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btrfs: zstd: fix lost wakeup when waiting for a workspace

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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: 3f93aef535 ("btrfs: add zstd compression level support")
Assisted-by: Claude:claude-opus-5
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: FAN YE <fy15309206903@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
FAN YE 2026-08-21 17:50:09 +00:00 committed by David Sterba
parent cacf358322
commit 2acb9f3d1c
1 changed files with 10 additions and 1 deletions
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11
fs/btrfs/zstd.c
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@ -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;
}