On Wed, Jul 15, 2020 at 02:54:07PM +0800, Leo Yan wrote: > On Wed, Jul 15, 2020 at 07:56:50AM +0200, Ahmed S. Darwish wrote: > > On Wed, Jul 15, 2020 at 10:05:07AM +0800, Leo Yan wrote: > > > From: Peter Zijlstra <pet...@infradead.org> > > > > > ... > > > > > > Provide struct clock_read_data and two (seqcount) helpers so that > > > architectures (arm64 in specific) can expose the numbers to userspace. > > > > > ... > > > > > > +struct clock_read_data *sched_clock_read_begin(unsigned int *seq) > > > +{ > > > + *seq = raw_read_seqcount(&cd.seq); > > > + return cd.read_data + (*seq & 1); > > > +} > > > + > > ... > > > > Hmm, this seqcount_t is actually a latch seqcount. I know the original > > code also used raw_read_seqcount(), but while at it, let's use the > > proper read API for seqcount_t latchers: raw_read_seqcount_latch(). > > Good point. To be honest, I think myself cannot give a good judgement > for memory barrier related thing :) > > I read a bit the document for the latch technique [1], comparing to > raw_read_seqcount_latch(), the function raw_read_seqcount() contains > smp_rmb(), IIUC, the *read* memory barrier is used to support for > kcsan. >
The smp_rmb() has no relation whatsoever to KCSAN. It pairs with the write memory barriers in the seqcount_t write path. AFAIK, PeterZ is the author of this patch, so let's wait for his input here. Thanks, -- Ahmed S. Darwish Linutronix GmbH