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authorJeff Dike <jdike@addtoit.com>2007-05-10 22:22:34 -0700
committerLinus Torvalds <torvalds@woody.linux-foundation.org>2007-05-11 08:29:34 -0700
commitc14b84949e127560084c7c56b365931c71c60768 (patch)
tree88bce4993779078856612b6a32f65f14ab379d85 /arch/um/kernel
parent2ea5bc5e5bb51492f189bba44045e0de7decf4a0 (diff)
uml: iRQ stacks
Add a separate IRQ stack. This differs from i386 in having the entire interrupt run on a separate stack rather than starting on the normal kernel stack and switching over once some preparation has been done. The underlying mechanism, is of course, sigaltstack. Another difference is that interrupts that happen in userspace are handled on the normal kernel stack. These cause a wait wakeup instead of a signal delivery so there is no point in trying to switch stacks for these. There's no other stuff on the stack, so there is no extra stack consumption. This quirk makes it possible to have the entire interrupt run on a separate stack - process preemption (and calls to schedule()) happens on a normal kernel stack. If we enable CONFIG_PREEMPT, this will need to be rethought. The IRQ stack for CPU 0 is declared in the same way as the initial kernel stack. IRQ stacks for other CPUs will be allocated dynamically. An extra field was added to the thread_info structure. When the active thread_info is copied to the IRQ stack, the real_thread field points back to the original stack. This makes it easy to tell where to copy the thread_info struct back to when the interrupt is finished. It also serves as a marker of a nested interrupt. It is NULL for the first interrupt on the stack, and non-NULL for any nested interrupts. Care is taken to behave correctly if a second interrupt comes in when the thread_info structure is being set up or taken down. I could just disable interrupts here, but I don't feel like giving up any of the performance gained by not flipping signals on and off. If an interrupt comes in during these critical periods, the handler can't run because it has no idea what shape the stack is in. So, it sets a bit for its signal in a global mask and returns. The outer handler will deal with this signal itself. Atomicity is had with xchg. A nested interrupt that needs to bail out will xchg its signal mask into pending_mask and repeat in case yet another interrupt hit at the same time, until the mask stabilizes. The outermost interrupt will set up the thread_info and xchg a zero into pending_mask when it is done. At this point, nested interrupts will look at ->real_thread and see that no setup needs to be done. They can just continue normally. Similar care needs to be taken when exiting the outer handler. If another interrupt comes in while it is copying the thread_info, it will drop a bit into pending_mask. The outer handler will check this and if it is non-zero, will loop, set up the stack again, and handle the interrupt. Signed-off-by: Jeff Dike <jdike@linux.intel.com> Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'arch/um/kernel')
-rw-r--r--arch/um/kernel/dyn.lds.S2
-rw-r--r--arch/um/kernel/init_task.c16
-rw-r--r--arch/um/kernel/irq.c111
-rw-r--r--arch/um/kernel/skas/process.c4
-rw-r--r--arch/um/kernel/uml.lds.S2
5 files changed, 129 insertions, 6 deletions
diff --git a/arch/um/kernel/dyn.lds.S b/arch/um/kernel/dyn.lds.S
index e36f92b463ce..87a4e4427d8d 100644
--- a/arch/um/kernel/dyn.lds.S
+++ b/arch/um/kernel/dyn.lds.S
@@ -97,6 +97,8 @@ SECTIONS
.data : {
. = ALIGN(KERNEL_STACK_SIZE); /* init_task */
*(.data.init_task)
+ . = ALIGN(KERNEL_STACK_SIZE);
+ *(.data.init_irqstack)
*(.data .data.* .gnu.linkonce.d.*)
SORT(CONSTRUCTORS)
}
diff --git a/arch/um/kernel/init_task.c b/arch/um/kernel/init_task.c
index f5385a3799bb..d4f1d1ab252b 100644
--- a/arch/um/kernel/init_task.c
+++ b/arch/um/kernel/init_task.c
@@ -1,5 +1,5 @@
-/*
- * Copyright (C) 2000 Jeff Dike (jdike@karaya.com)
+/*
+ * Copyright (C) 2000 - 2007 Jeff Dike (jdike@{addtoit,intel.linux}.com)
* Licensed under the GPL
*/
@@ -33,14 +33,18 @@ EXPORT_SYMBOL(init_task);
/*
* Initial thread structure.
*
- * We need to make sure that this is 16384-byte aligned due to the
+ * We need to make sure that this is aligned due to the
* way process stacks are handled. This is done by having a special
* "init_task" linker map entry..
*/
-union thread_union init_thread_union
-__attribute__((__section__(".data.init_task"))) =
-{ INIT_THREAD_INFO(init_task) };
+union thread_union init_thread_union
+ __attribute__((__section__(".data.init_task"))) =
+ { INIT_THREAD_INFO(init_task) };
+
+union thread_union cpu0_irqstack
+ __attribute__((__section__(".data.init_irqstack"))) =
+ { INIT_THREAD_INFO(init_task) };
void unprotect_stack(unsigned long stack)
{
diff --git a/arch/um/kernel/irq.c b/arch/um/kernel/irq.c
index a9651a175eb5..dba04d88b432 100644
--- a/arch/um/kernel/irq.c
+++ b/arch/um/kernel/irq.c
@@ -32,6 +32,7 @@
#include "sigio.h"
#include "um_malloc.h"
#include "misc_constants.h"
+#include "as-layout.h"
/*
* Generic, controller-independent functions:
@@ -468,3 +469,113 @@ int init_aio_irq(int irq, char *name, irq_handler_t handler)
out:
return err;
}
+
+/*
+ * IRQ stack entry and exit:
+ *
+ * Unlike i386, UML doesn't receive IRQs on the normal kernel stack
+ * and switch over to the IRQ stack after some preparation. We use
+ * sigaltstack to receive signals on a separate stack from the start.
+ * These two functions make sure the rest of the kernel won't be too
+ * upset by being on a different stack. The IRQ stack has a
+ * thread_info structure at the bottom so that current et al continue
+ * to work.
+ *
+ * to_irq_stack copies the current task's thread_info to the IRQ stack
+ * thread_info and sets the tasks's stack to point to the IRQ stack.
+ *
+ * from_irq_stack copies the thread_info struct back (flags may have
+ * been modified) and resets the task's stack pointer.
+ *
+ * Tricky bits -
+ *
+ * What happens when two signals race each other? UML doesn't block
+ * signals with sigprocmask, SA_DEFER, or sa_mask, so a second signal
+ * could arrive while a previous one is still setting up the
+ * thread_info.
+ *
+ * There are three cases -
+ * The first interrupt on the stack - sets up the thread_info and
+ * handles the interrupt
+ * A nested interrupt interrupting the copying of the thread_info -
+ * can't handle the interrupt, as the stack is in an unknown state
+ * A nested interrupt not interrupting the copying of the
+ * thread_info - doesn't do any setup, just handles the interrupt
+ *
+ * The first job is to figure out whether we interrupted stack setup.
+ * This is done by xchging the signal mask with thread_info->pending.
+ * If the value that comes back is zero, then there is no setup in
+ * progress, and the interrupt can be handled. If the value is
+ * non-zero, then there is stack setup in progress. In order to have
+ * the interrupt handled, we leave our signal in the mask, and it will
+ * be handled by the upper handler after it has set up the stack.
+ *
+ * Next is to figure out whether we are the outer handler or a nested
+ * one. As part of setting up the stack, thread_info->real_thread is
+ * set to non-NULL (and is reset to NULL on exit). This is the
+ * nesting indicator. If it is non-NULL, then the stack is already
+ * set up and the handler can run.
+ */
+
+static unsigned long pending_mask;
+
+unsigned long to_irq_stack(int sig, unsigned long *mask_out)
+{
+ struct thread_info *ti;
+ unsigned long mask, old;
+ int nested;
+
+ mask = xchg(&pending_mask, 1 << sig);
+ if(mask != 0){
+ /* If any interrupts come in at this point, we want to
+ * make sure that their bits aren't lost by our
+ * putting our bit in. So, this loop accumulates bits
+ * until xchg returns the same value that we put in.
+ * When that happens, there were no new interrupts,
+ * and pending_mask contains a bit for each interrupt
+ * that came in.
+ */
+ old = 1 << sig;
+ do {
+ old |= mask;
+ mask = xchg(&pending_mask, old);
+ } while(mask != old);
+ return 1;
+ }
+
+ ti = current_thread_info();
+ nested = (ti->real_thread != NULL);
+ if(!nested){
+ struct task_struct *task;
+ struct thread_info *tti;
+
+ task = cpu_tasks[ti->cpu].task;
+ tti = task_thread_info(task);
+ *ti = *tti;
+ ti->real_thread = tti;
+ task->stack = ti;
+ }
+
+ mask = xchg(&pending_mask, 0);
+ *mask_out |= mask | nested;
+ return 0;
+}
+
+unsigned long from_irq_stack(int nested)
+{
+ struct thread_info *ti, *to;
+ unsigned long mask;
+
+ ti = current_thread_info();
+
+ pending_mask = 1;
+
+ to = ti->real_thread;
+ current->stack = to;
+ ti->real_thread = NULL;
+ *to = *ti;
+
+ mask = xchg(&pending_mask, 0);
+ return mask & ~1;
+}
+
diff --git a/arch/um/kernel/skas/process.c b/arch/um/kernel/skas/process.c
index a96ae1a0610e..2a69a7ce5792 100644
--- a/arch/um/kernel/skas/process.c
+++ b/arch/um/kernel/skas/process.c
@@ -163,8 +163,12 @@ static int start_kernel_proc(void *unused)
extern int userspace_pid[];
+extern char cpu0_irqstack[];
+
int start_uml_skas(void)
{
+ stack_protections((unsigned long) &cpu0_irqstack);
+ set_sigstack(cpu0_irqstack, THREAD_SIZE);
if(proc_mm)
userspace_pid[0] = start_userspace(0);
diff --git a/arch/um/kernel/uml.lds.S b/arch/um/kernel/uml.lds.S
index f6301274cf3c..bc59f97e34d0 100644
--- a/arch/um/kernel/uml.lds.S
+++ b/arch/um/kernel/uml.lds.S
@@ -59,6 +59,8 @@ SECTIONS
{
. = ALIGN(KERNEL_STACK_SIZE); /* init_task */
*(.data.init_task)
+ . = ALIGN(KERNEL_STACK_SIZE);
+ *(.data.init_irqstack)
*(.data)
*(.gnu.linkonce.d*)
CONSTRUCTORS

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