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mm: shrinkers: fix race condition on debugfs cleanup

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When something registers and unregisters many shrinkers, such as:
    for x in $(seq 10000); do unshare -Ui true; done

Sometimes the following error is printed to the kernel log:
    debugfs: Directory '...' with parent 'shrinker' already present!

This occurs since commit badc28d492 ("mm: shrinkers: fix deadlock in
shrinker debugfs") / v6.2: Since the call to `debugfs_remove_recursive`
was moved outside the `shrinker_rwsem`/`shrinker_mutex` lock, but the call
to `ida_free` stayed inside, a newly registered shrinker can be
re-assigned that ID and attempt to create the debugfs directory before the
directory from the previous shrinker has been removed.

The locking changes in commit f95bdb700b ("mm: vmscan: make global slab
shrink lockless") made the race condition more likely, though it existed
before then.

Commit badc28d492 ("mm: shrinkers: fix deadlock in shrinker debugfs")
could be reverted since the issue is addressed should no longer occur
since the count and scan operations are lockless since commit 20cd1892fc
("mm: shrinkers: make count and scan in shrinker debugfs lockless"). 
However, since this is a contended lock, prefer instead moving `ida_free`
outside the lock to avoid the race.

Link: https://lkml.kernel.org/r/20230503013232.299211-1-joanbrugueram@gmail.com
Fixes: badc28d492 ("mm: shrinkers: fix deadlock in shrinker debugfs")
Signed-off-by: Joan Bruguera Micó <joanbrugueram@gmail.com>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Joan Bruguera Micó
2023-05-03 01:32:32 +00:00
committed by Andrew Morton
parent 0257d9908d
commit 26e239b37e
3 changed files with 24 additions and 9 deletions
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15
mm/shrinker_debug.c
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@@ -237,7 +237,8 @@ int shrinker_debugfs_rename(struct shrinker *shrinker, const char *fmt, ...)
}
EXPORT_SYMBOL(shrinker_debugfs_rename);
struct dentry *shrinker_debugfs_remove(struct shrinker *shrinker)
struct dentry *shrinker_debugfs_detach(struct shrinker *shrinker,
int *debugfs_id)
{
struct dentry *entry = shrinker->debugfs_entry;
@@ -246,14 +247,18 @@ struct dentry *shrinker_debugfs_remove(struct shrinker *shrinker)
kfree_const(shrinker->name);
shrinker->name = NULL;
if (entry) {
ida_free(&shrinker_debugfs_ida, shrinker->debugfs_id);
shrinker->debugfs_entry = NULL;
}
*debugfs_id = entry ? shrinker->debugfs_id : -1;
shrinker->debugfs_entry = NULL;
return entry;
}
void shrinker_debugfs_remove(struct dentry *debugfs_entry, int debugfs_id)
{
debugfs_remove_recursive(debugfs_entry);
ida_free(&shrinker_debugfs_ida, debugfs_id);
}
static int __init shrinker_debugfs_init(void)
{
struct shrinker *shrinker;