Re: [PATCH v4 0/5] fsnotify: fix softlockups iterating over d_subdirs

Stephen Brennan
2022-11-11T22:09:29Z

Stephen Brennan <stephen.s.brennan@oracle.com> writes:

> Hi Jan, Amir, Al,
>
> Here's my v4 patch series that aims to eliminate soft lockups when updating
> dentry flags in fsnotify. I've incorporated Jan's suggestion of simply
> allowing the flag to be lazily cleared in the fsnotify_parent() function,
> via Amir's patch. This allowed me to drop patch #2 from my previous series
> (fsnotify: Protect i_fsnotify_mask and child flags with inode rwsem). I
> replaced it with "fsnotify: require inode lock held during child flag
> update", patch #5 in this series. I also added "dnotify: move
> fsnotify_recalc_mask() outside spinlock" to address the sleep-during-atomic
> issues with dnotify.
>
> Jan expressed concerns about lock ordering of the inode rwsem with the
> fsnotify group mutex. I built this with lockdep enabled (see below for the
> lock debugging .config section -- I'm not too familiar with lockdep so I
> wanted a sanity check). I ran all the fanotify, inotify, and dnotify tests
> I could find in LTP, with no lockdep splats to be found. I don't know that
> this can completely satisfy the concerns about lock ordering: I'm reading
> through the code to better understand the concern about "the removal of
> oneshot mark during modify event generation". But I'm encouraged by the
> LTP+lockdep results.

Of course, I forgot to append the .config section:

#
# Lock Debugging (spinlocks, mutexes, etc...)
#
CONFIG_LOCK_DEBUGGING_SUPPORT=y
CONFIG_PROVE_LOCKING=y
# CONFIG_PROVE_RAW_LOCK_NESTING is not set
CONFIG_LOCK_STAT=y
CONFIG_DEBUG_RT_MUTEXES=y
CONFIG_DEBUG_SPINLOCK=y
CONFIG_DEBUG_MUTEXES=y
CONFIG_DEBUG_WW_MUTEX_SLOWPATH=y
CONFIG_DEBUG_RWSEMS=y
CONFIG_DEBUG_LOCK_ALLOC=y
CONFIG_LOCKDEP=y
CONFIG_LOCKDEP_BITS=15
CONFIG_LOCKDEP_CHAINS_BITS=16
CONFIG_LOCKDEP_STACK_TRACE_BITS=19
CONFIG_LOCKDEP_STACK_TRACE_HASH_BITS=14
CONFIG_LOCKDEP_CIRCULAR_QUEUE_BITS=12
CONFIG_DEBUG_LOCKDEP=y
CONFIG_DEBUG_ATOMIC_SLEEP=y
# CONFIG_DEBUG_LOCKING_API_SELFTESTS is not set
# CONFIG_LOCK_TORTURE_TEST is not set
# CONFIG_WW_MUTEX_SELFTEST is not set
# CONFIG_SCF_TORTURE_TEST is not set
CONFIG_CSD_LOCK_WAIT_DEBUG=y
# end of Lock Debugging (spinlocks, mutexes, etc...)

>
> I also went ahead and did my negative dentry oriented testing. Of course
> the fsnotify_parent() issue is fully resolved, and when I tested several
> processes all using inotifywait on the same directory full of negative
> dentries, I was able to use ftrace to confirm that
> fsnotify_update_children_dentry_flags() was called exactly once for all
> processes. No softlockups occurred!
>
> I originally wrote this series to make the last patch (#5) optional: if for
> some reason we didn't think it was necessary to hold the inode rwsem, then
> we could omit it -- the main penalty being the race condition described in
> the patch description. I tested without the last patch and LTP passed also
> with lockdep enabled, but of course when multiple tasks did an inotifywait
> on the same directory (with many negative dentries) only the first waited
> for the flag updates, the rest of the tasks immediately returned despite
> the flags not being ready.
>
> I agree with Amir that as long as the lock ordering is fine, we should keep
> patch #5. And if that's the case, I can reorder the series a bit to make it
> a bit more logical, and eliminate logic in
> fsnotify_update_children_dentry_flags() for handling d_move/cursor races,
> which I promptly delete later in the series.
>
> 1. fsnotify: clear PARENT_WATCHED flags lazily
> 2. fsnotify: Use d_find_any_alias to get dentry associated with inode
> 3. dnotify: move fsnotify_recalc_mask() outside spinlock
> 4. fsnotify: require inode lock held during child flag update
> 5. fsnotify: allow sleepable child flag update
>
> Thanks for continuing to read this series, I hope we're making progress
> toward a simpler way to fix these scaling issues!
>
> Stephen
>
> Amir Goldstein (1):
> fsnotify: clear PARENT_WATCHED flags lazily
>
> Stephen Brennan (4):
> fsnotify: Use d_find_any_alias to get dentry associated with inode
> dnotify: move fsnotify_recalc_mask() outside spinlock
> fsnotify: allow sleepable child flag update
> fsnotify: require inode lock held during child flag update
>
> fs/notify/dnotify/dnotify.c | 28 ++++++---
> fs/notify/fsnotify.c | 101 ++++++++++++++++++++++---------
> fs/notify/fsnotify.h | 3 +-
> fs/notify/mark.c | 40 +++++++++++-
> include/linux/fsnotify_backend.h | 8 ++-
> 5 files changed, 136 insertions(+), 44 deletions(-)
>
> --
> 2.34.1