Re: [PATCH for-5.0 2/2] block: Fix blk->in_flight during blk_wait_while_drained()

[Kevin Wolf <kwolf@redhat.com>]

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Am 03.04.2020 um 14:42 hat Max Reitz geschrieben:
> On 03.04.20 12:44, Kevin Wolf wrote:
> > Calling blk_wait_while_drained() while blk->in_flight is increased for
> > the current request is wrong because it will cause the drain operation
> > to deadlock.
> > 
> > Many callers of blk_wait_while_drained() have already increased
> > blk->in_flight when called in a blk_aio_*() path, but can also be called
> > in synchonous code paths where blk->in_flight isn't increased. This
> > means that these calls of blk_wait_while_drained() are wrong at least in
> > some cases.
> > 
> > In order to fix this, increase blk->in_flight even for synchronous
> > operations and temporarily decrease the counter again in
> > blk_wait_while_drained().
> > 
> > Fixes: cf3129323f900ef5ddbccbe86e4fa801e88c566e
> > Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> > ---
> >  block/block-backend.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> blk_co_pdiscard() and blk_co_flush() are called from outside of
> block-backend.c (namely from mirror.c and nbd/server.c).  Is that OK?

Hm... I think you're right that the NBD server has a problem now because
we might now decrease blk->in_flight without having increased it.
(Mirror should be fine anyway because it sets disable_request_queuing.)

At first I was going to suggest that we could do the opposite of this
patch and just move the dec/wait/inc sequence (which this patch removes
for read/write) to all coroutine entry functions, so direct calls
wouldn't incorrectly decrease the counter.

But this is not what we want either, we do want to queue requests for
drained BlockBackends even in the blk_co_*() API.

Do you have another idea or do we have to turn blk_co_*() into wrappers
around the existing functions, which would gain an additional bool
parameter that tells whether we need to dec/inc or not?

Kevin

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