| Message ID | 1507573730-8083-12-git-send-email-mark.rutland@arm.com |
|---|---|
| State | New |
| Headers | show |
| Series | Preparatory work to kill off ACCESS_ONCE() | expand |
Mark Rutland <mark.rutland@arm.com> writes: > For several reasons, it is desirable to use {READ,WRITE}_ONCE() in > preference to ACCESS_ONCE(), and new code is expected to use one of the > former. So far, there's been no reason to change most existing uses of > ACCESS_ONCE(), as these aren't currently harmful. > > However, for some features it is necessary to instrument reads and > writes separately, which is not possible with ACCESS_ONCE(). This > distinction is critical to correct operation. > > The bulk of the kernel code can be transformed via Coccinelle to use > {READ,WRITE}_ONCE(), though this only modifies users of ACCESS_ONCE(), > and not the implementation itself. As such, it has the potential to > break homebrew ACCESS_ONCE() macros seen in some user code in the kernel > tree (e.g. the virtio code, as fixed in commit ea9156fb3b71d9f7). > > To avoid fragility if/when that transformation occurs, and to align with > the preferred usage of {READ,WRITE}_ONCE(), this patch updates the DSCR > selftest code to use READ_ONCE() rather than ACCESS_ONCE(). There should > be no functional change as a result of this patch. > > Signed-off-by: Mark Rutland <mark.rutland@arm.com> > Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> > Cc: Michael Ellerman <mpe@ellerman.id.au> > Cc: Paul Mackerras <paulus@samba.org> > Cc: Shuah Khan <shuah@kernel.org> > --- > tools/testing/selftests/powerpc/dscr/dscr.h | 2 +- > tools/testing/selftests/powerpc/dscr/dscr_default_test.c | 2 +- > 2 files changed, 2 insertions(+), 2 deletions(-) Acked-by: Michael Ellerman <mpe@ellerman.id.au> cheers
On Tue, Oct 10, 2017 at 11:45:04AM +1100, Michael Ellerman wrote: > Mark Rutland <mark.rutland@arm.com> writes: > > > For several reasons, it is desirable to use {READ,WRITE}_ONCE() in > > preference to ACCESS_ONCE(), and new code is expected to use one of the > > former. So far, there's been no reason to change most existing uses of > > ACCESS_ONCE(), as these aren't currently harmful. > > > > However, for some features it is necessary to instrument reads and > > writes separately, which is not possible with ACCESS_ONCE(). This > > distinction is critical to correct operation. > > > > The bulk of the kernel code can be transformed via Coccinelle to use > > {READ,WRITE}_ONCE(), though this only modifies users of ACCESS_ONCE(), > > and not the implementation itself. As such, it has the potential to > > break homebrew ACCESS_ONCE() macros seen in some user code in the kernel > > tree (e.g. the virtio code, as fixed in commit ea9156fb3b71d9f7). > > > > To avoid fragility if/when that transformation occurs, and to align with > > the preferred usage of {READ,WRITE}_ONCE(), this patch updates the DSCR > > selftest code to use READ_ONCE() rather than ACCESS_ONCE(). There should > > be no functional change as a result of this patch. > > > > Signed-off-by: Mark Rutland <mark.rutland@arm.com> > > Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> > > Cc: Michael Ellerman <mpe@ellerman.id.au> > > Cc: Paul Mackerras <paulus@samba.org> > > Cc: Shuah Khan <shuah@kernel.org> > > --- > > tools/testing/selftests/powerpc/dscr/dscr.h | 2 +- > > tools/testing/selftests/powerpc/dscr/dscr_default_test.c | 2 +- > > 2 files changed, 2 insertions(+), 2 deletions(-) > > Acked-by: Michael Ellerman <mpe@ellerman.id.au> Thanks! Mark.
diff --git a/tools/testing/selftests/powerpc/dscr/dscr.h b/tools/testing/selftests/powerpc/dscr/dscr.h index 18ea223b..cdb840b 100644 --- a/tools/testing/selftests/powerpc/dscr/dscr.h +++ b/tools/testing/selftests/powerpc/dscr/dscr.h @@ -39,7 +39,7 @@ #define rmb() asm volatile("lwsync":::"memory") #define wmb() asm volatile("lwsync":::"memory") -#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x)) +#define READ_ONCE(x) (*(volatile typeof(x) *)&(x)) /* Prilvilege state DSCR access */ inline unsigned long get_dscr(void) diff --git a/tools/testing/selftests/powerpc/dscr/dscr_default_test.c b/tools/testing/selftests/powerpc/dscr/dscr_default_test.c index df17c3b..9e1a37e 100644 --- a/tools/testing/selftests/powerpc/dscr/dscr_default_test.c +++ b/tools/testing/selftests/powerpc/dscr/dscr_default_test.c @@ -27,7 +27,7 @@ static void *do_test(void *in) unsigned long d, cur_dscr, cur_dscr_usr; unsigned long s1, s2; - s1 = ACCESS_ONCE(sequence); + s1 = READ_ONCE(sequence); if (s1 & 1) continue; rmb();
For several reasons, it is desirable to use {READ,WRITE}_ONCE() in preference to ACCESS_ONCE(), and new code is expected to use one of the former. So far, there's been no reason to change most existing uses of ACCESS_ONCE(), as these aren't currently harmful. However, for some features it is necessary to instrument reads and writes separately, which is not possible with ACCESS_ONCE(). This distinction is critical to correct operation. The bulk of the kernel code can be transformed via Coccinelle to use {READ,WRITE}_ONCE(), though this only modifies users of ACCESS_ONCE(), and not the implementation itself. As such, it has the potential to break homebrew ACCESS_ONCE() macros seen in some user code in the kernel tree (e.g. the virtio code, as fixed in commit ea9156fb3b71d9f7). To avoid fragility if/when that transformation occurs, and to align with the preferred usage of {READ,WRITE}_ONCE(), this patch updates the DSCR selftest code to use READ_ONCE() rather than ACCESS_ONCE(). There should be no functional change as a result of this patch. Signed-off-by: Mark Rutland <mark.rutland@arm.com> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> Cc: Michael Ellerman <mpe@ellerman.id.au> Cc: Paul Mackerras <paulus@samba.org> Cc: Shuah Khan <shuah@kernel.org> --- tools/testing/selftests/powerpc/dscr/dscr.h | 2 +- tools/testing/selftests/powerpc/dscr/dscr_default_test.c | 2 +- 2 files changed, 2 insertions(+), 2 deletions(-) -- 1.9.1