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[ Upstream commit a124692b698b00026a58d89831ceda2331b2e1d0 ] Custom trampolines can only be enabled if there is only a single ops attached to it. If there's only a single callback registered to a function, and the ops has a trampoline registered for it, then we can call the trampoline directly. This is very useful for improving the performance of ftrace and livepatch. If more than one callback is registered to a function, the general trampoline is used, and the custom trampoline is not restored back to the direct call even if all the other callbacks were unregistered and we are back to one callback for the function. To fix this, set FTRACE_FL_TRAMP flag if rec count is decremented to one, and the ops that left has a trampoline. Testing After this patch : insmod livepatch_unshare_files.ko cat /sys/kernel/debug/tracing/enabled_functions unshare_files (1) R I tramp: 0xffffffffc0000000(klp_ftrace_handler+0x0/0xa0) ->ftrace_ops_assist_func+0x0/0xf0 echo unshare_files > /sys/kernel/debug/tracing/set_ftrace_filter echo function > /sys/kernel/debug/tracing/current_tracer cat /sys/kernel/debug/tracing/enabled_functions unshare_files (2) R I ->ftrace_ops_list_func+0x0/0x150 echo nop > /sys/kernel/debug/tracing/current_tracer cat /sys/kernel/debug/tracing/enabled_functions unshare_files (1) R I tramp: 0xffffffffc0000000(klp_ftrace_handler+0x0/0xa0) ->ftrace_ops_assist_func+0x0/0xf0 Link: http://lkml.kernel.org/r/1556969979-111047-1-git-send-email-cj.chengjian@huawei.com Signed-off-by: Cheng Jian <cj.chengjian@huawei.com> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org> Signed-off-by: Sasha Levin <sashal@kernel.org>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.