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Commit 4eacdf18374e5d7d21a728b46dfec269ac8ef55c

Authored by Frederic Weisbecker
Committed by Paul E. McKenney
1 parent 90f45e4e72
Exists in smarc-l5.0.0_1.0.0-ga and in 5 other branches imx_3.10.17_1.0.1_ga, imx_3.10.53_1.1.0_ga, imx_3.14.28_1.0.0_ga, smarc-imx_3.10.53_1.1.0_ga, smarc-imx_3.14.28_1.0.0_ga

context_tracking: Add comments on interface and internals 

This subsystem lacks many explanations on its purpose and
design. Add these missing comments.

v4: Document function parameter to be more kernel-doc
friendly, as per Namhyung suggestion.

Reported-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Alessio Igor Bogani <abogani@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Chris Metcalf <cmetcalf@tilera.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Geoff Levand <geoff@infradead.org>
Cc: Gilad Ben Yossef <gilad@benyossef.com>
Cc: Hakan Akkan <hakanakkan@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Li Zhong <zhong@linux.vnet.ibm.com>
Cc: Namhyung Kim <namhyung.kim@lge.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Showing 1 changed file with 65 additions and 10 deletions Inline Diff

kernel/context_tracking.c
Diff comments   View file @ 4eacdf1
1 /*
2 * Context tracking: Probe on high level context boundaries such as kernel
3 * and userspace. This includes syscalls and exceptions entry/exit.
4 *
5 * This is used by RCU to remove its dependency on the timer tick while a CPU
6 * runs in userspace.
7 *
8 * Started by Frederic Weisbecker:
9 *
10 * Copyright (C) 2012 Red Hat, Inc., Frederic Weisbecker <fweisbec@redhat.com>
11 *
12 * Many thanks to Gilad Ben-Yossef, Paul McKenney, Ingo Molnar, Andrew Morton,
13 * Steven Rostedt, Peter Zijlstra for suggestions and improvements.
14 *
15 */
16
1 #include <linux/context_tracking.h> 17 #include <linux/context_tracking.h>
2 #include <linux/rcupdate.h> 18 #include <linux/rcupdate.h>
3 #include <linux/sched.h> 19 #include <linux/sched.h>
4 #include <linux/percpu.h> 20 #include <linux/percpu.h>
5 #include <linux/hardirq.h> 21 #include <linux/hardirq.h>
6 22
7 struct context_tracking { 23 struct context_tracking {
8 /* 24 /*
9 * When active is false, hooks are not set to 25 * When active is false, probes are unset in order
10 * minimize overhead: TIF flags are cleared 26 * to minimize overhead: TIF flags are cleared
11 * and calls to user_enter/exit are ignored. This 27 * and calls to user_enter/exit are ignored. This
12 * may be further optimized using static keys. 28 * may be further optimized using static keys.
13 */ 29 */
14 bool active; 30 bool active;
15 enum { 31 enum {
16 IN_KERNEL = 0, 32 IN_KERNEL = 0,
17 IN_USER, 33 IN_USER,
18 } state; 34 } state;
19 }; 35 };
20 36
21 static DEFINE_PER_CPU(struct context_tracking, context_tracking) = { 37 static DEFINE_PER_CPU(struct context_tracking, context_tracking) = {
22 #ifdef CONFIG_CONTEXT_TRACKING_FORCE 38 #ifdef CONFIG_CONTEXT_TRACKING_FORCE
23 .active = true, 39 .active = true,
24 #endif 40 #endif
25 }; 41 };
26 42
43 /**
44 * user_enter - Inform the context tracking that the CPU is going to
45 * enter userspace mode.
46 *
47 * This function must be called right before we switch from the kernel
48 * to userspace, when it's guaranteed the remaining kernel instructions
49 * to execute won't use any RCU read side critical section because this
50 * function sets RCU in extended quiescent state.
51 */
27 void user_enter(void) 52 void user_enter(void)
28 { 53 {
29 unsigned long flags; 54 unsigned long flags;
30 55
31 /* 56 /*
32 * Some contexts may involve an exception occuring in an irq, 57 * Some contexts may involve an exception occuring in an irq,
33 * leading to that nesting: 58 * leading to that nesting:
34 * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit() 59 * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit()
35 * This would mess up the dyntick_nesting count though. And rcu_irq_*() 60 * This would mess up the dyntick_nesting count though. And rcu_irq_*()
36 * helpers are enough to protect RCU uses inside the exception. So 61 * helpers are enough to protect RCU uses inside the exception. So
37 * just return immediately if we detect we are in an IRQ. 62 * just return immediately if we detect we are in an IRQ.
38 */ 63 */
39 if (in_interrupt()) 64 if (in_interrupt())
40 return; 65 return;
41 66
67 /* Kernel threads aren't supposed to go to userspace */
42 WARN_ON_ONCE(!current->mm); 68 WARN_ON_ONCE(!current->mm);
43 69
44 local_irq_save(flags); 70 local_irq_save(flags);
45 if (__this_cpu_read(context_tracking.active) && 71 if (__this_cpu_read(context_tracking.active) &&
46 __this_cpu_read(context_tracking.state) != IN_USER) { 72 __this_cpu_read(context_tracking.state) != IN_USER) {
47 __this_cpu_write(context_tracking.state, IN_USER); 73 __this_cpu_write(context_tracking.state, IN_USER);
74 /*
75 * At this stage, only low level arch entry code remains and
76 * then we'll run in userspace. We can assume there won't be
77 * any RCU read-side critical section until the next call to
78 * user_exit() or rcu_irq_enter(). Let's remove RCU's dependency
79 * on the tick.
80 */
48 rcu_user_enter(); 81 rcu_user_enter();
49 } 82 }
50 local_irq_restore(flags); 83 local_irq_restore(flags);
51 } 84 }
52 85
86
87 /**
88 * user_exit - Inform the context tracking that the CPU is
89 * exiting userspace mode and entering the kernel.
90 *
91 * This function must be called after we entered the kernel from userspace
92 * before any use of RCU read side critical section. This potentially include
93 * any high level kernel code like syscalls, exceptions, signal handling, etc...
94 *
95 * This call supports re-entrancy. This way it can be called from any exception
96 * handler without needing to know if we came from userspace or not.
97 */
53 void user_exit(void) 98 void user_exit(void)
54 { 99 {
55 unsigned long flags; 100 unsigned long flags;
56 101
57 /*
58 * Some contexts may involve an exception occuring in an irq,
59 * leading to that nesting:
60 * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit()
61 * This would mess up the dyntick_nesting count though. And rcu_irq_*()
62 * helpers are enough to protect RCU uses inside the exception. So
63 * just return immediately if we detect we are in an IRQ.
64 */
65 if (in_interrupt()) 102 if (in_interrupt())
66 return; 103 return;
67 104
68 local_irq_save(flags); 105 local_irq_save(flags);
69 if (__this_cpu_read(context_tracking.state) == IN_USER) { 106 if (__this_cpu_read(context_tracking.state) == IN_USER) {
70 __this_cpu_write(context_tracking.state, IN_KERNEL); 107 __this_cpu_write(context_tracking.state, IN_KERNEL);
108 /*
109 * We are going to run code that may use RCU. Inform
110 * RCU core about that (ie: we may need the tick again).
111 */
71 rcu_user_exit(); 112 rcu_user_exit();
72 } 113 }
73 local_irq_restore(flags); 114 local_irq_restore(flags);
74 } 115 }
75 116
117
118 /**
119 * context_tracking_task_switch - context switch the syscall callbacks
120 * @prev: the task that is being switched out
121 * @next: the task that is being switched in
122 *
123 * The context tracking uses the syscall slow path to implement its user-kernel
124 * boundaries probes on syscalls. This way it doesn't impact the syscall fast
125 * path on CPUs that don't do context tracking.
126 *
127 * But we need to clear the flag on the previous task because it may later
128 * migrate to some CPU that doesn't do the context tracking. As such the TIF
129 * flag may not be desired there.
130 */
76 void context_tracking_task_switch(struct task_struct *prev, 131 void context_tracking_task_switch(struct task_struct *prev,