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Date:   Thu, 5 Apr 2018 12:57:56 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, stable@...r.kernel.org,
        linux-kernel@...r.kernel.org, Boqun Feng <boqun.feng@...il.com>
Subject: Re: [PATCH] locking/qrwlock: Give priority to readers with irqs
 disabled to prevent deadlock

On 04.04.2018 19:25, Waiman Long wrote:
> On 04/04/2018 11:55 AM, Kirill Tkhai wrote:
>> On 04.04.2018 18:51, Kirill Tkhai wrote:
>>> On 04.04.2018 18:35, Peter Zijlstra wrote:
>>>> On Wed, Apr 04, 2018 at 06:24:39PM +0300, Kirill Tkhai wrote:
>>>>> The following situation leads to deadlock:
>>>>>
>>>>> [task 1]                          [task 2]                         [task 3]
>>>>> kill_fasync()                     mm_update_next_owner()           copy_process()
>>>>>  spin_lock_irqsave(&fa->fa_lock)   read_lock(&tasklist_lock)        write_lock_irq(&tasklist_lock)
>>>>>   send_sigio()                    <IRQ>                             ...
>>>>>    read_lock(&fown->lock)         kill_fasync()                     ...
>>>>>     read_lock(&tasklist_lock)      spin_lock_irqsave(&fa->fa_lock)  ...
>>>>>
>>>>> Task 1 can't acquire read locked tasklist_lock, since there is
>>>>> already task 3 expressed its wish to take the lock exclusive.
>>>>> Task 2 holds the read locked lock, but it can't take the spin lock.
>>>>>
>>>>> The patch makes queued_read_lock_slowpath() to give task 1 the same
>>>>> priority as it was an interrupt handler, and to take the lock
>>>> That re-introduces starvation scenarios. And the above looks like a
>>>> proper deadlock that should be sorted by fixing the locking order.
>>> We can move tasklist_lock out of send_sigio(), but I'm not sure
>>> it's possible for read_lock(&fown->lock).
>>>
>>> Is there another solution? Is there reliable way to iterate do_each_pid_task()
>>> with rcu_read_lock()?
>> In case of &fown->lock we may always disable irqs for all the places, where it's
>> taken for read, i.e. read_lock_irqsave(&fown->lock). This seems to fix the problem
>> for this lock.
> 
> One possible solution is add a flag in send_sigio() to use a
> read_trylock(&tasklist_lock) instead of read_lock(). If the trylock
> fails, returns an error and have the caller (kill_fasync) release
> fa->fa_lock and retry again. Task 1 has 3 levels of nested locking and
> so it should be the one that does a retry if the innermost locking
> fails. An warning can be printed if the retry count is too large.

send_sigio() is called from several places, and the context, which calls
it, in general is unknown. In case of dnotify_handle_event(), which calls
it under spinlock, trylock will act as busy loop. This may block some
smp/stop machine primitives, and I'm not sure, this can't bring to some
currently unvisible deadlocks.

There is a solution, I'm analysing in the moment, when we can convert
fasync_struct::fa_lock to read/write lock, then we can take it from
interrupt without problems.

Kirill

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