Messages in this thread | | | From | David Laight <> | Subject | RE: [PATCH] ntp: use memset and offsetof init | Date | Mon, 8 Feb 2021 09:28:25 +0000 |
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From: Thomas Gleixner > Sent: 05 February 2021 17:34 > > On Wed, Jan 20 2021 at 10:51, Yejune Deng wrote: > > In pps_fill_timex(), use memset and offsetof instead of '= 0'. > > > > Signed-off-by: Yejune Deng <yejune.deng@gmail.com> > > --- > > kernel/time/ntp.c | 13 +++++-------- > > 1 file changed, 5 insertions(+), 8 deletions(-) > > > > diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c > > index 87389b9e21ab..3416c0381104 100644 > > --- a/kernel/time/ntp.c > > +++ b/kernel/time/ntp.c > > @@ -225,14 +225,11 @@ static inline int is_error_status(int status) > > static inline void pps_fill_timex(struct __kernel_timex *txc) > > { > > /* PPS is not implemented, so these are zero */ > > - txc->ppsfreq = 0; > > - txc->jitter = 0; > > - txc->shift = 0; > > - txc->stabil = 0; > > - txc->jitcnt = 0; > > - txc->calcnt = 0; > > - txc->errcnt = 0; > > - txc->stbcnt = 0; > > + int offset, len; > > + > > + offset = offsetof(struct __kernel_timex, ppsfreq); > > + len = offsetof(struct __kernel_timex, tai) - offset; > > + memset(txc + offset, 0, len); > > That zeros bytes at a memory location which is > > (offset) * sizeof(struct __kernel_timex) > > bytes away from txc. How did this every boot? > > And no, even if you fix that pointer math problem then this kind of > calculation from the middle of a struct is error prone.
It is also, at best, a code size optimisation. If memset() is actually called (not inlined) then you get a whole lot of tests against the size and alignment before any writes of zero happen - which will be the same ones as in the inline code.
It can be worth using memcpy to copy part of a structure but usually for one with lots of small fields (especially bitfields).
David
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