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    Subject[tip:x86/fpu] x86/fpu: Don't abuse drop_init_fpu() in flush_thread()
    Commit-ID:  f893959b0898bd876673adbeb6798bdf25c034d7
    Gitweb: http://git.kernel.org/tip/f893959b0898bd876673adbeb6798bdf25c034d7
    Author: Oleg Nesterov <oleg@redhat.com>
    AuthorDate: Fri, 13 Mar 2015 18:30:30 +0100
    Committer: Ingo Molnar <mingo@kernel.org>
    CommitDate: Mon, 23 Mar 2015 10:13:58 +0100

    x86/fpu: Don't abuse drop_init_fpu() in flush_thread()

    flush_thread() -> drop_init_fpu() is suboptimal and confusing. It does
    drop_fpu() or restore_init_xstate() depending on !use_eager_fpu(). But
    flush_thread() too checks eagerfpu right after that, and if it is true
    then restore_init_xstate() just burns CPU for no reason. We are going to
    load init_xstate_buf again after we set used_math()/user_has_fpu(), until
    then the FPU state can't survive after switch_to().

    Remove it, and change the "if (!use_eager_fpu())" to call drop_fpu().
    While at it, clean up the tsk/current usage.

    Signed-off-by: Oleg Nesterov <oleg@redhat.com>
    Signed-off-by: Borislav Petkov <bp@suse.de>
    Cc: Andy Lutomirski <luto@amacapital.net>
    Cc: Dave Hansen <dave.hansen@intel.com>
    Cc: Fenghua Yu <fenghua.yu@intel.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Pekka Riikonen <priikone@iki.fi>
    Cc: Quentin Casasnovas <quentin.casasnovas@oracle.com>
    Cc: Rik van Riel <riel@redhat.com>
    Cc: Suresh Siddha <sbsiddha@gmail.com>
    Link: http://lkml.kernel.org/r/20150313173030.GA31217@redhat.com
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    ---
    arch/x86/kernel/process.c | 15 ++++++---------
    1 file changed, 6 insertions(+), 9 deletions(-)

    diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
    index 6b05829..1d2ebad 100644
    --- a/arch/x86/kernel/process.c
    +++ b/arch/x86/kernel/process.c
    @@ -132,17 +132,14 @@ void flush_thread(void)
    flush_ptrace_hw_breakpoint(tsk);
    memset(tsk->thread.tls_array, 0, sizeof(tsk->thread.tls_array));

    - drop_init_fpu(tsk);
    - /*
    - * Free the FPU state for non xsave platforms. They get reallocated
    - * lazily at the first use.
    - */
    - if (!use_eager_fpu())
    + if (!use_eager_fpu()) {
    + /* FPU state will be reallocated lazily at the first use. */
    + drop_fpu(tsk);
    free_thread_xstate(tsk);
    - else if (!used_math()) {
    + } else if (!used_math()) {
    /* kthread execs. TODO: cleanup this horror. */
    - if (WARN_ON(init_fpu(current)))
    - force_sig(SIGKILL, current);
    + if (WARN_ON(init_fpu(tsk)))
    + force_sig(SIGKILL, tsk);
    user_fpu_begin();
    restore_init_xstate();
    }

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