From 03e0add80f4cf3f7393edb574eeb3a89a1db7758 Mon Sep 17 00:00:00 2001 From: "Darrick J. Wong" Date: Tue, 2 May 2023 09:16:05 +1000 Subject: xfs: explicitly specify cpu when forcing inodegc delayed work to run immediately I've been noticing odd racing behavior in the inodegc code that could only be explained by one cpu adding an inode to its inactivation llist at the same time that another cpu is processing that cpu's llist. Preemption is disabled between get/put_cpu_ptr, so the only explanation is scheduler mayhem. I inserted the following debug code into xfs_inodegc_worker (see the next patch): ASSERT(gc->cpu == smp_processor_id()); This assertion tripped during overnight tests on the arm64 machines, but curiously not on x86_64. I think we haven't observed any resource leaks here because the lockfree list code can handle simultaneous llist_add and llist_del_all functions operating on the same list. However, the whole point of having percpu inodegc lists is to take advantage of warm memory caches by inactivating inodes on the last processor to touch the inode. The incorrect scheduling seems to occur after an inodegc worker is subjected to mod_delayed_work(). This wraps mod_delayed_work_on with WORK_CPU_UNBOUND specified as the cpu number. Unbound allows for scheduling on any cpu, not necessarily the same one that scheduled the work. Because preemption is disabled for as long as we have the gc pointer, I think it's safe to use current_cpu() (aka smp_processor_id) to queue the delayed work item on the correct cpu. Fixes: 7cf2b0f9611b ("xfs: bound maximum wait time for inodegc work") Signed-off-by: Darrick J. Wong Reviewed-by: Dave Chinner Signed-off-by: Dave Chinner --- fs/xfs/xfs_icache.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) (limited to 'fs/xfs/xfs_icache.c') diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c index 351849fc18ff..58712113d5d6 100644 --- a/fs/xfs/xfs_icache.c +++ b/fs/xfs/xfs_icache.c @@ -2069,7 +2069,8 @@ xfs_inodegc_queue( queue_delay = 0; trace_xfs_inodegc_queue(mp, __return_address); - mod_delayed_work(mp->m_inodegc_wq, &gc->work, queue_delay); + mod_delayed_work_on(current_cpu(), mp->m_inodegc_wq, &gc->work, + queue_delay); put_cpu_ptr(gc); if (xfs_inodegc_want_flush_work(ip, items, shrinker_hits)) { @@ -2113,7 +2114,8 @@ xfs_inodegc_cpu_dead( if (xfs_is_inodegc_enabled(mp)) { trace_xfs_inodegc_queue(mp, __return_address); - mod_delayed_work(mp->m_inodegc_wq, &gc->work, 0); + mod_delayed_work_on(current_cpu(), mp->m_inodegc_wq, &gc->work, + 0); } put_cpu_ptr(gc); } -- cgit From b37c4c8339cd394ea6b8b415026603320a185651 Mon Sep 17 00:00:00 2001 From: "Darrick J. Wong" Date: Tue, 2 May 2023 09:16:12 +1000 Subject: xfs: check that per-cpu inodegc workers actually run on that cpu Now that we've allegedly worked out the problem of the per-cpu inodegc workers being scheduled on the wrong cpu, let's put in a debugging knob to let us know if a worker ever gets mis-scheduled again. Signed-off-by: Darrick J. Wong Reviewed-by: Dave Chinner Signed-off-by: Dave Chinner --- fs/xfs/xfs_icache.c | 2 ++ fs/xfs/xfs_mount.h | 3 +++ fs/xfs/xfs_super.c | 3 +++ 3 files changed, 8 insertions(+) (limited to 'fs/xfs/xfs_icache.c') diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c index 58712113d5d6..4b63c065ef19 100644 --- a/fs/xfs/xfs_icache.c +++ b/fs/xfs/xfs_icache.c @@ -1856,6 +1856,8 @@ xfs_inodegc_worker( struct xfs_inode *ip, *n; unsigned int nofs_flag; + ASSERT(gc->cpu == smp_processor_id()); + WRITE_ONCE(gc->items, 0); if (!node) diff --git a/fs/xfs/xfs_mount.h b/fs/xfs/xfs_mount.h index f3269c0626f0..aaaf5ec13492 100644 --- a/fs/xfs/xfs_mount.h +++ b/fs/xfs/xfs_mount.h @@ -66,6 +66,9 @@ struct xfs_inodegc { /* approximate count of inodes in the list */ unsigned int items; unsigned int shrinker_hits; +#if defined(DEBUG) || defined(XFS_WARN) + unsigned int cpu; +#endif }; /* diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c index 4d2e87462ac4..7e706255f165 100644 --- a/fs/xfs/xfs_super.c +++ b/fs/xfs/xfs_super.c @@ -1095,6 +1095,9 @@ xfs_inodegc_init_percpu( for_each_possible_cpu(cpu) { gc = per_cpu_ptr(mp->m_inodegc, cpu); +#if defined(DEBUG) || defined(XFS_WARN) + gc->cpu = cpu; +#endif init_llist_head(&gc->list); gc->items = 0; INIT_DELAYED_WORK(&gc->work, xfs_inodegc_worker); -- cgit From 2254a7396a0ca6309854948ee1c0a33fa4268cec Mon Sep 17 00:00:00 2001 From: "Darrick J. Wong" Date: Tue, 2 May 2023 09:16:14 +1000 Subject: xfs: fix xfs_inodegc_stop racing with mod_delayed_work syzbot reported this warning from the faux inodegc shrinker that tries to kick off inodegc work: ------------[ cut here ]------------ WARNING: CPU: 1 PID: 102 at kernel/workqueue.c:1445 __queue_work+0xd44/0x1120 kernel/workqueue.c:1444 RIP: 0010:__queue_work+0xd44/0x1120 kernel/workqueue.c:1444 Call Trace: __queue_delayed_work+0x1c8/0x270 kernel/workqueue.c:1672 mod_delayed_work_on+0xe1/0x220 kernel/workqueue.c:1746 xfs_inodegc_shrinker_scan fs/xfs/xfs_icache.c:2212 [inline] xfs_inodegc_shrinker_scan+0x250/0x4f0 fs/xfs/xfs_icache.c:2191 do_shrink_slab+0x428/0xaa0 mm/vmscan.c:853 shrink_slab+0x175/0x660 mm/vmscan.c:1013 shrink_one+0x502/0x810 mm/vmscan.c:5343 shrink_many mm/vmscan.c:5394 [inline] lru_gen_shrink_node mm/vmscan.c:5511 [inline] shrink_node+0x2064/0x35f0 mm/vmscan.c:6459 kswapd_shrink_node mm/vmscan.c:7262 [inline] balance_pgdat+0xa02/0x1ac0 mm/vmscan.c:7452 kswapd+0x677/0xd60 mm/vmscan.c:7712 kthread+0x2e8/0x3a0 kernel/kthread.c:376 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:308 This warning corresponds to this code in __queue_work: /* * For a draining wq, only works from the same workqueue are * allowed. The __WQ_DESTROYING helps to spot the issue that * queues a new work item to a wq after destroy_workqueue(wq). */ if (unlikely(wq->flags & (__WQ_DESTROYING | __WQ_DRAINING) && WARN_ON_ONCE(!is_chained_work(wq)))) return; For this to trip, we must have a thread draining the inodedgc workqueue and a second thread trying to queue inodegc work to that workqueue. This can happen if freezing or a ro remount race with reclaim poking our faux inodegc shrinker and another thread dropping an unlinked O_RDONLY file: Thread 0 Thread 1 Thread 2 xfs_inodegc_stop xfs_inodegc_shrinker_scan xfs_is_inodegc_enabled xfs_clear_inodegc_enabled xfs_inodegc_queue_all xfs_inodegc_queue xfs_is_inodegc_enabled drain_workqueue llist_empty mod_delayed_work_on(..., 0) __queue_work In other words, everything between the access to inodegc_enabled state and the decision to poke the inodegc workqueue requires some kind of coordination to avoid the WQ_DRAINING state. We could perhaps introduce a lock here, but we could also try to eliminate WQ_DRAINING from the picture. We could replace the drain_workqueue call with a loop that flushes the workqueue and queues workers as long as there is at least one inode present in the per-cpu inodegc llists. We've disabled inodegc at this point, so we know that the number of queued inodes will eventually hit zero as long as xfs_inodegc_start cannot reactivate the workers. There are four callers of xfs_inodegc_start. Three of them come from the VFS with s_umount held: filesystem thawing, failed filesystem freezing, and the rw remount transition. The fourth caller is mounting rw (no remount or freezing possible). There are three callers ofs xfs_inodegc_stop. One is unmounting (no remount or thaw possible). Two of them come from the VFS with s_umount held: fs freezing and ro remount transition. Hence, it is correct to replace the drain_workqueue call with a loop that drains the inodegc llists. Fixes: 6191cf3ad59f ("xfs: flush inodegc workqueue tasks before cancel") Signed-off-by: Darrick J. Wong Reviewed-by: Dave Chinner Signed-off-by: Dave Chinner --- fs/xfs/xfs_icache.c | 32 +++++++++++++++++++++++++++----- 1 file changed, 27 insertions(+), 5 deletions(-) (limited to 'fs/xfs/xfs_icache.c') diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c index 4b63c065ef19..0f60e301eb1f 100644 --- a/fs/xfs/xfs_icache.c +++ b/fs/xfs/xfs_icache.c @@ -435,18 +435,23 @@ xfs_iget_check_free_state( } /* Make all pending inactivation work start immediately. */ -static void +static bool xfs_inodegc_queue_all( struct xfs_mount *mp) { struct xfs_inodegc *gc; int cpu; + bool ret = false; for_each_online_cpu(cpu) { gc = per_cpu_ptr(mp->m_inodegc, cpu); - if (!llist_empty(&gc->list)) + if (!llist_empty(&gc->list)) { mod_delayed_work_on(cpu, mp->m_inodegc_wq, &gc->work, 0); + ret = true; + } } + + return ret; } /* @@ -1911,24 +1916,41 @@ xfs_inodegc_flush( /* * Flush all the pending work and then disable the inode inactivation background - * workers and wait for them to stop. + * workers and wait for them to stop. Caller must hold sb->s_umount to + * coordinate changes in the inodegc_enabled state. */ void xfs_inodegc_stop( struct xfs_mount *mp) { + bool rerun; + if (!xfs_clear_inodegc_enabled(mp)) return; + /* + * Drain all pending inodegc work, including inodes that could be + * queued by racing xfs_inodegc_queue or xfs_inodegc_shrinker_scan + * threads that sample the inodegc state just prior to us clearing it. + * The inodegc flag state prevents new threads from queuing more + * inodes, so we queue pending work items and flush the workqueue until + * all inodegc lists are empty. IOWs, we cannot use drain_workqueue + * here because it does not allow other unserialized mechanisms to + * reschedule inodegc work while this draining is in progress. + */ xfs_inodegc_queue_all(mp); - drain_workqueue(mp->m_inodegc_wq); + do { + flush_workqueue(mp->m_inodegc_wq); + rerun = xfs_inodegc_queue_all(mp); + } while (rerun); trace_xfs_inodegc_stop(mp, __return_address); } /* * Enable the inode inactivation background workers and schedule deferred inode - * inactivation work if there is any. + * inactivation work if there is any. Caller must hold sb->s_umount to + * coordinate changes in the inodegc_enabled state. */ void xfs_inodegc_start( -- cgit