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Commit 5b3eec0c authored by Ingo Molnar's avatar Ingo Molnar
Browse files

x86: ret_from_fork - get rid of jump back 



Impact: remove dead code

If we take a closer look at the rff_trace/rff_action ret_from_fork code,
we have to realize that it does all the wrong things: for example it
checks the TIF flag - while later on jumping back to the ret-from-syscall
path - duplicating the check needlessly.

But checking for _TIF_SYSCALL_TRACE is completely unnecessary here because
we clear that flag for every freshly forked task. So the whole "tracing"
code here, for which there is a out of line jump optimization that makes
it even harder to read, is in reality completely dead code ...

Reported-by: default avatarCyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
Tested-by: default avatarCyrill Gorcunov <gorcunov@gmail.com>
parent 3bdae4f4

arch/x86/kernel/entry_64.S

+16 −15
Original line number Diff line number Diff line
@@ -361,34 +361,35 @@ ENTRY(save_paranoid) 
END(save_paranoid)



/*

 * A newly forked process directly context switches into this.

 * A newly forked process directly context switches into this address.

 *

 * rdi: prev task we switched from

 */

/* rdi:	prev */

ENTRY(ret_from_fork)

	DEFAULT_FRAME



	push kernel_eflags(%rip)

	CFI_ADJUST_CFA_OFFSET 8

	popf					# reset kernel eflags

	CFI_ADJUST_CFA_OFFSET -8

	call schedule_tail



	call schedule_tail			# rdi: 'prev' task parameter



	GET_THREAD_INFO(%rcx)

	testl $(_TIF_SYSCALL_TRACE|_TIF_SYSCALL_AUDIT),TI_flags(%rcx)



	CFI_REMEMBER_STATE

	jnz rff_trace

rff_action:

	RESTORE_REST



	testl $3, CS-ARGOFFSET(%rsp)		# from kernel_thread?

	je   int_ret_from_sys_call

	testl $_TIF_IA32,TI_flags(%rcx)



	testl $_TIF_IA32, TI_flags(%rcx)	# 32-bit compat task needs IRET

	jnz  int_ret_from_sys_call



	RESTORE_TOP_OF_STACK %rdi, -ARGOFFSET

	jmp ret_from_sys_call

	jmp ret_from_sys_call			# go to the SYSRET fastpath



	CFI_RESTORE_STATE

rff_trace:

	movq %rsp,%rdi

	call syscall_trace_leave

	GET_THREAD_INFO(%rcx)

	jmp rff_action

	CFI_ENDPROC

END(ret_from_fork)