From: Peter Zijlstra <peterz@infradead.org>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@kernel.org>,
Will Deacon <will@kernel.org>,
"Paul E . McKenney" <paulmck@kernel.org>,
Joel Fernandes <joel@joelfernandes.org>,
Steven Rostedt <rostedt@goodmis.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Thomas Gleixner <tglx@linutronix.de>,
Kurt Schwemmer <kurt.schwemmer@microsemi.com>,
Logan Gunthorpe <logang@deltatee.com>,
Bjorn Helgaas <bhelgaas@google.com>,
linux-pci@vger.kernel.org
Subject: Re: [PATCH 3/9] pci/switchtec: Don't abuse completion wait queue for poll
Date: Fri, 13 Mar 2020 20:31:32 +0100 [thread overview]
Message-ID: <20200313193132.GE12521@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20200313174701.148376-4-bigeasy@linutronix.de>
On Fri, Mar 13, 2020 at 06:46:55PM +0100, Sebastian Andrzej Siewior wrote:
> The poll callback is abusing the completion wait queue and sticks it into
> poll_wait() to wake up pollers after a command has completed.
>
> First of all it's a layering violation as it imposes restrictions on the
> inner workings of completions. Just because C allows to do so does not
> justify that in any way. The proper way to do such things is to post
> patches which extend the core infrastructure and not by silently abusing
> it.
>
> Aside of that the implementation is seriously broken:
>
> 1) It cannot work with EPOLLEXCLUSIVE
>
> 2) It's racy:
>
> poll() write()
> switchtec_dev_poll() switchtec_dev_write()
> poll_wait(&s->comp.wait); mrpc_queue_cmd()
> init_completion(&s->comp)
> init_waitqueue_head(&s->comp.wait)
>
> Replace it with a regular wait queue which removes the completion abuse and
> cures #1 and #2 above.
>
> Cc: Kurt Schwemmer <kurt.schwemmer@microsemi.com>
> Cc: Logan Gunthorpe <logang@deltatee.com>
> Cc: Bjorn Helgaas <bhelgaas@google.com>
> Cc: linux-pci@vger.kernel.org
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Relying on implementation details of locking primitives like that is
yuck.
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
next prev parent reply other threads:[~2020-03-13 19:31 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-13 17:46 [PATCH 0/9] Lock ordering documentation and annotation for lockdep Sebastian Andrzej Siewior
2020-03-13 17:46 ` [PATCH 1/9] Documentation: Add lock ordering and nesting documentation Sebastian Andrzej Siewior
2020-03-14 22:57 ` Randy Dunlap
2020-03-16 10:34 ` Sebastian Andrzej Siewior
2020-03-16 15:13 ` Randy Dunlap
2020-03-13 17:46 ` [PATCH 2/9] timekeeping: Split jiffies seqlock Sebastian Andrzej Siewior
2020-03-13 17:46 ` [PATCH 3/9] pci/switchtec: Don't abuse completion wait queue for poll Sebastian Andrzej Siewior
2020-03-13 18:11 ` Logan Gunthorpe
2020-03-14 0:23 ` Thomas Gleixner
2020-03-14 6:01 ` Logan Gunthorpe
2020-03-16 18:52 ` Thomas Gleixner
2020-03-16 19:24 ` Logan Gunthorpe
2020-03-16 19:34 ` Thomas Gleixner
2020-03-16 21:53 ` Logan Gunthorpe
2020-03-17 0:17 ` Thomas Gleixner
2020-03-17 1:15 ` Logan Gunthorpe
2020-03-13 19:31 ` Peter Zijlstra [this message]
2020-03-13 17:46 ` [PATCH 4/9] sched/swait: Prepare usage in completions Sebastian Andrzej Siewior
2020-03-14 0:26 ` Thomas Gleixner
2020-03-13 17:46 ` [PATCH 5/9] completion: Use simple wait queues Sebastian Andrzej Siewior
2020-03-14 15:40 ` Linus Torvalds
2020-03-16 18:55 ` Thomas Gleixner
2020-03-13 17:46 ` [PATCH 6/9] lockdep: Introduce wait-type checks Sebastian Andrzej Siewior
2020-03-31 13:25 ` Geert Uytterhoeven
2020-03-31 13:42 ` Sebastian Andrzej Siewior
2020-03-31 14:55 ` Peter Zijlstra
2020-03-31 15:28 ` Peter Zijlstra
2020-03-31 15:37 ` Frederic Weisbecker
2020-03-13 17:46 ` [PATCH 7/9] lockdep: Add hrtimer context tracing bits Sebastian Andrzej Siewior
2020-03-13 17:47 ` [PATCH 8/9] lockdep: Annotate irq_work Sebastian Andrzej Siewior
2020-03-13 17:47 ` [PATCH 9/9] lockdep: Add posixtimer context tracing bits Sebastian Andrzej Siewior
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