Re: [PATCH 3/4] export/fuse: Safe termination for FUSE-uring

[Brian Song <hibriansong@gmail.com>]

On 9/9/25 3:33 PM, Stefan Hajnoczi wrote:
> On Fri, Aug 29, 2025 at 10:50:24PM -0400, Brian Song wrote:
>> @@ -901,24 +941,15 @@ static void fuse_export_shutdown(BlockExport *blk_exp)
>>            */
>>           g_hash_table_remove(exports, exp->mountpoint);
>>       }
>> -}
>> -
>> -static void fuse_export_delete(BlockExport *blk_exp)
>> -{
>> -    FuseExport *exp = container_of(blk_exp, FuseExport, common);
>>   
>> -    for (int i = 0; i < exp->num_queues; i++) {
>> +    for (size_t i = 0; i < exp->num_queues; i++) {
>>           FuseQueue *q = &exp->queues[i];
>>   
>>           /* Queue 0's FD belongs to the FUSE session */
>>           if (i > 0 && q->fuse_fd >= 0) {
>>               close(q->fuse_fd);
> 
> This changes the behavior of the non-io_uring code. Now all fuse fds and
> fuse_session are closed while requests are potentially still being
> processed.
> 
> There is a race condition: if an IOThread is processing a request here
> then it may invoke a system call on q->fuse_fd just after it has been
> closed but not set to -1. If another thread has also opened a new file
> then the fd could be reused, resulting in an accidental write(2) to the
> new file. I'm not sure whether there is a way to trigger this in
> practice, but it looks like a problem waiting to happen.
> 
> Simply setting q->fuse_fd to -1 here doesn't fix the race. It would be
> necessary to stop processing fuse_fd in the thread before closing it
> here or to schedule a BH in each thread so that fuse_fd can be closed
> in the thread that uses the fd.

I get what you mean. This newly introduced cleanup code was originally 
in the deletion section, after the reconf counter decreased to 0, and it 
was meant to cancel the pending SQEs. But now we've moved it to the 
shutdown section, which may introduce a potential problem. How do you 
think we should fix it? This is the last week of GSoC, I'm already busy 
on weekdays since the new term has started.