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btrfs: fix lockup in find_free_extent with read-only block groups
If we have a block group that is all of the following: 1) uncached in memory 2) is read-only 3) has a disk cache state that indicates we need to recreate the cache AND the file system has enough free space fragmentation such that the request for an extent of a given size can't be honored; AND have a single CPU core; AND it's the block group with the highest starting offset such that there are no opportunities (like reading from disk) for the loop to yield the CPU; We can end up with a lockup. The root cause is simple. Once we're in the position that we've read in all of the other block groups directly and none of those block groups can honor the request, there are no more opportunities to sleep. We end up trying to start a caching thread which never gets run if we only have one core. This *should* present as a hung task waiting on the caching thread to make some progress, but it doesn't. Instead, it degrades into a busy loop because of the placement of the read-only check. During the first pass through the loop, block_group->cached will be set to BTRFS_CACHE_STARTED and have_caching_bg will be set. Then we hit the read-only check and short circuit the loop. We're not yet in LOOP_CACHING_WAIT, so we skip that loop back before going through the loop again for other raid groups. Then we move to LOOP_CACHING_WAIT state. During the this pass through the loop, ->cached will still be BTRFS_CACHE_STARTED, which means it's not cached, so we'll enter cache_block_group, do a lot of nothing, and return, and also set have_caching_bg again. Then we hit the read-only check and short circuit the loop. The same thing happens as before except now we DO trigger the LOOP_CACHING_WAIT && have_caching_bg check and loop back up to the top. We do this forever. There are two fixes in this patch since they address the same underlying bug. The first is to add a cond_resched to the end of the loop to ensure that the caching thread always has an opportunity to run. This will fix the soft lockup issue, but find_free_extent will still loop doing nothing until the thread has completed. The second is to move the read-only check to the top of the loop. We're never going to return an allocation within a read-only block group so we may as well skip it early. The check for ->cached == BTRFS_CACHE_ERROR would cause the same problem except that BTRFS_CACHE_ERROR is considered a "done" state and we won't re-set have_caching_bg again. Many thanks to Stephan Kulow <coolo@suse.de> for his excellent help in the testing process. Signed-off-by:Jeff Mahoney <jeffm@suse.com> Reviewed-by: