NMIs has
> nothing to do with locking, or corrupting the buffers. It has to do with
> the dynamic code modification. Whenever we modify code, we must guarantee
> that it will not be executed on another CPU.
>
> Kstop_machine serves this purpose rather well. We can modify code without
> worrying it will be executed on another CPU, except for NMIs. The problem
> now comes where an NMI can come in and execute the code being modified.
> That's why I put in all the notrace, lines. But it gets difficult because
> of nmi_notifier can call all over the kernel. Perhaps, we can simply
> disable the nmi-notifier when we are doing the kstop_machine call?
>

Or use this code, based on a temporary breakpoint, to do the code
patching (part of the -lttng tree). It does not require stop_machine at
all and is nmi safe.

Mathieu

Immediate Values - x86 Optimization NMI and MCE support

x86 optimization of the immediate values which uses a movl with code patching
to set/unset the value used to populate the register used as variable source.
It uses a breakpoint to bypass the instruction being changed, which lessens the
interrupt latency of the operation and protects against NMIs and MCE.

- More reentrant immediate value : uses a breakpoint. Needs to know the
instruction's first byte. This is why we keep the "instruction size"
variable, so we can support the REX prefixed instructions too.

Changelog:
- Change the immediate.c update code to support variable length opcodes.
- Use text_poke_early with cr0 WP save/restore to patch the bypass. We are doing
non atomic writes to a code region only touched by us (nobody can execute it
since we are protected by the imv_mutex).
- Add x86_64 support, ready for i386+x86_64 -> x86 merge.
- Use asm-x86/asm.h.
- Change the immediate.c update code to support variable length opcodes.
- Use imv_* instead of immediate_*.
- Use kernel_wp_disable/enable instead of save/restore.
- Fix 1 byte immediate value so it declares its instruction size.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
CC: Andi Kleen <ak@muc.de>
CC: "H. Peter Anvin" <hpa@zytor.com>
CC: Chuck Ebbert <cebbert@redhat.com>
CC: Christoph Hellwig <hch@infradead.org>
CC: Jeremy Fitzhardinge <jeremy@goop.org>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Ingo Molnar <mingo@redhat.com>
---
arch/x86/kernel/Makefile | 1
arch/x86/kernel/immediate.c | 291 ++++++++++++++++++++++++++++++++++++++++++++
arch/x86/kernel/traps_32.c | 8 -
include/asm-x86/immediate.h | 48 ++++++-
4 files changed, 338 insertions(+), 10 deletions(-)

Index: linux-2.6-lttng/include/asm-x86/immediate.h
===================================================================
--- linux-2.6-lttng.orig/include/asm-x86/immediate.h 2008-08-06 01:32:23.000000000 -0400
+++ linux-2.6-lttng/include/asm-x86/immediate.h 2008-08-06 02:35:56.000000000 -0400
@@ -12,6 +12,18 @@

#include <asm/asm.h>

+struct __imv {
+ unsigned long var; /* Pointer to the identifier variable of the
+ * immediate value
+ */
+ unsigned long imv; /*
+ * Pointer to the memory location of the
+ * immediate value within the instruction.
+ */
+ unsigned char size; /* Type size. */
+ unsigned char insn_size;/* Instruction size. */
+} __attribute__ ((packed));
+
/**
* imv_read - read immediate variable
* @name: immediate value name
@@ -26,6 +38,11 @@
* what will generate an instruction with 8 bytes immediate value (not the REX.W
* prefixed one that loads a sign extended 32 bits immediate value in a r64
* register).
+ *
+ * Create the instruction in a discarded section to calculate its size. This is
+ * how we can align the beginning of the instruction on an address that will
+ * permit atomic modification of the immediate value without knowing the size of
+ * the opcode used by the compiler. The operand size is known in advance.
*/
#define imv_read(name) \
({ \
@@ -33,9 +50,14 @@
BUILD_BUG_ON(sizeof(value) > 8); \
switch (sizeof(value)) { \
case 1: \
- asm(".section __imv,\"aw\",@progbits\n\t" \
+ asm(".section __discard,\"\",@progbits\n\t" \
+ "1:\n\t" \
+ "mov $0,%0\n\t" \
+ "2:\n\t" \
+ ".previous\n\t" \
+ ".section __imv,\"aw\",@progbits\n\t" \
_ASM_PTR "%c1, (3f)-%c2\n\t" \
- ".byte %c2\n\t" \
+ ".byte %c2, (2b-1b)\n\t" \
".previous\n\t" \
"mov $0,%0\n\t" \
"3:\n\t" \
@@ -45,10 +67,16 @@
break; \
case 2: \
case 4: \
- asm(".section __imv,\"aw\",@progbits\n\t" \
+ asm(".section __discard,\"\",@progbits\n\t" \
+ "1:\n\t" \
+ "mov $0,%0\n\t" \
+ "2:\n\t" \
+ ".previous\n\t" \
+ ".section __imv,\"aw\",@progbits\n\t" \
_ASM_PTR "%c1, (3f)-%c2\n\t" \
- ".byte %c2\n\t" \
+ ".byte %c2, (2b-1b)\n\t" \
".previous\n\t" \
+ ".org . + ((-.-(2b-1b)) & (%c2-1)), 0x90\n\t" \
"mov $0,%0\n\t" \
"3:\n\t" \
: "=r" (value) \
@@ -60,10 +88,16 @@
value = name##__imv; \
break; \
} \
- asm(".section __imv,\"aw\",@progbits\n\t" \
+ asm(".section __discard,\"\",@progbits\n\t" \
+ "1:\n\t" \
+ "mov $0xFEFEFEFE01010101,%0\n\t" \
+ "2:\n\t" \
+ ".previous\n\t" \
+ ".section __imv,\"aw\",@progbits\n\t" \
_ASM_PTR "%c1, (3f)-%c2\n\t" \
- ".byte %c2\n\t" \
+ ".byte %c2, (2b-1b)\n\t" \
".previous\n\t" \
+ ".org . + ((-.-(2b-1b)) & (%c2-1)), 0x90\n\t" \
"mov $0xFEFEFEFE01010101,%0\n\t" \
"3:\n\t" \
: "=r" (value) \
@@ -74,4 +108,6 @@
value; \
})

+extern int arch_imv_update(const struct __imv *imv, int early);
+
#endif /* _ASM_X86_IMMEDIATE_H */
Index: linux-2.6-lttng/arch/x86/kernel/traps_32.c
===================================================================
--- linux-2.6-lttng.orig/arch/x86/kernel/traps_32.c 2008-08-06 00:44:22.000000000 -0400
+++ linux-2.6-lttng/arch/x86/kernel/traps_32.c 2008-08-06 02:35:57.000000000 -0400
@@ -576,7 +576,7 @@ void do_##name(struct pt_regs *regs, lon
}

DO_VM86_ERROR_INFO(0, SIGFPE, "divide error", divide_error, FPE_INTDIV, regs->ip)
-#ifndef CONFIG_KPROBES
+#if !defined(CONFIG_KPROBES) && !defined(CONFIG_IMMEDIATE)
DO_VM86_ERROR(3, SIGTRAP, "int3", int3)
#endif
DO_VM86_ERROR(4, SIGSEGV, "overflow", overflow)
@@ -850,7 +850,7 @@ void restart_nmi(void)
acpi_nmi_enable();
}

-#ifdef CONFIG_KPROBES
+#if defined(CONFIG_KPROBES) || defined(CONFIG_IMMEDIATE)
void __kprobes do_int3(struct pt_regs *regs, long error_code)
{
trace_hardirqs_fixup();
@@ -859,8 +859,8 @@ void __kprobes do_int3(struct pt_regs *r
== NOTIFY_STOP)
return;
/*
- * This is an interrupt gate, because kprobes wants interrupts
- * disabled. Normal trap handlers don't.
+ * This is an interrupt gate, because kprobes and immediate values wants
+ * interrupts disabled. Normal trap handlers don't.
*/
restore_interrupts(regs);

Index: linux-2.6-lttng/arch/x86/kernel/Makefile
===================================================================
--- linux-2.6-lttng.orig/arch/x86/kernel/Makefile 2008-08-06 00:41:34.000000000 -0400
+++ linux-2.6-lttng/arch/x86/kernel/Makefile 2008-08-06 02:35:57.000000000 -0400
@@ -73,6 +73,7 @@ obj-$(CONFIG_X86_SUMMIT_NUMA) += summit_
obj-y += vsmp_64.o
obj-$(CONFIG_KPROBES) += kprobes.o
obj-$(CONFIG_MODULES) += module_$(BITS).o
+obj-$(CONFIG_IMMEDIATE) += immediate.o
obj-$(CONFIG_EFI) += efi.o efi_$(BITS).o efi_stub_$(BITS).o
obj-$(CONFIG_DOUBLEFAULT) += doublefault_32.o
obj-$(CONFIG_KGDB) += kgdb.o
Index: linux-2.6-lttng/arch/x86/kernel/immediate.c
===================================================================
--- /dev/null 1970-01-01 00:00:00.000000000 +0000
+++ linux-2.6-lttng/arch/x86/kernel/immediate.c 2008-08-06 02:36:33.000000000 -0400
@@ -0,0 +1,291 @@
+/*
+ * Immediate Value - x86 architecture specific code.
+ *
+ * Rationale
+ *
+ * Required because of :
+ * - Erratum 49 fix for Intel PIII.
+ * - Still present on newer processors : Intel Core 2 Duo Processor for Intel
+ * Centrino Duo Processor Technology Specification Update, AH33.
+ * Unsynchronized Cross-Modifying Code Operations Can Cause Unexpected
+ * Instruction Execution Results.
+ *
+ * Permits immediate value modification by XMC with correct serialization.
+ *
+ * Reentrant for NMI and trap handler instrumentation. Permits XMC to a
+ * location that has preemption enabled because it involves no temporary or
+ * reused data structure.
+ *
+ * Quoting Richard J Moore, source of the information motivating this
+ * implementation which differs from the one proposed by Intel which is not
+ * suitable for kernel context (does not support NMI and would require disabling
+ * interrupts on every CPU for a long period) :
+ *
+ * "There is another issue to consider when looking into using probes other
+ * then int3:
+ *
+ * Intel erratum 54 - Unsynchronized Cross-modifying code - refers to the
+ * practice of modifying code on one processor where another has prefetched
+ * the unmodified version of the code. Intel states that unpredictable general
+ * protection faults may result if a synchronizing instruction (iret, int,
+ * int3, cpuid, etc ) is not executed on the second processor before it
+ * executes the pre-fetched out-of-date copy of the instruction.
+ *
+ * When we became aware of this I had a long discussion with Intel's
+ * microarchitecture guys. It turns out that the reason for this erratum
+ * (which incidentally Intel does not intend to fix) is because the trace
+ * cache - the stream of micro-ops resulting from instruction interpretation -
+ * cannot be guaranteed to be valid. Reading between the lines I assume this
+ * issue arises because of optimization done in the trace cache, where it is
+ * no longer possible to identify the original instruction boundaries. If the
+ * CPU discoverers that the trace cache has been invalidated because of
+ * unsynchronized cross-modification then instruction execution will be
+ * aborted with a GPF. Further discussion with Intel revealed that replacing
+ * the first opcode byte with an int3 would not be subject to this erratum.
+ *
+ * So, is cmpxchg reliable? One has to guarantee more than mere atomicity."
+ *
+ * Overall design
+ *
+ * The algorithm proposed by Intel applies not so well in kernel context: it
+ * would imply disabling interrupts and looping on every CPUs while modifying
+ * the code and would not support instrumentation of code called from interrupt
+ * sources that cannot be disabled.
+ *
+ * Therefore, we use a different algorithm to respect Intel's erratum (see the
+ * quoted discussion above). We make sure that no CPU sees an out-of-date copy
+ * of a pre-fetched instruction by 1 - using a breakpoint, which skips the
+ * instruction that is going to be modified, 2 - issuing an IPI to every CPU to
+ * execute a sync_core(), to make sure that even when the breakpoint is removed,
+ * no cpu could possibly still have the out-of-date copy of the instruction,
+ * modify the now unused 2nd byte of the instruction, and then put back the
+ * original 1st byte of the instruction.
+ *
+ * It has exactly the same intent as the algorithm proposed by Intel, but
+ * it has less side-effects, scales better and supports NMI, SMI and MCE.
+ *
+ * Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
+ */
+
+#include <linux/preempt.h>
+#include <linux/smp.h>
+#include <linux/notifier.h>
+#include <linux/module.h>
+#include <linux/immediate.h>
+#include <linux/kdebug.h>
+#include <linux/rcupdate.h>
+#include <linux/kprobes.h>
+#include <linux/io.h>
+
+#include <asm/cacheflush.h>
+
+#define BREAKPOINT_INSTRUCTION 0xcc
+#define BREAKPOINT_INS_LEN 1
+#define NR_NOPS 10
+
+static unsigned long target_after_int3; /* EIP of the target after the int3 */
+static unsigned long bypass_eip; /* EIP of the bypass. */
+static unsigned long bypass_after_int3; /* EIP after the end-of-bypass int3 */
+static unsigned long after_imv; /*
+ * EIP where to resume after the
+ * single-stepping.
+ */
+
+/*
+ * Internal bypass used during value update. The bypass is skipped by the
+ * function in which it is inserted.
+ * No need to be aligned because we exclude readers from the site during
+ * update.
+ * Layout is:
+ * (10x nop) int3
+ * (maximum size is 2 bytes opcode + 8 bytes immediate value for long on x86_64)
+ * The nops are the target replaced by the instruction to single-step.
+ * Align on 16 bytes to make sure the nops fit within a single page so remapping
+ * it can be done easily.
+ */
+static inline void _imv_bypass(unsigned long *bypassaddr,
+ unsigned long *breaknextaddr)
+{
+ asm volatile("jmp 2f;\n\t"
+ ".align 16;\n\t"
+ "0:\n\t"
+ ".space 10, 0x90;\n\t"
+ "1:\n\t"
+ "int3;\n\t"
+ "2:\n\t"
+ "mov $(0b),%0;\n\t"
+ "mov $((1b)+1),%1;\n\t"
+ : "=r" (*bypassaddr),
+ "=r" (*breaknextaddr));
+}
+
+static void imv_synchronize_core(void *info)
+{
+ sync_core(); /* use cpuid to stop speculative execution */
+}
+
+/*
+ * The eip value points right after the breakpoint instruction, in the second
+ * byte of the movl.
+ * Disable preemption in the bypass to make sure no thread will be preempted in
+ * it. We can then use synchronize_sched() to make sure every bypass users have
+ * ended.
+ */
+static int imv_notifier(struct notifier_block *nb,
+ unsigned long val, void *data)
+{
+ enum die_val die_val = (enum die_val) val;
+ struct die_args *args = data;
+
+ if (!args->regs || user_mode_vm(args->regs))
+ return NOTIFY_DONE;
+
+ if (die_val == DIE_INT3) {
+ if (args->regs->ip == target_after_int3) {
+ preempt_disable();
+ args->regs->ip = bypass_eip;
+ return NOTIFY_STOP;
+ } else if (args->regs->ip == bypass_after_int3) {
+ args->regs->ip = after_imv;
+ preempt_enable();
+ return NOTIFY_STOP;
+ }
+ }
+ return NOTIFY_DONE;
+}
+
+static struct notifier_block imv_notify = {
+ .notifier_call = imv_notifier,
+ .priority = 0x7fffffff, /* we need to be notified first */
+};
+
+/**
+ * arch_imv_update - update one immediate value
+ * @imv: pointer of type const struct __imv to update
+ * @early: early boot (1) or normal (0)
+ *
+ * Update one immediate value. Must be called with imv_mutex held.
+ */
+__kprobes int arch_imv_update(const struct __imv *imv, int early)
+{
+ int ret;
+ unsigned char opcode_size = imv->insn_size - imv->size;
+ unsigned long insn = imv->imv - opcode_size;
+ unsigned long len;
+ char *vaddr;
+ struct page *pages[1];
+
+#ifdef CONFIG_KPROBES
+ /*
+ * Fail if a kprobe has been set on this instruction.
+ * (TODO: we could eventually do better and modify all the (possibly
+ * nested) kprobes for this site if kprobes had an API for this.
+ */
+ if (unlikely(!early
+ && *(unsigned char *)insn == BREAKPOINT_INSTRUCTION)) {
+ printk(KERN_WARNING "Immediate value in conflict with kprobe. "
+ "Variable at %p, "
+ "instruction at %p, size %hu\n",
+ (void *)imv->imv,
+ (void *)imv->var, imv->size);
+ return -EBUSY;
+ }
+#endif
+
+ /*
+ * If the variable and the instruction have the same value, there is
+ * nothing to do.
+ */
+ switch (imv->size) {
+ case 1: if (*(uint8_t *)imv->imv
+ == *(uint8_t *)imv->var)
+ return 0;
+ break;
+ case 2: if (*(uint16_t *)imv->imv
+ == *(uint16_t *)imv->var)
+ return 0;
+ break;
+ case 4: if (*(uint32_t *)imv->imv
+ == *(uint32_t *)imv->var)
+ return 0;
+ break;
+#ifdef CONFIG_X86_64
+ case 8: if (*(uint64_t *)imv->imv
+ == *(uint64_t *)imv->var)
+ return 0;
+ break;
+#endif
+ default:return -EINVAL;
+ }
+
+ if (!early) {
+ /* bypass is 10 bytes long for x86_64 long */
+ WARN_ON(imv->insn_size > 10);
+ _imv_bypass(&bypass_eip, &bypass_after_int3);
+
+ after_imv = imv->imv + imv->size;
+
+ /*
+ * Using the _early variants because nobody is executing the
+ * bypass code while we patch it. It is protected by the
+ * imv_mutex. Since we modify the instructions non atomically
+ * (for nops), we have to use the _early variant.
+ * We must however deal with RO pages.
+ * Use a single page : 10 bytes are aligned on 16 bytes
+ * boundaries.
+ */
+ pages[0] = virt_to_page((void *)bypass_eip);
+ vaddr = vmap(pages, 1, VM_MAP, PAGE_KERNEL);
+ BUG_ON(!vaddr);
+ text_poke_early(&vaddr[bypass_eip & ~PAGE_MASK],
+ (void *)insn, imv->insn_size);
+ /*
+ * Fill the rest with nops.
+ */
+ len = NR_NOPS - imv->insn_size;
+ add_nops((void *)
+ &vaddr[(bypass_eip & ~PAGE_MASK) + imv->insn_size],
+ len);
+ vunmap(vaddr);
+
+ target_after_int3 = insn + BREAKPOINT_INS_LEN;
+ /* register_die_notifier has memory barriers */
+ register_die_notifier(&imv_notify);
+ /* The breakpoint will single-step the bypass */
+ text_poke((void *)insn,
+ ((unsigned char[]){BREAKPOINT_INSTRUCTION}), 1);
+ /*
+ * Make sure the breakpoint is set before we continue (visible
+ * to other CPUs and interrupts).
+ */
+ wmb();
+ /*
+ * Execute serializing instruction on each CPU.
+ */
+ ret = on_each_cpu(imv_synchronize_core, NULL, 1);
+ BUG_ON(ret != 0);
+
+ text_poke((void *)(insn + opcode_size), (void *)imv->var,
+ imv->size);
+ /*
+ * Make sure the value can be seen from other CPUs and
+ * interrupts.
+ */
+ wmb();
+ text_poke((void *)insn, (unsigned char *)bypass_eip, 1);
+ /*
+ * Wait for all int3 handlers to end (interrupts are disabled in
+ * int3). This CPU is clearly not in a int3 handler, because
+ * int3 handler is not preemptible and there cannot be any more
+ * int3 handler called for this site, because we placed the
+ * original instruction back. synchronize_sched has memory
+ * barriers.
+ */
+ synchronize_sched();
+ unregister_die_notifier(&imv_notify);
+ /* unregister_die_notifier has memory barriers */
+ } else
+ text_poke_early((void *)imv->imv, (void *)imv->var,
+ imv->size);
+ return 0;
+}

--
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68