| Message ID | 20240620175703.605111-34-yury.norov@gmail.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | lib/find: add atomic find_bit() primitives | expand |
Hi Yury, thanks for your patch! On Thu, 2024-06-20 at 10:56 -0700, Yury Norov wrote: > Simplify ilsel_enable() by using find_and_set_bit(). > > Geert also pointed the bug in the old implementation: > > I don't think the old code worked as intended: the first time > no free bit is found, bit would have been ILSEL_LEVELS, and > test_and_set_bit() would have returned false, thus terminating > the loop, and continuing with an out-of-range bit value? Hence > to work correctly, bit ILSEL_LEVELS of ilsel_level_map should > have been initialized to one? Or am I missing something? > > The new code does not have that issue. > > CC: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de> > Signed-off-by: Yury Norov <yury.norov@gmail.com> > Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be> > --- > arch/sh/boards/mach-x3proto/ilsel.c | 5 +++-- > 1 file changed, 3 insertions(+), 2 deletions(-) > > diff --git a/arch/sh/boards/mach-x3proto/ilsel.c b/arch/sh/boards/mach-x3proto/ilsel.c > index f0d5eb41521a..35b585e154f0 100644 > --- a/arch/sh/boards/mach-x3proto/ilsel.c > +++ b/arch/sh/boards/mach-x3proto/ilsel.c > @@ -8,6 +8,7 @@ > */ > #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt > > +#include <linux/find_atomic.h> > #include <linux/init.h> > #include <linux/kernel.h> > #include <linux/module.h> > @@ -99,8 +100,8 @@ int ilsel_enable(ilsel_source_t set) > } > > do { > - bit = find_first_zero_bit(&ilsel_level_map, ILSEL_LEVELS); > - } while (test_and_set_bit(bit, &ilsel_level_map)); > + bit = find_and_set_bit(&ilsel_level_map, ILSEL_LEVELS); > + } while (bit >= ILSEL_LEVELS); > > __ilsel_enable(set, bit); I will need to take a closer look at the whole code in ilsel_enable() to understand what's happening here. If Geert's explanation is correct, it sounds more like you're fixing a bug and saying you're optimizing the function in the patch subject would sound more like an euphemism. Also, I think we should add a Fixes tag if possible in case your patch fixes an actual bug. I will have a closer look over the weekend. Adrian
On Fri, Jun 21, 2024 at 10:48:44AM +0200, John Paul Adrian Glaubitz wrote: > Hi Yury, > > thanks for your patch! > > On Thu, 2024-06-20 at 10:56 -0700, Yury Norov wrote: > > Simplify ilsel_enable() by using find_and_set_bit(). > > > > Geert also pointed the bug in the old implementation: > > > > I don't think the old code worked as intended: the first time > > no free bit is found, bit would have been ILSEL_LEVELS, and > > test_and_set_bit() would have returned false, thus terminating > > the loop, and continuing with an out-of-range bit value? Hence > > to work correctly, bit ILSEL_LEVELS of ilsel_level_map should > > have been initialized to one? Or am I missing something? > > > > The new code does not have that issue. > > > > CC: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de> > > Signed-off-by: Yury Norov <yury.norov@gmail.com> > > Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be> > > --- > > arch/sh/boards/mach-x3proto/ilsel.c | 5 +++-- > > 1 file changed, 3 insertions(+), 2 deletions(-) > > > > diff --git a/arch/sh/boards/mach-x3proto/ilsel.c b/arch/sh/boards/mach-x3proto/ilsel.c > > index f0d5eb41521a..35b585e154f0 100644 > > --- a/arch/sh/boards/mach-x3proto/ilsel.c > > +++ b/arch/sh/boards/mach-x3proto/ilsel.c > > @@ -8,6 +8,7 @@ > > */ > > #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt > > > > +#include <linux/find_atomic.h> > > #include <linux/init.h> > > #include <linux/kernel.h> > > #include <linux/module.h> > > @@ -99,8 +100,8 @@ int ilsel_enable(ilsel_source_t set) > > } > > > > do { > > - bit = find_first_zero_bit(&ilsel_level_map, ILSEL_LEVELS); > > - } while (test_and_set_bit(bit, &ilsel_level_map)); > > + bit = find_and_set_bit(&ilsel_level_map, ILSEL_LEVELS); > > + } while (bit >= ILSEL_LEVELS); > > > > __ilsel_enable(set, bit); > > I will need to take a closer look at the whole code in ilsel_enable() to understand what's > happening here. If Geert's explanation is correct, it sounds more like you're fixing a bug > and saying you're optimizing the function in the patch subject would sound more like an > euphemism. > > Also, I think we should add a Fixes tag if possible in case your patch fixes an actual bug. > > I will have a closer look over the weekend. Hi John, The problem is that if the ilsel_level_map if dense, the @bit will be set to ILSEL_LEVELS. The following test_and_set_bit() will therefore access a bit beyond the end of bitmap. Which in turn is undef. I'm not familiar to the subsystem as whole, so I can't say if it's ever possible to have the ilsel_level_map all set. If you take a look that would be great. If this series will not move, the fix for this code would be: do { bit = find_first_zero_bit(&ilsel_level_map, ILSEL_LEVELS); } while (bit >= ILSEL_LEVELS || test_and_set_bit(bit, &ilsel_level_map)); It would work, but because find_first_zero_bit() is not designed to work correctly in concurrent environment, it may trigger KCSAN and/or return something non-relevant. See cover letter on this series for details. Thanks, Yury
diff --git a/arch/sh/boards/mach-x3proto/ilsel.c b/arch/sh/boards/mach-x3proto/ilsel.c index f0d5eb41521a..35b585e154f0 100644 --- a/arch/sh/boards/mach-x3proto/ilsel.c +++ b/arch/sh/boards/mach-x3proto/ilsel.c @@ -8,6 +8,7 @@ */ #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt +#include <linux/find_atomic.h> #include <linux/init.h> #include <linux/kernel.h> #include <linux/module.h> @@ -99,8 +100,8 @@ int ilsel_enable(ilsel_source_t set) } do { - bit = find_first_zero_bit(&ilsel_level_map, ILSEL_LEVELS); - } while (test_and_set_bit(bit, &ilsel_level_map)); + bit = find_and_set_bit(&ilsel_level_map, ILSEL_LEVELS); + } while (bit >= ILSEL_LEVELS); __ilsel_enable(set, bit);