From 2acb9f3d1cc8f65dc81ed55e238cbf8e5b60bff7 Mon Sep 17 00:00:00 2001 From: FAN YE Date: Fri, 21 Aug 2026 17:50:09 +0000 Subject: [PATCH] btrfs: zstd: fix lost wakeup when waiting for a workspace A writer can sleep forever in zstd_get_workspace() even though a workspace is free. When zstd_alloc_workspace() fails, the task is queued on zwsm->wait and schedules unconditionally, never re-testing the pool. zstd_put_workspace() publishes the workspace and then calls cond_wake_up(), which only wakes when a sleeper is already visible, so a workspace returned between the failed allocation and prepare_to_wait() wakes nobody. The window is wide: zstd_alloc_workspace() goes through kvmalloc() and may enter reclaim. Only a max level workspace triggers the wakeup and one is deliberately kept allocated as the fallback every waiter waits for, so once its wakeup is lost the writer stays in TASK_UNINTERRUPTIBLE until some other task happens to return one. Re-check the pool after prepare_to_wait() has published the waiter, and use the workspace if one turned up. Fixes: 3f93aef535c8 ("btrfs: add zstd compression level support") Assisted-by: Claude:claude-opus-5 Reviewed-by: Qu Wenruo Signed-off-by: FAN YE Signed-off-by: David Sterba --- fs/btrfs/zstd.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) diff --git a/fs/btrfs/zstd.c b/fs/btrfs/zstd.c index 86919293fd54..58d9ff76fe07 100644 --- a/fs/btrfs/zstd.c +++ b/fs/btrfs/zstd.c @@ -307,8 +307,17 @@ struct list_head *zstd_get_workspace(struct btrfs_fs_info *fs_info, int level) DEFINE_WAIT(wait); prepare_to_wait(&zwsm->wait, &wait, TASK_UNINTERRUPTIBLE); - schedule(); + /* + * Re-check after being queued: zstd_put_workspace() only wakes + * a queue that already has a sleeper, so a workspace returned + * since the failed allocation woke nobody. + */ + ws = zstd_find_workspace(fs_info, level); + if (!ws) + schedule(); finish_wait(&zwsm->wait, &wait); + if (ws) + return ws; goto again; }