Re: [Qestion] Softlockup when send IPI to other CPUs

[Catalin Marinas <catalin.marinas@arm.com>]

On Tue, Jan 22, 2019 at 05:44:02AM +0000, Will Deacon wrote:
> On Mon, Jan 21, 2019 at 02:21:28PM +0000, Catalin Marinas wrote:
> > arm64: Do not issue IPIs for user executable ptes
> > 
> > From: Catalin Marinas <catalin.marinas@arm.com>
> > 
> > Commit 3b8c9f1cdfc5 ("arm64: IPI each CPU after invalidating the I-cache
> > for kernel mappings") was aimed at fixing the I-cache invalidation for
> > kernel mappings. However, it inadvertently caused all cache maintenance
> > for user mappings via set_pte_at() -> __sync_icache_dcache() to call
> > kick_all_cpus_sync().
> > 
> > Fixes: 3b8c9f1cdfc5 ("arm64: IPI each CPU after invalidating the I-cache for kernel mappings")
> > Cc: <stable@vger.kernel.org> # 4.19.x-
> > Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> > ---
> >  arch/arm64/include/asm/cacheflush.h |    2 +-
> >  arch/arm64/kernel/probes/uprobes.c  |    2 +-
> >  arch/arm64/mm/flush.c               |   14 ++++++++++----
> >  3 files changed, 12 insertions(+), 6 deletions(-)
> > 
> > diff --git a/arch/arm64/include/asm/cacheflush.h b/arch/arm64/include/asm/cacheflush.h
> > index 19844211a4e6..18e92d9dacd4 100644
> > --- a/arch/arm64/include/asm/cacheflush.h
> > +++ b/arch/arm64/include/asm/cacheflush.h
> > @@ -80,7 +80,7 @@ extern void __clean_dcache_area_poc(void *addr, size_t len);
> >  extern void __clean_dcache_area_pop(void *addr, size_t len);
> >  extern void __clean_dcache_area_pou(void *addr, size_t len);
> >  extern long __flush_cache_user_range(unsigned long start, unsigned long end);
> > -extern void sync_icache_aliases(void *kaddr, unsigned long len);
> > +extern void sync_icache_aliases(void *kaddr, unsigned long len, bool sync);
> 
> I'd much prefer just adding something like sync_user_icache_aliases(), which
> would avoid the IPI, since bool arguments tend to make the callsites
> unreadable imo.

I wonder whether we need the IPI for uprobes and ptrace. I would say we
can avoid it for ptrace since normally the debugged thread should be
stopped. I think it's different for uprobes but changing the text of a
live thread doesn't come with many guarantees anyway. So I'm tempted to
simply change sync_icache_aliases() to not issue an IPI at all, in which
case we wouldn't even need the bool argument; it's only used for user
addresses. So the new diff would be (the text is the same):

diff --git a/arch/arm64/mm/flush.c b/arch/arm64/mm/flush.c
index 30695a868107..5c9073bace83 100644
--- a/arch/arm64/mm/flush.c
+++ b/arch/arm64/mm/flush.c
@@ -33,7 +33,11 @@ void sync_icache_aliases(void *kaddr, unsigned long len)
 		__clean_dcache_area_pou(kaddr, len);
 		__flush_icache_all();
 	} else {
-		flush_icache_range(addr, addr + len);
+		/*
+		 * Don't issue kick_all_cpus_sync() after I-cache invalidation
+		 * for user mappings.
+		 */
+		__flush_icache_range(addr, addr + len);
 	}
 }