Re: [PATCH 2/5] io_uring: serialize ctx->rings->sq_flags with cmpxchg()

[Jens Axboe <axboe@kernel.dk>]

On 4/23/22 7:07 AM, Jens Axboe wrote:
> On 4/23/22 2:06 AM, Pavel Begunkov wrote:
>> On 4/22/22 22:42, Jens Axboe wrote:
>>> Rather than require ctx->completion_lock for ensuring that we don't
>>> clobber the flags, use try_cmpxchg() instead. This removes the need
>>> to grab the completion_lock, in preparation for needing to set or
>>> clear sq_flags when we don't know the status of this lock.
>>>
>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>> ---
>>>   fs/io_uring.c | 54 ++++++++++++++++++++++++++++++---------------------
>>>   1 file changed, 32 insertions(+), 22 deletions(-)
>>>
>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>> index 626bf840bed2..38e58fe4963d 100644
>>> --- a/fs/io_uring.c
>>> +++ b/fs/io_uring.c
>>> @@ -1999,6 +1999,34 @@ static void io_cqring_ev_posted_iopoll(struct io_ring_ctx *ctx)
>>>           io_cqring_wake(ctx);
>>>   }
>>>   +static void io_ring_sq_flag_clear(struct io_ring_ctx *ctx, unsigned int flag)
>>> +{
>>> +    struct io_rings *rings = ctx->rings;
>>> +    unsigned int oldf, newf;
>>> +
>>> +    do {
>>> +        oldf = READ_ONCE(rings->sq_flags);
>>> +
>>> +        if (!(oldf & flag))
>>> +            break;
>>> +        newf = oldf & ~flag;
>>> +    } while (!try_cmpxchg(&rings->sq_flags, &oldf, newf));
>>> +}
>>> +
>>> +static void io_ring_sq_flag_set(struct io_ring_ctx *ctx, unsigned int flag)
>>> +{
>>> +    struct io_rings *rings = ctx->rings;
>>> +    unsigned int oldf, newf;
>>> +
>>> +    do {
>>> +        oldf = READ_ONCE(rings->sq_flags);
>>> +
>>> +        if (oldf & flag)
>>> +            break;
>>> +        newf = oldf | flag;
>>> +    } while (!try_cmpxchg(&rings->sq_flags, &oldf, newf));
>>> +}
>>
>> atomic and/or might be a better fit
> 
> That would indeed be great, but the type is fixed. Not sure if all archs
> end up with a plain int as the type?

Looks like it actually is defined generically, so this could work.

-- 
Jens Axboe