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

[Waiman Long]

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