linux-kernel - Re: [PATCH] sys_ioprio

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Date:	Tue, 22 Aug 2006 21:57:04 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Andrew Morton <akpm@...l.org>
Cc:	Jens Axboe <axboe@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sys_ioprio_set: don't disable irqs

On 08/21, Andrew Morton wrote:
>
> On Mon, 21 Aug 2006 00:50:34 +0400
> Oleg Nesterov <oleg@...sign.ru> wrote:
>
> > Question: why do we need to disable irqs in exit_io_context() ?
>
> iirc it was to prevent IRQ-context code from getting a hold on
> current->io_context and then playing around with it while it's getting
> freed.
>
> In practice, a preempt_disable() there would probably suffice (ie: if this
> CPU is running an ISR, it won't be running exit_io_context as well).  But
> local_irq_disable() is clearer, albeit more expensive.

Looks like my understanding of block I/O is even less than nothing :(

irq_disable() can't prevent from IRQ-context code playing with our io_context
on other CPUs. But this doesn't matter, we are only changing ioc->task.

What does matter, we are clearing the pointer to it: task_struct->io_context,
and IRQ should not look at it, no?

Or... Do you mean it is possible to submit I/O from IRQ on behalf of current ?????
In that case current_io_context() will re-instantiate ->io_context after irq_enable().
What is exit_io_context() for then? It is only called from do_exit() when we know
the task won't start IO.

(please don't beat a newbie)

> > Why do we need ->alloc_lock to clear io_context->task ?
>
> To prevent races against elv_unregister(), I guess.

elv_unregister() takes task_lock(), should see ->io_context == NULL.

Oleg.

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