The patch titled Subject: mm: shrinkers: fix deadlock in shrinker debugfs has been added to the -mm mm-unstable branch. Its filename is mm-shrinkers-fix-deadlock-in-shrinker-debugfs.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Qi Zheng zhengqi.arch@bytedance.com Subject: mm: shrinkers: fix deadlock in shrinker debugfs Date: Thu, 2 Feb 2023 18:56:12 +0800

The debugfs_remove_recursive() is invoked by unregister_shrinker(), which is holding the write lock of shrinker_rwsem. It will waits for the handler of debugfs file complete. The handler also needs to hold the read lock of shrinker_rwsem to do something. So it may cause the following deadlock:

CPU0 CPU1

debugfs_file_get() shrinker_debugfs_count_show()/shrinker_debugfs_scan_write()

unregister_shrinker() --> down_write(&shrinker_rwsem); debugfs_remove_recursive() // wait for (A) --> wait_for_completion();

// wait for (B) --> down_read_killable(&shrinker_rwsem) debugfs_file_put() -- (A)

up_write() -- (B)

The down_read_killable() can be killed, so that the above deadlock can be recovered. But it still requires an extra kill action, otherwise it will block all subsequent shrinker-related operations, so it's better to fix it.

Link: https://lkml.kernel.org/r/20230202105612.64641-1-zhengqi.arch@bytedance.com Fixes: 5035ebc644ae ("mm: shrinkers: introduce debugfs interface for memory shrinkers") Signed-off-by: Qi Zheng zhengqi.arch@bytedance.com Cc: Roman Gushchin roman.gushchin@linux.dev Cc: Kent Overstreet kent.overstreet@gmail.com Cc: Muchun Song songmuchun@bytedance.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

--- a/include/linux/shrinker.h~mm-shrinkers-fix-deadlock-in-shrinker-debugfs +++ a/include/linux/shrinker.h @@ -107,7 +107,7 @@ extern void synchronize_shrinkers(void);

#ifdef CONFIG_SHRINKER_DEBUG extern int shrinker_debugfs_add(struct shrinker *shrinker); -extern void shrinker_debugfs_remove(struct shrinker *shrinker); +extern struct dentry *shrinker_debugfs_remove(struct shrinker *shrinker); extern int __printf(2, 3) shrinker_debugfs_rename(struct shrinker *shrinker, const char *fmt, ...); #else /* CONFIG_SHRINKER_DEBUG */ @@ -115,7 +115,7 @@ static inline int shrinker_debugfs_add(s { return 0; } -static inline void shrinker_debugfs_remove(struct shrinker *shrinker) +static inline struct dentry *shrinker_debugfs_remove(struct shrinker *shrinker) { } static inline __printf(2, 3) --- a/mm/shrinker_debug.c~mm-shrinkers-fix-deadlock-in-shrinker-debugfs +++ a/mm/shrinker_debug.c @@ -246,18 +246,21 @@ int shrinker_debugfs_rename(struct shrin } EXPORT_SYMBOL(shrinker_debugfs_rename);

-void shrinker_debugfs_remove(struct shrinker *shrinker) +struct dentry *shrinker_debugfs_remove(struct shrinker *shrinker) { + struct dentry *entry = shrinker->debugfs_entry; + lockdep_assert_held(&shrinker_rwsem);

kfree_const(shrinker->name); shrinker->name = NULL;

- if (!shrinker->debugfs_entry) - return; + if (entry) { + ida_free(&shrinker_debugfs_ida, shrinker->debugfs_id); + shrinker->debugfs_entry = NULL; + }

- debugfs_remove_recursive(shrinker->debugfs_entry); - ida_free(&shrinker_debugfs_ida, shrinker->debugfs_id); + return entry; }

static int __init shrinker_debugfs_init(void) --- a/mm/vmscan.c~mm-shrinkers-fix-deadlock-in-shrinker-debugfs +++ a/mm/vmscan.c @@ -747,6 +747,8 @@ EXPORT_SYMBOL(register_shrinker); */ void unregister_shrinker(struct shrinker *shrinker) { + struct dentry *debugfs_entry; + if (!(shrinker->flags & SHRINKER_REGISTERED)) return;

@@ -755,9 +757,11 @@ void unregister_shrinker(struct shrinker shrinker->flags &= ~SHRINKER_REGISTERED; if (shrinker->flags & SHRINKER_MEMCG_AWARE) unregister_memcg_shrinker(shrinker); - shrinker_debugfs_remove(shrinker); + debugfs_entry = shrinker_debugfs_remove(shrinker); up_write(&shrinker_rwsem);

+ debugfs_remove_recursive(debugfs_entry); + kfree(shrinker->nr_deferred); shrinker->nr_deferred = NULL; } _

Patches currently in -mm which might be from zhengqi.arch@bytedance.com are

mm-shrinkers-fix-deadlock-in-shrinker-debugfs.patch