Messages in this thread Patch in this message | | | Date | Fri, 29 Aug 2014 20:16:03 +0200 | From | Oleg Nesterov <> | Subject | [PATCH 1/4] x86, fpu: introduce per-cpu "bool in_kernel_fpu" |
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interrupted_kernel_fpu_idle() tries to detect if kernel_fpu_begin() is safe or not. In particulat it should obviously deny the nested kernel_fpu_begin() and this logic doesn't look clean.
If use_eager_fpu() == T we rely on a) __thread_has_fpu() check in interrupted_kernel_fpu_idle(), and b) on the fact that _begin() does __thread_clear_has_fpu().
Otherwise we demand that the interrupted task has no FPU if it is in kernel mode, this works becase __kernel_fpu_begin() does clts().
Add the per-cpu "bool in_kernel_fpu" variable, and change this code to check/set/clear it. This allows to do some cleanups (see the next changes) and fixes.
Note that the current code looks racy. Say, kernel_fpu_begin() right after math_state_restore()->__thread_fpu_begin() will overwrite the regs we are going to restore. This patch doesn't even try to fix this, it just adds the comment, but "in_kernel_fpu" can also be used to implement kernel_fpu_disable() / kernel_fpu_enable().
Signed-off-by: Oleg Nesterov <oleg@redhat.com> --- arch/x86/include/asm/i387.h | 2 +- arch/x86/kernel/i387.c | 10 ++++++++++ 2 files changed, 11 insertions(+), 1 deletions(-)
diff --git a/arch/x86/include/asm/i387.h b/arch/x86/include/asm/i387.h index ed8089d..5e275d3 100644 --- a/arch/x86/include/asm/i387.h +++ b/arch/x86/include/asm/i387.h @@ -40,8 +40,8 @@ extern void __kernel_fpu_end(void); static inline void kernel_fpu_begin(void) { - WARN_ON_ONCE(!irq_fpu_usable()); preempt_disable(); + WARN_ON_ONCE(!irq_fpu_usable()); __kernel_fpu_begin(); } diff --git a/arch/x86/kernel/i387.c b/arch/x86/kernel/i387.c index d5dd808..8fb8868 100644 --- a/arch/x86/kernel/i387.c +++ b/arch/x86/kernel/i387.c @@ -19,6 +19,8 @@ #include <asm/fpu-internal.h> #include <asm/user.h> +static DEFINE_PER_CPU(bool, in_kernel_fpu); + /* * Were we in an interrupt that interrupted kernel mode? * @@ -33,6 +35,9 @@ */ static inline bool interrupted_kernel_fpu_idle(void) { + if (this_cpu_read(in_kernel_fpu)) + return false; + if (use_eager_fpu()) return __thread_has_fpu(current); @@ -73,6 +78,9 @@ void __kernel_fpu_begin(void) { struct task_struct *me = current; + this_cpu_write(in_kernel_fpu, true); + + /* FIXME: race with math_state_restore()-like code */ if (__thread_has_fpu(me)) { __thread_clear_has_fpu(me); __save_init_fpu(me); @@ -99,6 +107,8 @@ void __kernel_fpu_end(void) } else { stts(); } + + this_cpu_write(in_kernel_fpu, false); } EXPORT_SYMBOL(__kernel_fpu_end); -- 1.5.5.1
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