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

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Date:   Wed, 1 Aug 2018 12:49:38 -0700
From:   Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:     Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
        <linux-snps-arc@...ts.infradead.org>
CC:     <linux-kernel@...r.kernel.org>,
        Vineet Gupta <Vineet.Gupta1@...opsys.com>
Subject: Re: [PATCH] ARC: Improve handling of fatal signals in do_page_fault()

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.

>  	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.

>  			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