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

[Jens Axboe <axboe@kernel.dk>]

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?

-- 
Jens Axboe