linux-kernel - Re: [PATCH 4/4] ublk: improve handling of saturated queues when ublk server exits

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Message-ID: <Z+SI4x+0J52rCJpN@dev-ushankar.dev.purestorage.com>
Date: Wed, 26 Mar 2025 17:08:19 -0600
From: Uday Shankar <ushankar@...estorage.com>
To: Ming Lei <ming.lei@...hat.com>
Cc: Shuah Khan <shuah@...nel.org>, Jens Axboe <axboe@...nel.dk>,
	linux-block@...r.kernel.org, linux-kselftest@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] ublk: improve handling of saturated queues when ublk
 server exits

On Wed, Mar 26, 2025 at 12:56:56PM -0600, Uday Shankar wrote:
> On Wed, Mar 26, 2025 at 11:54:16AM -0600, Uday Shankar wrote:
> > > ublk_abort_requests() should be called only in case of queue dying,
> > > since ublk server may open & close the char device multiple times.
> > 
> > Sure that is technically possible, however is any real ublk server doing
> > this? Seems like a strange thing to do, and seems reasonable for the
> > driver to transition the device to the nosrv state (dead or recovery,
> > depending on flags) when the char device is closed, since in this case,
> > no one can be handling I/O anymore.
> 
> I see ublksrv itself is doing this :(
> 
> /* Wait until ublk device is setup by udev */
> static void ublksrv_check_dev(const struct ublksrv_ctrl_dev_info *info)
> {
> 	unsigned int max_time = 1000000, wait = 0;
> 	char buf[64];
> 
> 	snprintf(buf, 64, "%s%d", "/dev/ublkc", info->dev_id);
> 
> 	while (wait < max_time) {
> 		int fd = open(buf, O_RDWR);
> 
> 		if (fd > 0) {
> 			close(fd);
> 			break;
> 		}
> 
> 		usleep(100000);
> 		wait += 100000;
> 	}
> }
> 
> This seems related to some failures in ublksrv tests

Actually this is the only issue I'm seeing - after patching this up in
ublksrv, make T=generic test appears to pass - I don't see any logs
indicating failures, and no kernel panics.

So the question is, does this patch break existing ublk servers? It does
break ublksrv as shown above, but I think one could argue that the above
code is just testing for file existence, and it's a bit weird to do that
by opening and closing the file (especially given that it's a device
special file). It can be patched to just use access or something
instead.