x86/fpu: Document user_fpu_begin()

oleg-nesterov · Ingo Molnar · commit fb14b4eadf73 · 2015-03-23T10:13:58.000+01:00
Currently, user_fpu_begin() has a single caller and it is not clear why do we actually need it and why we should not worry about preemption right after preempt_enable(). 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/20150311173409.GC5032@redhat.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/arch/x86/include/asm/fpu-internal.h b/arch/x86/include/asm/fpu-internal.h
@@ -508,10 +508,12 @@ static inline int restore_xstate_sig(void __user *buf, int ia32_frame)
 }
 
 /*
- * Need to be preemption-safe.
+ * Needs to be preemption-safe.
  *
  * NOTE! user_fpu_begin() must be used only immediately before restoring
- * it. This function does not do any save/restore on their own.
+ * the save state. It does not do any saving/restoring on its own. In
+ * lazy FPU mode, it is just an optimization to avoid a #NM exception,
+ * the task can lose the FPU right after preempt_enable().
  */
 static inline void user_fpu_begin(void)
 {

Original file line number	Diff line number	Diff line change
`@@ -508,10 +508,12 @@ static inline int restore_xstate_sig(void __user *buf, int ia32_frame)`
`508`	`508`	`}`
`509`	`509`
`510`	`510`	`/*`
`511`		`- * Need to be preemption-safe.`
	`511`	`+ * Needs to be preemption-safe.`
`512`	`512`	`*`
`513`	`513`	`* NOTE! user_fpu_begin() must be used only immediately before restoring`
`514`		`- * it. This function does not do any save/restore on their own.`
	`514`	`+ * the save state. It does not do any saving/restoring on its own. In`
	`515`	`+ * lazy FPU mode, it is just an optimization to avoid a #NM exception,`
	`516`	`+ * the task can lose the FPU right after preempt_enable().`
`515`	`517`	`*/`
`516`	`518`	`static inline void user_fpu_begin(void)`
`517`	`519`	`{`