On Wed, Nov 28 2018, David Howells wrote: > NeilBrown <neilb@xxxxxxxx> wrote: > >> It was observed that a process blocked indefintely in >> __fscache_read_or_alloc_page(), waiting for FSCACHE_COOKIE_LOOKING_UP >> to be cleared via fscache_wait_for_deferred_lookup(). > > Was there a backtrace for that? Yes. PID: 29360 TASK: ffff881ff2ac0f80 CPU: 3 COMMAND: "zsh" #0 [ffff881ff43efbf8] schedule at ffffffff815e56f1 #1 [ffff881ff43efc58] bit_wait at ffffffff815e64ed #2 [ffff881ff43efc68] __wait_on_bit at ffffffff815e61b8 #3 [ffff881ff43efca0] out_of_line_wait_on_bit at ffffffff815e625e #4 [ffff881ff43efd08] fscache_wait_for_deferred_lookup at ffffffffa04f2e8f [fscache] #5 [ffff881ff43efd18] __fscache_read_or_alloc_page at ffffffffa04f2ffe [fscache] #6 [ffff881ff43efd58] __nfs_readpage_from_fscache at ffffffffa0679668 [nfs] #7 [ffff881ff43efd78] nfs_readpage at ffffffffa067092b [nfs] #8 [ffff881ff43efda0] generic_file_read_iter at ffffffff81187a73 #9 [ffff881ff43efe50] nfs_file_read at ffffffffa066544b [nfs] #10 [ffff881ff43efe70] __vfs_read at ffffffff811fc756 #11 [ffff881ff43efee8] vfs_read at ffffffff811fccfa #12 [ffff881ff43eff18] sys_read at ffffffff811fda62 #13 [ffff881ff43eff50] entry_SYSCALL_64_fastpath at ffffffff815e986e This is in SLES12-SP2 which is based on Linux-4.4 > >> This leaves a window where something else can set FSCACHE_COOKIE_LOOKING_UP > > Attempting to enable the cookie again would have that effect. > >> There is some uncertainty in this analysis, but it seems to be fit the >> observations. Adding the wake in this patch will be handled correctly >> by __fscache_read_or_alloc_page(), as it checks if ->backing_objects >> is empty again, after waiting. > > That should work. The read op should just then fail with -ENOBUFS, causing > the netfs to issue the read over the network instead. > > I think there's a more elegant solution to be had, but it'll take some thought > as to how to achieve it. Please let me know when you come up with something. Thanks, NeilBrown
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