From: Chris Wilson <chris@chris-wilson.co.uk>
To: Matthew Wilcox <willy@infradead.org>,
dri-devel@lists.freedesktop.org, linux-mm@kvack.org,
"Souptick Joarder" <jrdr.linux@gmail.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: Signal handling in a page fault handler
Date: Tue, 03 Apr 2018 13:33:15 +0100 [thread overview]
Message-ID: <152275879566.32747.9293394837417347482@mail.alporthouse.com> (raw)
In-Reply-To: <20180402141058.GL13332@bombadil.infradead.org>
Quoting Matthew Wilcox (2018-04-02 15:10:58)
>
> Souptick and I have been auditing the various page fault handler routines
> and we've noticed that graphics drivers assume that a signal should be
> able to interrupt a page fault. In contrast, the page cache takes great
> care to allow only fatal signals to interrupt a page fault.
>
> I believe (but have not verified) that a non-fatal signal being delivered
> to a task which is in the middle of a page fault may well end up in an
> infinite loop, attempting to handle the page fault and failing forever.
>
> Here's one of the simpler ones:
>
> ret = mutex_lock_interruptible(&etnaviv_obj->lock);
> if (ret)
> return VM_FAULT_NOPAGE;
>
> (many other drivers do essentially the same thing including i915)
>
> On seeing NOPAGE, the fault handler believes the PTE is in the page
> table, so does nothing before it returns to arch code at which point
> I get lost in the magic assembler macros. I believe it will end up
> returning to userspace if the signal is non-fatal, at which point it'll
> go right back into the page fault handler, and mutex_lock_interruptible()
> will immediately fail. So we've converted a sleeping lock into the most
> expensive spinlock.
I'll ask the obvious question: why isn't the signal handled on return to
userspace?
> I don't think the graphics drivers really want to be interrupted by
> any signal.
Assume the worst case and we may block for 10s. Even a 10ms delay may be
unacceptable to some signal handlers (one presumes). For the number one
^C usecase, yes that may be reduced to only bother if it's killable, but
I wonder if there are not timing loops (e.g. sigitimer in Xorg < 1.19)
that want to be able to interrupt random blockages.
-Chris
next prev parent reply other threads:[~2018-04-03 12:33 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-04-02 14:10 Signal handling in a page fault handler Matthew Wilcox
2018-04-03 12:33 ` Chris Wilson [this message]
2018-04-03 13:10 ` Matthew Wilcox
[not found] ` <152276164305.32747.4969221700358143640@mail.alporthouse.com>
2018-04-03 13:48 ` Matthew Wilcox
2018-04-03 13:12 ` Thomas Hellstrom
2018-04-03 14:48 ` Matthew Wilcox
2018-04-03 15:12 ` Daniel Vetter
2018-04-04 9:32 ` Daniel Vetter
2018-04-04 14:39 ` Matthew Wilcox
2018-04-04 15:15 ` Daniel Vetter
2018-04-04 16:24 ` Matthew Wilcox
2018-04-04 17:45 ` Daniel Vetter
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