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| -rw-r--r-- | include/linux/jump_label.h | 32 |
1 files changed, 32 insertions, 0 deletions
diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h index 5ce8b140428f..f7c69580fea7 100644 --- a/include/linux/jump_label.h +++ b/include/linux/jump_label.h @@ -1,6 +1,38 @@ #ifndef _LINUX_JUMP_LABEL_H #define _LINUX_JUMP_LABEL_H +/* + * Jump label support + * + * Copyright (C) 2009-2012 Jason Baron <jbaron@redhat.com> + * Copyright (C) 2011-2012 Peter Zijlstra <pzijlstr@redhat.com> + * + * Jump labels provide an interface to generate dynamic branches using + * self-modifying code. Assuming toolchain and architecture support the result + * of a "if (static_branch(&key))" statement is a unconditional branch (which + * defaults to false - and the true block is placed out of line). + * + * However at runtime we can change the 'static' branch target using + * jump_label_{inc,dec}(). These function as a 'reference' count on the key + * object and for as long as there are references all branches referring to + * that particular key will point to the (out of line) true block. + * + * Since this relies on modifying code the jump_label_{inc,dec}() functions + * must be considered absolute slow paths (machine wide synchronization etc.). + * OTOH, since the affected branches are unconditional their runtime overhead + * will be absolutely minimal, esp. in the default (off) case where the total + * effect is a single NOP of appropriate size. The on case will patch in a jump + * to the out-of-line block. + * + * When the control is directly exposed to userspace it is prudent to delay the + * decrement to avoid high frequency code modifications which can (and do) + * cause significant performance degradation. Struct jump_label_key_deferred and + * jump_label_dec_deferred() provide for this. + * + * Lacking toolchain and or architecture support, it falls back to a simple + * conditional branch. + */ + #include <linux/types.h> #include <linux/compiler.h> #include <linux/workqueue.h> |