Re: [PATCH v6 04/46] percpu_rwlock: Implement the core design of Per-CPU Reader-Writer Locks

[Michel Lespinasse <walken@google.com>]

On Tue, Feb 19, 2013 at 1:56 AM, Srivatsa S. Bhat
<srivatsa.bhat@linux.vnet.ibm.com> wrote:
> On 02/18/2013 09:51 PM, Srivatsa S. Bhat wrote:
>> On 02/18/2013 09:15 PM, Michel Lespinasse wrote:
>>> I don't see anything preventing a race with the corresponding code in
>>> percpu_write_unlock() that sets writer_signal back to false. Did I
>>> miss something here ? It seems to me we don't have any guarantee that
>>> all writer signals will be set to true at the end of the loop...
>>
>> Ah, thanks for pointing that out! IIRC Oleg had pointed this issue in the last
>> version, but back then, I hadn't fully understood what he meant. Your
>> explanation made it clear. I'll work on fixing this.
>
> We can fix this by using the simple patch (untested) shown below.
> The alternative would be to acquire the rwlock for write, update the
> ->writer_signal values, release the lock, wait for readers to switch,
> again acquire the rwlock for write with interrupts disabled etc... which
> makes it kinda messy, IMHO. So I prefer the simple version shown below.

Looks good.

Another alternative would be to make writer_signal an atomic integer
instead of a bool. That way writers can increment it before locking
and decrement it while unlocking.

To reduce the number of atomic ops during writer lock/unlock, the
writer_signal could also be a global read_mostly variable (I don't see
any downsides to that compared to having it percpu - or is it because
you wanted all the fastpath state to be in one single cacheline ?)

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.