Re: [PATCH 05/27] block: ensure that async polled IO is marked REQ_NOWAIT

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On Fri, 2018-11-30 at 09:56 -0700, Jens Axboe wrote:
+AD4 We can't wait for polled events to complete, as they may require active
+AD4 polling from whoever submitted it. If that is the same task that is
+AD4 submitting new IO, we could deadlock waiting for IO to complete that
+AD4 this task is supposed to be completing itself.
+AD4 
+AD4 Signed-off-by: Jens Axboe +ADw-axboe+AEA-kernel.dk+AD4
+AD4 ---
+AD4  fs/block+AF8-dev.c +AHw 10 +-+-+-+-+-+-+-+-+--
+AD4  1 file changed, 9 insertions(+-), 1 deletion(-)
+AD4 
+AD4 diff --git a/fs/block+AF8-dev.c b/fs/block+AF8-dev.c
+AD4 index 6de8d35f6e41..ebc3d5a0f424 100644
+AD4 --- a/fs/block+AF8-dev.c
+AD4 +-+-+- b/fs/block+AF8-dev.c
+AD4 +AEAAQA -402,8 +-402,16 +AEAAQA +AF8AXw-blkdev+AF8-direct+AF8-IO(struct kiocb +ACo-iocb, struct iov+AF8-iter +ACo-iter, int nr+AF8-pages)
+AD4  
+AD4  		nr+AF8-pages +AD0 iov+AF8-iter+AF8-npages(iter, BIO+AF8-MAX+AF8-PAGES)+ADs
+AD4  		if (+ACE-nr+AF8-pages) +AHs
+AD4 -			if (iocb-+AD4-ki+AF8-flags +ACY IOCB+AF8-HIPRI)
+AD4 +-			if (iocb-+AD4-ki+AF8-flags +ACY IOCB+AF8-HIPRI) +AHs
+AD4  				bio-+AD4-bi+AF8-opf +AHwAPQ REQ+AF8-HIPRI+ADs
+AD4 +-				/+ACo
+AD4 +-				 +ACo For async polled IO, we can't wait for
+AD4 +-				 +ACo requests to complete, as they may also be
+AD4 +-				 +ACo polled and require active reaping.
+AD4 +-				 +ACo-/
+AD4 +-				if (+ACE-is+AF8-sync)
+AD4 +-					bio-+AD4-bi+AF8-opf +AHwAPQ REQ+AF8-NOWAIT+ADs
+AD4 +-			+AH0
+AD4  
+AD4  			qc +AD0 submit+AF8-bio(bio)+ADs
+AD4  			WRITE+AF8-ONCE(iocb-+AD4-ki+AF8-cookie, qc)+ADs

Setting REQ+AF8-NOWAIT from inside the block layer will make the code that
submits requests harder to review. Have you considered to make this code
fail I/O if REQ+AF8-NOWAIT has not been set and to require that the context
that submits I/O sets REQ+AF8-NOWAIT?

Thanks,

Bart.



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