x86, fpu: fix CONFIG_PREEMPT=y corruption of application's FPU stack

Jürgen Mell reported an FPU state corruption bug under CONFIG_PREEMPT,
and bisected it to commit v2.6.19-1363-gacc20761, "i386: add sleazy FPU
optimization".

Add tsk_used_math() checks to prevent calling math_state_restore()
which can sleep in the case of !tsk_used_math(). This prevents
making a blocking call in __switch_to().

Apparently "fpu_counter > 5" check is not enough, as in some signal handling
and fork/exec scenarios, fpu_counter > 5 and !tsk_used_math() is possible.

It's a side effect though. This is the failing scenario:

process 'A' in save_i387_ia32() just after clear_used_math()

Got an interrupt and pre-empted out.

At the next context switch to process 'A' again, kernel tries to restore
the math state proactively and sees a fpu_counter > 0 and !tsk_used_math()

This results in init_fpu() during the __switch_to()'s math_state_restore()

And resulting in fpu corruption which will be saved/restored
(save_i387_fxsave and restore_i387_fxsave) during the remaining
part of the signal handling after the context switch.
Bisected-by: Jürgen Mell <j.mell@t-online.de>
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Tested-by: Jürgen Mell <j.mell@t-online.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@kernel.org

arch/x86/kernel/process_32.c

@@ -649,8 +649,11 @@ struct task_struct * __switch_to(struct task_struct *prev_p, struct task_struct
 	/* If the task has used fpu the last 5 timeslices, just do a full
 	 * restore of the math state immediately to avoid the trap; the
 	 * chances of needing FPU soon are obviously high now
+	 *
+	 * tsk_used_math() checks prevent calling math_state_restore(),
+	 * which can sleep in the case of !tsk_used_math()
 	 */
-	if (next_p->fpu_counter > 5)
+	if (tsk_used_math(next_p) && next_p->fpu_counter > 5)
 		math_state_restore();
 
 	/*

arch/x86/kernel/process_64.c

@@ -658,8 +658,11 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
 	/* If the task has used fpu the last 5 timeslices, just do a full
 	 * restore of the math state immediately to avoid the trap; the
 	 * chances of needing FPU soon are obviously high now
+	 *
+	 * tsk_used_math() checks prevent calling math_state_restore(),
+	 * which can sleep in the case of !tsk_used_math()
 	 */
-	if (next_p->fpu_counter>5)
+	if (tsk_used_math(next_p) && next_p->fpu_counter > 5)
 		math_state_restore();
 	return prev_p;
 }