On 04/09/2015 04:14 PM, Peter Zijlstra wrote:
On Thu, Apr 09, 2015 at 04:07:55PM -0400, Waiman Long wrote:
The qrwlock is fair in the process context, but becoming unfair when
in the interrupt context to support use cases like the tasklist_lock.
However, the unfair code in the interrupt context has problem that
may cause deadlock.
The fast path increments the reader count. In the interrupt context,
the reader in the slowpath will wait until the writer release the
lock. However, if other readers have the lock and the writer is just
in the waiting mode. It will never get the write lock because the
that interrupt context reader has increment the count. This will
cause deadlock.
This patch fixes this problem by checking the state of the
reader/writer count retrieved at the fast path. If the writer
is in waiting mode, the reader will get the lock immediately and
return. Otherwise, it will wait until the writer release the lock
like before.
A little word on how you found this issue would be nice.
It is not found by testing. I didn't see any problem with a running
Linux kernel so far.
I am in the process of trying to make the qrwlock lock unfair in virt.
When I inspect the code, I found out that the interrupt code didn't look
right. That is why I send out a patch to fix that.
Regards,
Longman
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