[PATCH] btrfs: zstd: fix lost wakeup when waiting for a workspace
FAN YE <[email protected]>
| Newsgroups | org.kernel.feeds.b4-sent,org.kernel.vger.linux-btrfs,org.kernel.vger.linux-kernel |
|---|---|
| Message-ID | <[email protected]> |
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
Signed-off-by: FAN YE <[email protected]>
---
Reproduced under QEMU/TCG: CONFIG_FAULT_INJECTION_STACKTRACE_FILTER forces
zstd_alloc_workspace() to fail exactly once and widens the pre-wait window
to 400ms while six concurrent zstd:15 writers race it. Unpatched, a
btrfs-delalloc kworker hangs in zstd_get_workspace()'s schedule() (hung_task
warning, >120s); the identical race against the patched code does not hang.
Compile-tested (W=1, x86_64 defconfig + CONFIG_BTRFS_FS=y).
---
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..cb15cbd737c4 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;
}
---
base-commit: 531ed942bb0df04f6747983fecdedce76a22d07f
change-id: 20260821-btrfs-zstd-lost-wakeup-0b0ee88ed52f
Best regards,
--
FAN YE <[email protected]>