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

Catalin Marinas
2019-01-21T14:21:38Z

On Sat, Jan 19, 2019 at 11:58:27PM +0000, Will Deacon wrote:
> On Thu, Jan 17, 2019 at 07:42:44AM +0000, chenwandun wrote:
> > Recently, I do some tests on linux-4.19 and hit a softlockup issue.
> >
> > I find some CPUs get the spinlock in the __split_huge_pmd function and
> > then send IPI to other CPUs, waiting the response, while several CPUs
> > enter the __split_huge_pmd function, want to get the spinlock, but always
> > in queued_spin_lock_slowpath,
> >
> > Because long time no response to the IPI, that results in a softlockup.
> >
> > As to sending IPI, it was in the patch
> > 3b8c9f1cdfc506e94e992ae66b68bbe416f89610. The patch is mean to send IPI
> > to each CPU after invalidating the I-cache for kernel mappings. In this
> > case, after modify pmd, it sends IPI to other CPUS to sync memory
> > mappings.
> >
> > No stable test case to repeat the result, it is hard to repeat the test procedure.
> >
> > The environment is arm64, 64 CPUs. Except for idle CPU, there are 6 kind
> > of callstacks in total.
>
> This looks like another lockup that would be solved if we deferred our
> I-cache invalidation when mapping user-executable pages, and instead
> performed the invalidation off the back of a UXN permission fault, where we
> could avoid holding any locks.

Looking back at commit 3b8c9f1cdfc5 ("arm64: IPI each CPU after
invalidating the I-cache for kernel mappings"), the text implies that it
should only do this for kernel mappings. I don't think we need this for
user mappings. We have a few scenarios where we invoke set_pte_at() with
exec permission:

1. Page faulted in - the pte was not previously accessible and the CPU
should not have stale decoded instructions (my interpretation of the
ARM ARM).

2. huge pmd splitting - there is no change to the underlying data. I
have a suspicion here that we shouldn't even call
sync_icache_aliases() but probably the PG_arch_1 isn't carried over
from the head compound page to the small pages (needs checking).

3. page migration - there is no change to the underlying data and
instructions, so I don't think we need the all CPUs sync.

4. mprotect() setting exec permission - that's normally the
responsibility of the user to ensure cache maintenance

(I can add more text to the patch below but need to get to the bottom of
this first)

---------8<-------------------------------------------------
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);

static inline void flush_icache_range(unsigned long start, unsigned long end)
{
diff --git a/arch/arm64/kernel/probes/uprobes.c b/arch/arm64/kernel/probes/uprobes.c
index 636ca0119c0e..595e8c8f41cd 100644
--- a/arch/arm64/kernel/probes/uprobes.c
+++ b/arch/arm64/kernel/probes/uprobes.c
@@ -24,7 +24,7 @@ void arch_uprobe_copy_ixol(struct page *page, unsigned long vaddr,
memcpy(dst, src, len);

/* flush caches (dcache/icache) */
- sync_icache_aliases(dst, len);
+ sync_icache_aliases(dst, len, true);

kunmap_atomic(xol_page_kaddr);
}
diff --git a/arch/arm64/mm/flush.c b/arch/arm64/mm/flush.c
index 30695a868107..5c2f23a92d14 100644
--- a/arch/arm64/mm/flush.c
+++ b/arch/arm64/mm/flush.c
@@ -25,15 +25,17 @@
#include <asm/cache.h>
#include <asm/tlbflush.h>

-void sync_icache_aliases(void *kaddr, unsigned long len)
+void sync_icache_aliases(void *kaddr, unsigned long len, bool sync)
{
unsigned long addr = (unsigned long)kaddr;

if (icache_is_aliasing()) {
__clean_dcache_area_pou(kaddr, len);
__flush_icache_all();
- } else {
+ } else if (sync) {
flush_icache_range(addr, addr + len);
+ } else {
+ __flush_icache_range(addr, addr + len);
}
}

@@ -42,7 +44,7 @@ static void flush_ptrace_access(struct vm_area_struct *vma, struct page *page,
unsigned long len)
{
if (vma->vm_flags & VM_EXEC)
- sync_icache_aliases(kaddr, len);
+ sync_icache_aliases(kaddr, len, true);
}

/*
@@ -63,8 +65,12 @@ void __sync_icache_dcache(pte_t pte)
struct page *page = pte_page(pte);

if (!test_and_set_bit(PG_dcache_clean, &page->flags))
+ /*
+ * Don't issue kick_all_cpus_sync() after I-cache invalidation
+ * when setting a user executable pte.
+ */
sync_icache_aliases(page_address(page),
- PAGE_SIZE << compound_order(page));
+ PAGE_SIZE << compound_order(page), false);
}
EXPORT_SYMBOL_GPL(__sync_icache_dcache);