[PATCH] x86_64: save FPU context slightly later

Touching of the floating point state in a kernel debugger must be
NMI-safe, specifically math_state_restore() must be able to deal with
being called out of an NMI context. In order to do that reliably, the
context switch code must take care to not leave a window open where
the current task's TS_USEDFPU flag and CR0.TS could get out of sync.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 4 insertions, 3 deletions

diff --git a/arch/x86_64/kernel/process.c b/arch/x86_64/kernel/process.c
index 0cb93abbf564..81111835722d 100644
--- a/arch/x86_64/kernel/process.c
+++ b/arch/x86_64/kernel/process.c
@@ -527,8 +527,6 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
         int cpu = smp_processor_id();  
         struct tss_struct *tss = &per_cpu(init_tss, cpu);
 
-        unlazy_fpu(prev_p);
-
         /*
          * Reload esp0, LDT and the page table pointer:
          */
@@ -586,11 +584,14 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
         }
 
         /* 
-         * Switch the PDA context.
+         * Switch the PDA and FPU contexts.
          */
         prev->userrsp = read_pda(oldrsp); 
         write_pda(oldrsp, next->userrsp); 
         write_pda(pcurrent, next_p); 
+        /* This must be here to ensure both math_state_restore() and
+           kernel_fpu_begin() work consistently. */
+        unlazy_fpu(prev_p);
         write_pda(kernelstack,
                   task_stack_page(next_p) + THREAD_SIZE - PDA_STACKOFFSET);