[Devel] [PATCH] fuse kio: Fix deadlock at pcs_fuse_submit() error path

[Kirill Tkhai]

On 17.10.2018 18:43, Kirill Tkhai wrote:
> On 17.10.2018 18:06, Pavel Butsykin wrote:
>>
>>
>> On 17.10.2018 16:57, Kirill Tkhai wrote:
>>> request_end() takes fc->lock, so we in case of error we bump
>>> into deadlock:
>>>
>>> Call Trace:
>>>    [<ffffffffb3bb63f5>] _raw_spin_lock+0x75/0xc0
>>>    [<ffffffffc170871b>] spin_lock+0x18/0x1b [fuse]
>>>    [<ffffffffc170ba63>] request_end+0x265/0x72b [fuse]
>>>    [<ffffffffc18a1b8d>] pcs_fuse_submit+0x9fb/0xaa3 [fuse_kio_pcs]
>>>    [<ffffffffc18a35c4>] kpcs_req_send+0x793/0xa60 [fuse_kio_pcs]
>>>    [<ffffffffc170b6ca>] flush_bg_queue+0x14f/0x283 [fuse]
>>>    [<ffffffffc170d4d4>] fuse_request_send_background_locked+0x50b/0x512 [fuse]
>>>    [<ffffffffc170d844>] fuse_request_send_background+0x369/0x43f [fuse]
>>>    [<ffffffffc173028b>] fuse_send_readpages+0x372/0x3b5 [fuse]
>>>    [<ffffffffc1730c3c>] fuse_readpages+0x28c/0x2f0 [fuse]
>>>    [<ffffffffb296ba58>] __do_page_cache_readahead+0x518/0x6d0
>>>
>>> Fix this by unlocking fc->lock before request_end() call. Note,
>>> that it may look strange to have two same lk parameters in
>>> pcs_fuse_submit(pfc, req, lk, lk), but the current design
>>> interprets requests submitted with locked lk as async and
>>> we keep this logic.
>>>
>>> Generally, I feel we need to improve design in a thing
>>> of queueing requests and locking, but we need more
>>> inverstigation and thinking here, so let's delay this
>>> to next VZ update.
>>>
>>> https://pmc.acronis.com/browse/VSTOR-16246
>>>
>>> Signed-off-by: Kirill Tkhai <ktkhai at virtuozzo.com>
>>> ---
>>>   fs/fuse/kio/pcs/pcs_fuse_kdirect.c |   10 +++++++---
>>>   1 file changed, 7 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/fs/fuse/kio/pcs/pcs_fuse_kdirect.c b/fs/fuse/kio/pcs/pcs_fuse_kdirect.c
>>> index b286a956a751..61415e029c45 100644
>>> --- a/fs/fuse/kio/pcs/pcs_fuse_kdirect.c
>>> +++ b/fs/fuse/kio/pcs/pcs_fuse_kdirect.c
>>> @@ -883,7 +883,7 @@ static int pcs_fuse_prep_rw(struct pcs_fuse_req *r)
>>>   	return ret;
>>>   }
>>>   
>>> -static void pcs_fuse_submit(struct pcs_fuse_cluster *pfc, struct fuse_req *req, int async)
>>> +static void pcs_fuse_submit(struct pcs_fuse_cluster *pfc, struct fuse_req *req, bool async, bool lk)
>>>   {
>>>   	struct pcs_fuse_req *r = pcs_req_from_fuse(req);
>>>   	struct fuse_inode *fi = get_fuse_inode(req->io_inode);
>>> @@ -963,7 +963,11 @@ static void pcs_fuse_submit(struct pcs_fuse_cluster *pfc, struct fuse_req *req,
>>>   error:
>>>   	DTRACE("do fuse_request_end req:%p op:%d err:%d\n", &r->req, r->req.in.h.opcode, r->req.out.h.error);
>>>   
>>> +	if (lk)
>>> +		spin_unlock(&pfc->fc->lock);
>>
>> We can't unlock fc->lock inside fuse_request_send_background_locked(),
>> because it breaks compatibility with fuse_set_nowrite(). We must
>> ensure that no one pending requests should not be between
>> fuse_set_nowrite() and fuse_release_nowrite(). But since fc unlock
>> inside fuse_request_send_background_locked() this promise can be broken.
> 
> No, this is not true, and this does not introduce new races.
> fuse_set_nowrite() does not really wait for all pending requests,
> since parallel kpcs_req_send() is possible after fuse_set_nowrite()
> released the lock. There is no protection. This is what about I say
> we need to redesign this thing. Also, keep in mind, that failing
> a request with request_end() is legitimate outside the lock, and
> this is just ordinary behavior we already have.
> 
> The change I did is similar to unlocking fc->lock after iteration

...iteration in flush_bg_queue()

> on some req, and it's definitely safe in current terms. If you
> can see new races introduced, just try to draw functions calls
> to verify that.
>  
>>>   	request_end(pfc->fc, &r->req);
>>> +	if (lk)
>>> +		spin_lock(&pfc->fc->lock);
>>>   	return;
>>>   
>>>   submit:
>>> @@ -1027,7 +1031,7 @@ static void _pcs_shrink_end(struct fuse_conn *fc, struct fuse_req *req)
>>>   
>>>   		TRACE("resubmit %p\n", &r->req);
>>>   		list_del_init(&ireq->list);
>>> -		pcs_fuse_submit(pfc, &r->req, 1);
>>> +		pcs_fuse_submit(pfc, &r->req, true, false);
>>>   	}
>>>   }
>>>   
>>> @@ -1174,7 +1178,7 @@ static int kpcs_req_send(struct fuse_conn* fc, struct fuse_req *req, bool bg, bo
>>>   	}
>>>   	__clear_bit(FR_PENDING, &req->flags);
>>>   
>>> -	pcs_fuse_submit(pfc, req, lk);
>>> +	pcs_fuse_submit(pfc, req, lk, lk);
>>>   	if (!bg)
>>>   		wait_event(req->waitq,
>>>   			   test_bit(FR_FINISHED, &req->flags) && !req->end);
>>>