Re: çå: Re: [PATCH v2] scsi: Introduce sdev_printk_ratelimited to throttlefrequent printk

From: Petr Mladek
Date: Fri Apr 06 2018 - 04:30:29 EST

On Tue 2018-04-03 14:19:43, wen.yang99@xxxxxxxxxx wrote:
> On the other handïqueue_lock is big, looping doing something under spinlock
> may locked many things and taking a long time, may cause some problems.
> So This code needs to be optimized later:
> scsi_request_fn()
> {
> for (;;) {
> int rtn;
> /*
> * get next queueable request. We do this early to make sure
> * that the request is fully prepared even if we cannot
> * accept it.
> */
> req = blk_peek_request(q);
> if (!req)
> break;
> if (unlikely(!scsi_device_online(sdev))) {
> sdev_printk(KERN_ERR, sdev,
> "rejected I/O to offline device\n");
> scsi_kill_request(req, q);
> continue;
> ^^^^^^^^^ still under spinlock
> }

I wonder if the following might be the best solution after all:

if (unlikely(!scsi_device_online(sdev))) {
scsi_kill_request(req, q);

* printk() might take a while on slow consoles.
* Prevent solftlockups by releasing the lock.
sdev_printk(KERN_ERR, sdev,
"rejecting I/O to offline device\n");

I see that the lock is released also in several other situations.
Therefore it looks safe. Also handling too many requests without
releasing the lock seems to be a bad idea in general. I think
that this solution was already suggested earlier.

Please, note that I moved scsi_kill_request() up. It looks natural
to remove it from the queue before we release the queue lock.

Best Regards,

BTW: Your mail had strange formatting. Please, try to avoid using