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charset="UTF-8" X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Sep 25, 2020 at 1:47 PM Catalin Marinas wrote: > > On Fri, Sep 25, 2020 at 01:26:02PM +0200, Andrey Konovalov wrote: > > On Fri, Sep 25, 2020 at 12:49 PM Catalin Marinas > > wrote: > > > > diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c > > > > index a3bd189602df..d110f382dacf 100644 > > > > --- a/arch/arm64/mm/fault.c > > > > +++ b/arch/arm64/mm/fault.c > > > > @@ -33,6 +33,7 @@ > > > > #include > > > > #include > > > > #include > > > > +#include > > > > #include > > > > #include > > > > #include > > > > @@ -294,6 +295,11 @@ static void die_kernel_fault(const char *msg, unsigned long addr, > > > > do_exit(SIGKILL); > > > > } > > > > > > > > +static void report_tag_fault(unsigned long addr, unsigned int esr, > > > > + struct pt_regs *regs) > > > > +{ > > > > +} > > > > > > Do we need to introduce report_tag_fault() in this patch? It's fine but > > > add a note in the commit log that it will be populated in a subsequent > > > patch. > > > > I did, see the last line of the commit description. > > Sorry, I missed that. No problem! > > > > + > > > > static void __do_kernel_fault(unsigned long addr, unsigned int esr, > > > > struct pt_regs *regs) > > > > { > > > > @@ -641,10 +647,40 @@ static int do_sea(unsigned long addr, unsigned int esr, struct pt_regs *regs) > > > > return 0; > > > > } > > > > > > > > +static void do_tag_recovery(unsigned long addr, unsigned int esr, > > > > + struct pt_regs *regs) > > > > +{ > > > > + static bool reported = false; > > > > + > > > > + if (!READ_ONCE(reported)) { > > > > + report_tag_fault(addr, esr, regs); > > > > + WRITE_ONCE(reported, true); > > > > + } > > > > > > I don't mind the READ_ONCE/WRITE_ONCE here but not sure what they help > > > with. > > > > The fault can happen on multiple cores at the same time, right? In > > that case without READ/WRITE_ONCE() we'll have a data-race here. > > READ/WRITE_ONCE won't magically solve such races. If two CPUs enter > simultaneously in do_tag_recovery(), they'd both read 'reported' as > false and both print the fault info. They won't solve the race condition, but they will solve the data race. I guess here we don't really care about the race condition, as printing a tag fault twice is OK. But having a data race here will lead to KCSAN reports, although won't probably break anything in practice. > If you really care about this race, you need to atomically both read and > update the variable with an xchg() or cmpxchg().