linux-kernel - Re: [PATCH-tip v7 01/20] locking/rwsem: Prevent decrement of reader count before increment

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Date:   Fri, 3 May 2019 09:32:35 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, Davidlohr Bueso <dave@...olabs.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        huang ying <huang.ying.caritas@...il.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH-tip v7 01/20] locking/rwsem: Prevent decrement of reader
 count before increment

On 5/3/19 8:06 AM, Peter Zijlstra wrote:
> On Sun, Apr 28, 2019 at 05:25:38PM -0400, Waiman Long wrote:
>> During my rwsem testing, it was found that after a down_read(), the
>> reader count may occasionally become 0 or even negative. Consequently,
>> a writer may steal the lock at that time and execute with the reader
>> in parallel thus breaking the mutual exclusion guarantee of the write
>> lock. In other words, both readers and writer can become rwsem owners
>> simultaneously.
>>
>> The current reader wakeup code does it in one pass to clear waiter->task
>> and put them into wake_q before fully incrementing the reader count.
>> Once waiter->task is cleared, the corresponding reader may see it,
>> finish the critical section and do unlock to decrement the count before
>> the count is incremented. This is not a problem if there is only one
>> reader to wake up as the count has been pre-incremented by 1.  It is
>> a problem if there are more than one readers to be woken up and writer
>> can steal the lock.
>>
>> The wakeup was actually done in 2 passes before the v4.9 commit
>> 70800c3c0cc5 ("locking/rwsem: Scan the wait_list for readers only
>> once"). To fix this problem, the wakeup is now done in two passes
>> again. In the first pass, we collect the readers and count them. The
>> reader count is then fully incremented. In the second pass, the
>> waiter->task is then cleared and they are put into wake_q to be woken
>> up later.
>>
>> Fixes: 70800c3c0cc5 ("locking/rwsem: Scan the wait_list for readers only once")
> It is effectively a revert of that patch, right? Just written more
> clever.
>
Yes, it is essentially a revert.

Cheers,
Longman