diff options
-rw-r--r-- | include/linux/jump_label.h | 17 |
1 files changed, 8 insertions, 9 deletions
diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h index 784304b222b3..98f923b6a0ea 100644 --- a/include/linux/jump_label.h +++ b/include/linux/jump_label.h | |||
@@ -8,28 +8,28 @@ | |||
8 | * Copyright (C) 2011-2012 Peter Zijlstra <pzijlstr@redhat.com> | 8 | * Copyright (C) 2011-2012 Peter Zijlstra <pzijlstr@redhat.com> |
9 | * | 9 | * |
10 | * Jump labels provide an interface to generate dynamic branches using | 10 | * Jump labels provide an interface to generate dynamic branches using |
11 | * self-modifying code. Assuming toolchain and architecture support the result | 11 | * self-modifying code. Assuming toolchain and architecture support, the result |
12 | * of a "if (static_key_false(&key))" statement is a unconditional branch (which | 12 | * of a "if (static_key_false(&key))" statement is an unconditional branch (which |
13 | * defaults to false - and the true block is placed out of line). | 13 | * defaults to false - and the true block is placed out of line). |
14 | * | 14 | * |
15 | * However at runtime we can change the branch target using | 15 | * However at runtime we can change the branch target using |
16 | * static_key_slow_{inc,dec}(). These function as a 'reference' count on the key | 16 | * static_key_slow_{inc,dec}(). These function as a 'reference' count on the key |
17 | * object and for as long as there are references all branches referring to | 17 | * object, and for as long as there are references all branches referring to |
18 | * that particular key will point to the (out of line) true block. | 18 | * that particular key will point to the (out of line) true block. |
19 | * | 19 | * |
20 | * Since this relies on modifying code the static_key_slow_{inc,dec}() functions | 20 | * Since this relies on modifying code, the static_key_slow_{inc,dec}() functions |
21 | * must be considered absolute slow paths (machine wide synchronization etc.). | 21 | * must be considered absolute slow paths (machine wide synchronization etc.). |
22 | * OTOH, since the affected branches are unconditional their runtime overhead | 22 | * OTOH, since the affected branches are unconditional, their runtime overhead |
23 | * will be absolutely minimal, esp. in the default (off) case where the total | 23 | * will be absolutely minimal, esp. in the default (off) case where the total |
24 | * effect is a single NOP of appropriate size. The on case will patch in a jump | 24 | * effect is a single NOP of appropriate size. The on case will patch in a jump |
25 | * to the out-of-line block. | 25 | * to the out-of-line block. |
26 | * | 26 | * |
27 | * When the control is directly exposed to userspace it is prudent to delay the | 27 | * When the control is directly exposed to userspace, it is prudent to delay the |
28 | * decrement to avoid high frequency code modifications which can (and do) | 28 | * decrement to avoid high frequency code modifications which can (and do) |
29 | * cause significant performance degradation. Struct static_key_deferred and | 29 | * cause significant performance degradation. Struct static_key_deferred and |
30 | * static_key_slow_dec_deferred() provide for this. | 30 | * static_key_slow_dec_deferred() provide for this. |
31 | * | 31 | * |
32 | * Lacking toolchain and or architecture support, it falls back to a simple | 32 | * Lacking toolchain and or architecture support, jump labels fall back to a simple |
33 | * conditional branch. | 33 | * conditional branch. |
34 | * | 34 | * |
35 | * struct static_key my_key = STATIC_KEY_INIT_TRUE; | 35 | * struct static_key my_key = STATIC_KEY_INIT_TRUE; |
@@ -43,8 +43,7 @@ | |||
43 | * | 43 | * |
44 | * Not initializing the key (static data is initialized to 0s anyway) is the | 44 | * Not initializing the key (static data is initialized to 0s anyway) is the |
45 | * same as using STATIC_KEY_INIT_FALSE. | 45 | * same as using STATIC_KEY_INIT_FALSE. |
46 | * | 46 | */ |
47 | */ | ||
48 | 47 | ||
49 | #include <linux/types.h> | 48 | #include <linux/types.h> |
50 | #include <linux/compiler.h> | 49 | #include <linux/compiler.h> |