linux-kernel - Re: [PATCH] ARC: Improve handling of fatal signals in do_page

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Date:   Thu, 2 Aug 2018 07:08:49 +0000
From:   Alexey Brodkin <Alexey.Brodkin@...opsys.com>
To:     Vineet Gupta <Vineet.Gupta1@...opsys.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>
Subject: Re: [PATCH] ARC: Improve handling of fatal signals in
 do_page_fault()

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