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

From: Peter Zijlstra
Date: Fri May 03 2019 - 08:07:44 EST

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