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    Subject[PATCH 3.16 101/254] MIPS: Fix a preemption issue with thread's FPU defaults
    3.16.55-rc1 review patch.  If anyone has any objections, please let me know.

    ------------------

    From: "Maciej W. Rozycki" <macro@linux-mips.org>

    commit 03dce595270f22d59a6f37e9170287c1afd94bc2 upstream.

    Fix "BUG: using smp_processor_id() in preemptible" reported in accesses
    to thread's FPU defaults: the value to initialise FSCR to at program
    startup, the FCSR r/w mask and the contents of FIR in full FPU
    emulation, removing a regression introduced with 9b26616c [MIPS: Respect
    the ISA level in FCSR handling] and f6843626 [MIPS: math-emu: Set FIR
    feature flags for full emulation].

    Use `boot_cpu_data' to obtain the data from, following the approach that
    `cpu_has_*' macros take and avoiding the call to `smp_processor_id' made
    in the reference to `current_cpu_data'. The contents of FSCR have to be
    consistent across processors in an SMP system, the settings there must
    not change as a thread is migrated across processors. And the contents
    of FIR are guaranteed to be consistent in FPU emulation, by definition.

    Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
    Tested-by: Ezequiel Garcia <ezequiel.garcia@imgtec.com>
    Tested-by: Paul Martin <paul.martin@codethink.co.uk>
    Cc: Markos Chandras <Markos.Chandras@imgtec.com>
    Cc: James Hogan <james.hogan@imgtec.com>
    Cc: linux-mips@linux-mips.org
    Patchwork: https://patchwork.linux-mips.org/patch/10030/
    Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
    [bwh: Backported to 3.16:
    - Drop change in cop1_cfc()
    - Adjust context]
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    --- a/arch/mips/include/asm/elf.h
    +++ b/arch/mips/include/asm/elf.h
    @@ -277,7 +277,7 @@ do { \
    \
    current->thread.abi = &mips_abi; \
    \
    - current->thread.fpu.fcr31 = current_cpu_data.fpu_csr31; \
    + current->thread.fpu.fcr31 = boot_cpu_data.fpu_csr31; \
    } while (0)

    #endif /* CONFIG_32BIT */
    @@ -337,7 +337,7 @@ do { \
    else \
    current->thread.abi = &mips_abi; \
    \
    - current->thread.fpu.fcr31 = current_cpu_data.fpu_csr31; \
    + current->thread.fpu.fcr31 = boot_cpu_data.fpu_csr31; \
    \
    p = personality(current->personality); \
    if (p != PER_LINUX32 && p != PER_LINUX) \
    --- a/arch/mips/kernel/ptrace.c
    +++ b/arch/mips/kernel/ptrace.c
    @@ -177,7 +177,7 @@ int ptrace_setfpregs(struct task_struct

    __get_user(value, data + 64);
    fcr31 = child->thread.fpu.fcr31;
    - mask = current_cpu_data.fpu_msk31;
    + mask = boot_cpu_data.fpu_msk31;
    child->thread.fpu.fcr31 = (value & ~mask) | (fcr31 & mask);

    /* FIR may not be written. */
    --- a/arch/mips/math-emu/cp1emu.c
    +++ b/arch/mips/math-emu/cp1emu.c
    @@ -934,7 +934,7 @@ emul:
    * Preserve read-only bits,
    * and convert to ieee library modes
    */
    - mask = current_cpu_data.fpu_msk31;
    + mask = boot_cpu_data.fpu_msk31;
    ctx->fcr31 = (value & ~(mask | FPU_CSR_RM)) |
    (ctx->fcr31 & mask) |
    modeindex(value);
    \
     
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