Re: [PATCH v3 1/1] io_uring: flush timeouts that should already have expired

[Pavel Begunkov <asml.silence@gmail.com>]

On 14/01/2021 21:40, Pavel Begunkov wrote:
> On 14/01/2021 15:50, Marcelo Diop-Gonzalez wrote:
>> Right now io_flush_timeouts() checks if the current number of events
>> is equal to ->timeout.target_seq, but this will miss some timeouts if
>> there have been more than 1 event added since the last time they were
>> flushed (possible in io_submit_flush_completions(), for example). Fix
>> it by recording the last sequence at which timeouts were flushed so
>> that the number of events seen can be compared to the number of events
>> needed without overflow.
> 
> Looks good, but there is a little change I'll ask you to make (see
> below). In a meanwhile I'll test it, so the patch on the fast track.
Tested, works good for me.

Apart from the small change I described before
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>

> 
>>
>> Signed-off-by: Marcelo Diop-Gonzalez <marcelo827@gmail.com>
>> ---
>>  fs/io_uring.c | 29 +++++++++++++++++++++++++----
>>  1 file changed, 25 insertions(+), 4 deletions(-)
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index 372be9caf340..71d8fa0733ad 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -354,6 +354,7 @@ struct io_ring_ctx {
>>  		unsigned		cq_entries;
>>  		unsigned		cq_mask;
>>  		atomic_t		cq_timeouts;
>> +		unsigned		cq_last_tm_flush;
>>  		unsigned long		cq_check_overflow;
>>  		struct wait_queue_head	cq_wait;
>>  		struct fasync_struct	*cq_fasync;
>> @@ -1639,19 +1640,36 @@ static void __io_queue_deferred(struct io_ring_ctx *ctx)
>>  
>>  static void io_flush_timeouts(struct io_ring_ctx *ctx)
>>  {
>> -	while (!list_empty(&ctx->timeout_list)) {
>> +	u32 seq = ctx->cached_cq_tail - atomic_read(&ctx->cq_timeouts);
> 
> This assignment should go after list_empty() -- because of the atomic part
> my compiler can't reshuffle them itself.
> 
>> +
>> +	if (list_empty(&ctx->timeout_list))
>> +		return;
> [...]
>>  static void io_commit_cqring(struct io_ring_ctx *ctx)
>> @@ -5837,6 +5855,9 @@ static int io_timeout(struct io_kiocb *req)
>>  	tail = ctx->cached_cq_tail - atomic_read(&ctx->cq_timeouts);
>>  	req->timeout.target_seq = tail + off;
>>  
>> +	/* Update the last seq here in case io_flush_timeouts() hasn't */
>> +	ctx->cq_last_tm_flush = tail;
> 
> Have to note that it's ok to do because we don't mix submissions and
> completions, so io_timeout should never fall under same completion_lock
> section as cq commit,
> 
> but otherwise some future locked version of io_timeout would be cutting
> off a part of the current flush window (i.e. this [last, cur] thing).
> 

-- 
Pavel Begunkov