netfs: Fix race between cache write completion and ALL_QUEUED being set

In the Linux kernel, the following vulnerability has been resolved:

netfs: Fix race between cache write completion and ALL_QUEUED being set

When netfslib is issuing subrequests, the subrequests start processing

immediately and may complete before we reach the end of the issuing

function. At the end of the issuing function we set NETFS_RREQ_ALL_QUEUED

to indicate to the collector that we aren't going to issue any more subreqs

and that it can do the final notifications and cleanup.

Now, this isn't a problem if the request is synchronous

(NETFS_RREQ_OFFLOAD_COLLECTION is unset) as the result collection will be

done in-thread and we're guaranteed an opportunity to run the collector.

However, if the request is asynchronous, collection is primarily triggered

by the termination of subrequests queuing it on a workqueue. Now, a race

can occur here if the app thread sets ALL_QUEUED after the last subrequest

terminates.

This can happen most easily with the copy2cache code (as used by Ceph)

where, in the collection routine of a read request, an asynchronous write

request is spawned to copy data to the cache. Folios are added to the

write request as they're unlocked, but there may be a delay before

ALL_QUEUED is set as the write subrequests may complete before we get

there.

If all the write subreqs have finished by the ALL_QUEUED point, no further

events happen and the collection never happens, leaving the request

hanging.

Fix this by queuing the collector after setting ALL_QUEUED. This is a bit

heavy-handed and it may be sufficient to do it only if there are no extant

subreqs.

Also add a tracepoint to cross-reference both requests in a copy-to-request

operation and add a trace to the netfs_rreq tracepoint to indicate the

setting of ALL_QUEUED.