linux-kernel - ->poll() instances shouldn't be indefinitely blocking

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Message-ID: <20151127050026.GX22011@ZenIV.linux.org.uk>
Date:	Fri, 27 Nov 2015 05:00:26 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Mauro Carvalho Chehab <mchehab@....samsung.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: ->poll() instances shouldn't be indefinitely blocking

Take a look at this:
static unsigned int gsc_m2m_poll(struct file *file,
                                        struct poll_table_struct *wait)
{
        struct gsc_ctx *ctx = fh_to_ctx(file->private_data);
        struct gsc_dev *gsc = ctx->gsc_dev;
        int ret;

        if (mutex_lock_interruptible(&gsc->lock))
                return -ERESTARTSYS;

        ret = v4l2_m2m_poll(file, ctx->m2m_ctx, wait);
        mutex_unlock(&gsc->lock);

        return ret;
}

a) ->poll() should not return -E...; callers expect just a bitmap of
POLL... values.

b) sure, it's nice that if this thing hangs, we'll be able to kill it.
However, if one's ->poll() can hang indefinitely, it means bad things
for poll(2), select(2), etc. semantics.  What the hell had been intended
there?

c) a bunch of v4l2_m2m_poll() callers are also taking some kind of
mutex; AFAICS, all of those appear bogus (the rest of them do not
play wiht ERESTARTSYS, just plain mutex_lock() for those).

What's going on there?
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