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Date:   Sun, 22 Jul 2018 14:49:58 +0100
From:   Al Viro <viro@ZenIV.linux.org.uk>
To:     Richard Weinberger <richard.weinberger@gmail.com>
Cc:     Theuns Verwoerd <Theuns.Verwoerd@alliedtelesis.co.nz>,
        "linux-mtd@lists.infradead.org" <linux-mtd@lists.infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        David Woodhouse <dwmw2@infradead.org>
Subject: Re: [PATCH 2/2] jffs2: Provide jffs2_sync files to track gc POLL
 progress
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On Sun, Jul 22, 2018 at 03:17:07PM +0200, Richard Weinberger wrote:

> On Fri, Jul 20, 2018 at 2:54 AM, Theuns Verwoerd
> <Theuns.Verwoerd@alliedtelesis.co.nz> wrote:
> >
> > On 07/20/2018 12:04 PM, Al Viro wrote:
> >> On Fri, Jul 20, 2018 at 11:50:12AM +1200, Theuns Verwoerd wrote:
> >>
> >>> +ssize_t jffs2_sync_file_read(struct file *f,
> >>> +          char __user *b, size_t len, loff_t *ofs)
> >>> +{
> >>> +    struct jffs2_sb_info *c = file_inode(f)->i_private;
> >>> +
> >>> +    while (c->tidemark)
> >>> +            schedule();
> >>> +
> >>> +    return 0;
> >>> +}
> >> Brilliant.  So when that gets called with c->tidemark being true and
> >> need_resched() - false, we shall...
> > By my reading, schedule() will happily force a reschedule (once) when
> > need_resched() is false.  Every time the process is granted a time
> > slice, it'll check the condition and surrender if not ready.

Except that the task remains runnable (and not using its timeslice up).
It won't sleep; there's nothing to drive it into TASK_NOT_RUNNING
state.

Try it and watch what's going on when you trigger that loop.  That's
really not a good way to wait for something to happen.