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.
Fixes: e2d46f2ec332 ("netfs: Change the read result collector to only use one work item") Reported-by: Max Kellermann max.kellermann@ionos.com Link: https://lore.kernel.org/r/CAKPOu+8z_ijTLHdiCYGU_Uk7yYD=shxyGLwfe-L7AV3DhebS3... Signed-off-by: David Howells dhowells@redhat.com cc: Paulo Alcantara pc@manguebit.org cc: Viacheslav Dubeyko slava@dubeyko.com cc: Alex Markuze amarkuze@redhat.com cc: Ilya Dryomov idryomov@gmail.com cc: netfs@lists.linux.dev cc: ceph-devel@vger.kernel.org cc: linux-fsdevel@vger.kernel.org cc: stable@vger.kernel.org --- fs/netfs/read_pgpriv2.c | 4 ++++ include/trace/events/netfs.h | 30 ++++++++++++++++++++++++++++++ 2 files changed, 34 insertions(+)
diff --git a/fs/netfs/read_pgpriv2.c b/fs/netfs/read_pgpriv2.c index 080d2a6a51d9..8097bc069c1d 100644 --- a/fs/netfs/read_pgpriv2.c +++ b/fs/netfs/read_pgpriv2.c @@ -111,6 +111,7 @@ static struct netfs_io_request *netfs_pgpriv2_begin_copy_to_cache( goto cancel_put;
__set_bit(NETFS_RREQ_OFFLOAD_COLLECTION, &creq->flags); + trace_netfs_copy2cache(rreq, creq); trace_netfs_write(creq, netfs_write_trace_copy_to_cache); netfs_stat(&netfs_n_wh_copy_to_cache); rreq->copy_to_cache = creq; @@ -155,6 +156,9 @@ void netfs_pgpriv2_end_copy_to_cache(struct netfs_io_request *rreq) netfs_issue_write(creq, &creq->io_streams[1]); smp_wmb(); /* Write lists before ALL_QUEUED. */ set_bit(NETFS_RREQ_ALL_QUEUED, &creq->flags); + trace_netfs_rreq(rreq, netfs_rreq_trace_end_copy_to_cache); + if (list_empty_careful(&creq->io_streams[1].subrequests)) + netfs_wake_collector(creq);
netfs_put_request(creq, netfs_rreq_trace_put_return); creq->copy_to_cache = NULL; diff --git a/include/trace/events/netfs.h b/include/trace/events/netfs.h index 73e96ccbe830..64a382fbc31a 100644 --- a/include/trace/events/netfs.h +++ b/include/trace/events/netfs.h @@ -55,6 +55,7 @@ EM(netfs_rreq_trace_copy, "COPY ") \ EM(netfs_rreq_trace_dirty, "DIRTY ") \ EM(netfs_rreq_trace_done, "DONE ") \ + EM(netfs_rreq_trace_end_copy_to_cache, "END-C2C") \ EM(netfs_rreq_trace_free, "FREE ") \ EM(netfs_rreq_trace_ki_complete, "KI-CMPL") \ EM(netfs_rreq_trace_recollect, "RECLLCT") \ @@ -559,6 +560,35 @@ TRACE_EVENT(netfs_write, __entry->start, __entry->start + __entry->len - 1) );
+TRACE_EVENT(netfs_copy2cache, + TP_PROTO(const struct netfs_io_request *rreq, + const struct netfs_io_request *creq), + + TP_ARGS(rreq, creq), + + TP_STRUCT__entry( + __field(unsigned int, rreq) + __field(unsigned int, creq) + __field(unsigned int, cookie) + __field(unsigned int, ino) + ), + + TP_fast_assign( + struct netfs_inode *__ctx = netfs_inode(rreq->inode); + struct fscache_cookie *__cookie = netfs_i_cookie(__ctx); + __entry->rreq = rreq->debug_id; + __entry->creq = creq->debug_id; + __entry->cookie = __cookie ? __cookie->debug_id : 0; + __entry->ino = rreq->inode->i_ino; + ), + + TP_printk("R=%08x CR=%08x c=%08x i=%x ", + __entry->rreq, + __entry->creq, + __entry->cookie, + __entry->ino) + ); + TRACE_EVENT(netfs_collect, TP_PROTO(const struct netfs_io_request *wreq),