On Wed, Mar 11, 2015 at 06:34:09PM +0100, Oleg Nesterov wrote: > Currently user_fpu_begin() has a single caller and it is not clear that > why do we actually need it, and why we should not worry about preemption > right after preempt_enable(). > > Signed-off-by: Oleg Nesterov <[email protected]> > --- > arch/x86/include/asm/fpu-internal.h | 4 +++- > 1 files changed, 3 insertions(+), 1 deletions(-) > > diff --git a/arch/x86/include/asm/fpu-internal.h > b/arch/x86/include/asm/fpu-internal.h > index 4bec98f..c615ae9 100644 > --- a/arch/x86/include/asm/fpu-internal.h > +++ b/arch/x86/include/asm/fpu-internal.h > @@ -464,7 +464,9 @@ static inline int restore_xstate_sig(void __user *buf, > int ia32_frame) > * Need 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. > + * it. This function does not do any save/restore on its own. In a lazy > + * fpu mode this is just optimization to avoid a dna fault, the task can > + * lose FPU right after preempt_enable(). > */
I cleaned it up a bit more, if you don't mind: --- From: Oleg Nesterov <[email protected]> Date: Wed, 11 Mar 2015 18:34:09 +0100 Subject: [PATCH] x86/fpu: Document user_fpu_begin() 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 <[email protected]> Cc: Andy Lutomirski <[email protected]> Cc: Linus Torvalds <[email protected]> Cc: Pekka Riikonen <[email protected]> Cc: Rik van Riel <[email protected]> Cc: Suresh Siddha <[email protected]> Cc: Fenghua Yu <[email protected]> Cc: Quentin Casasnovas <[email protected]> Cc: Dave Hansen <[email protected]> Cc: Ingo Molnar <[email protected]> Link: http://lkml.kernel.org/r/[email protected] Signed-off-by: --- arch/x86/include/asm/fpu-internal.h | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/arch/x86/include/asm/fpu-internal.h b/arch/x86/include/asm/fpu-internal.h index 810f20fd4e4e..e8ee3da3b924 100644 --- 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) { -- -- Regards/Gruss, Boris. ECO tip #101: Trim your mails when you reply. -- -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
