From: Nikolay Borisov nborisov@suse.com
commit 45da9c1767ac31857df572f0a909fbe88fd5a7e9 upstream.
Ordered work functions aren't guaranteed to be handled by the same thread which executed the normal work functions. The only way execution between normal/ordered functions is synchronized is via the WORK_DONE_BIT, unfortunately the used bitops don't guarantee any ordering whatsoever.
This manifested as seemingly inexplicable crashes on ARM64, where async_chunk::inode is seen as non-null in async_cow_submit which causes submit_compressed_extents to be called and crash occurs because async_chunk::inode suddenly became NULL. The call trace was similar to:
pc : submit_compressed_extents+0x38/0x3d0 lr : async_cow_submit+0x50/0xd0 sp : ffff800015d4bc20
<registers omitted for brevity>
Call trace: submit_compressed_extents+0x38/0x3d0 async_cow_submit+0x50/0xd0 run_ordered_work+0xc8/0x280 btrfs_work_helper+0x98/0x250 process_one_work+0x1f0/0x4ac worker_thread+0x188/0x504 kthread+0x110/0x114 ret_from_fork+0x10/0x18
Fix this by adding respective barrier calls which ensure that all accesses preceding setting of WORK_DONE_BIT are strictly ordered before setting the flag. At the same time add a read barrier after reading of WORK_DONE_BIT in run_ordered_work which ensures all subsequent loads would be strictly ordered after reading the bit. This in turn ensures are all accesses before WORK_DONE_BIT are going to be strictly ordered before any access that can occur in ordered_func.
Reported-by: Chris Murphy lists@colorremedies.com Fixes: 08a9ff326418 ("btrfs: Added btrfs_workqueue_struct implemented ordered execution based on kernel workqueue") CC: stable@vger.kernel.org # 4.4+ Link: https://bugzilla.redhat.com/show_bug.cgi?id=2011928 Reviewed-by: Josef Bacik josef@toxicpanda.com Tested-by: Chris Murphy chris@colorremedies.com Signed-off-by: Nikolay Borisov nborisov@suse.com Signed-off-by: David Sterba dsterba@suse.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- fs/btrfs/async-thread.c | 14 ++++++++++++++ 1 file changed, 14 insertions(+)
--- a/fs/btrfs/async-thread.c +++ b/fs/btrfs/async-thread.c @@ -283,6 +283,13 @@ static void run_ordered_work(struct __bt ordered_list); if (!test_bit(WORK_DONE_BIT, &work->flags)) break; + /* + * Orders all subsequent loads after reading WORK_DONE_BIT, + * paired with the smp_mb__before_atomic in btrfs_work_helper + * this guarantees that the ordered function will see all + * updates from ordinary work function. + */ + smp_rmb();
/* * we are going to call the ordered done function, but @@ -368,6 +375,13 @@ static void normal_work_helper(struct bt thresh_exec_hook(wq); work->func(work); if (need_order) { + /* + * Ensures all memory accesses done in the work function are + * ordered before setting the WORK_DONE_BIT. Ensuring the thread + * which is going to executed the ordered work sees them. + * Pairs with the smp_rmb in run_ordered_work. + */ + smp_mb__before_atomic(); set_bit(WORK_DONE_BIT, &work->flags); run_ordered_work(wq, work); }