Hi Vineet,
On Wed, 2018-08-01 at 12:49 -0700, Vineet Gupta wrote:
> Hi Alexey,
>
> I was finally forced to revisit this for my glibc tst-tls3-malloc deadlock. And
> indeed with this change we don'tsee the deadlock. But see below..
>
>
> > @@ -139,12 +139,16 @@ void do_page_fault(unsigned long address, struct pt_regs *regs)
> > */
> > fault = handle_mm_fault(vma, address, flags);
> >
> > - /* If Pagefault was interrupted by SIGKILL, exit page fault "early" */
> > + /* If we need to retry but a fatal signal is pending, handle the
> > + * signal first. We do not need to release the mmap_sem because
> > + * it would already be released in __lock_page_or_retry in
> > + * mm/filemap.c. */
>
> Right and we were already doing that: up_read() was called for !VM_FAULT_RETRY
> meaning we relied on the core mm to do that already for VM_FAULT_RETRY case.
>
> The issue here was additional check for VM_FAULT_ERROR. Typically this is not set
> by handle_mm_fault() meaning for common user faults with signal pending, we were
> not calling up_read, hence the ensuing deadlock.
Right.
> > if (unlikely(fatal_signal_pending(current))) {
> > - if ((fault & VM_FAULT_ERROR) && !(fault & VM_FAULT_RETRY))
> > - up_read(&mm->mmap_sem);
> > - if (user_mode(regs))
> > + if (fault & VM_FAULT_RETRY) {
> > + if (!user_mode(regs))
> > + goto no_context;
>
> Given this code is really tricky, lets only solve one problem with 1 one patch.
Agree.
> > return;
> > + }
> > }
>
> The fault handling is spaghetti mess of checks and more checks and has not really
> been touched since upstreaming. I need to clean it up and essentially rewrite it
> for v4.19
So would you like me to send a re-spin with less changes as discussed above so
we have something better for now and for back-porting to stable branches.
Or you're going to rewrite all that sometime soon yourself?
-Alexey
