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Sat, 22 Aug 2026 01:01:32 -0700 (PDT) Message-ID: <217f5236-e34d-46bb-97d7-2429f7a74ff6@suse.com> Date: Sat, 22 Aug 2026 17:31:26 +0930 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] btrfs: zstd: fix lost wakeup when waiting for a workspace To: fy15309206903@gmail.com, Nick Terrell , David Sterba , Chris Mason Cc: linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org References: <20260821-btrfs-zstd-lost-wakeup-v1-1-84f358d4ea67@gmail.com> Content-Language: en-US From: Qu Wenruo Autocrypt: addr=wqu@suse.com; keydata= xsBNBFnVga8BCACyhFP3ExcTIuB73jDIBA/vSoYcTyysFQzPvez64TUSCv1SgXEByR7fju3o 8RfaWuHCnkkea5luuTZMqfgTXrun2dqNVYDNOV6RIVrc4YuG20yhC1epnV55fJCThqij0MRL 1NxPKXIlEdHvN0Kov3CtWA+R1iNN0RCeVun7rmOrrjBK573aWC5sgP7YsBOLK79H3tmUtz6b 9Imuj0ZyEsa76Xg9PX9Hn2myKj1hfWGS+5og9Va4hrwQC8ipjXik6NKR5GDV+hOZkktU81G5 gkQtGB9jOAYRs86QG/b7PtIlbd3+pppT0gaS+wvwMs8cuNG+Pu6KO1oC4jgdseFLu7NpABEB AAHNGFF1IFdlbnJ1byA8d3F1QHN1c2UuY29tPsLAlAQTAQgAPgIbAwULCQgHAgYVCAkKCwIE FgIDAQIeAQIXgBYhBC3fcuWlpVuonapC4cI9kfOhJf6oBQJnEXVgBQkQ/lqxAAoJEMI9kfOh Jf6o+jIH/2KhFmyOw4XWAYbnnijuYqb/obGae8HhcJO2KIGcxbsinK+KQFTSZnkFxnbsQ+VY fvtWBHGt8WfHcNmfjdejmy9si2jyy8smQV2jiB60a8iqQXGmsrkuR+AM2V360oEbMF3gVvim 2VSX2IiW9KERuhifjseNV1HLk0SHw5NnXiWh1THTqtvFFY+CwnLN2GqiMaSLF6gATW05/sEd V17MdI1z4+WSk7D57FlLjp50F3ow2WJtXwG8yG8d6S40dytZpH9iFuk12Sbg7lrtQxPPOIEU rpmZLfCNJJoZj603613w/M8EiZw6MohzikTWcFc55RLYJPBWQ+9puZtx1DopW2jOwE0EWdWB rwEIAKpT62HgSzL9zwGe+WIUCMB+nOEjXAfvoUPUwk+YCEDcOdfkkM5FyBoJs8TCEuPXGXBO Cl5P5B8OYYnkHkGWutAVlUTV8KESOIm/KJIA7jJA+Ss9VhMjtePfgWexw+P8itFRSRrrwyUf E+0WcAevblUi45LjWWZgpg3A80tHP0iToOZ5MbdYk7YFBE29cDSleskfV80ZKxFv6koQocq0 vXzTfHvXNDELAuH7Ms/WJcdUzmPyBf3Oq6mKBBH8J6XZc9LjjNZwNbyvsHSrV5bgmu/THX2n g/3be+iqf6OggCiy3I1NSMJ5KtR0q2H2Nx2Vqb1fYPOID8McMV9Ll6rh8S8AEQEAAcLAfAQY AQgAJgIbDBYhBC3fcuWlpVuonapC4cI9kfOhJf6oBQJnEXWBBQkQ/lrSAAoJEMI9kfOhJf6o cakH+QHwDszsoYvmrNq36MFGgvAHRjdlrHRBa4A1V1kzd4kOUokongcrOOgHY9yfglcvZqlJ qfa4l+1oxs1BvCi29psteQTtw+memmcGruKi+YHD7793zNCMtAtYidDmQ2pWaLfqSaryjlzR /3tBWMyvIeWZKURnZbBzWRREB7iWxEbZ014B3gICqZPDRwwitHpH8Om3eZr7ygZck6bBa4MU o1XgbZcspyCGqu1xF/bMAY2iCDcq6ULKQceuKkbeQ8qxvt9hVxJC2W3lHq8dlK1pkHPDg9wO JoAXek8MF37R8gpLoGWl41FIUb3hFiu3zhDDvslYM4BmzI18QgQTQnotJH8= In-Reply-To: <20260821-btrfs-zstd-lost-wakeup-v1-1-84f358d4ea67@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit 在 2026/8/22 03:20, FAN YE via B4 Relay 写道: > From: FAN YE > > 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 > --- > 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; This doesn't work. Thread A (last ws holder) | Thread B (new ws user) -----------------------------------+-------------------------------- zstd_put_workspace() | | | zstd_get_workspace() | | |- zstd_find_workspace() | | | No WS found | | | | | |- zstd_alloc_workspace() | | | Allocation failure | | | | | |- zstd_find_workspace() | | | Still no WS found |- spin_lock_bh() | | |- spin_unlock_bh() | | |- cond_wake_up() | | No one is waiting | |- prepare_to_wait() | |- schedule() | |- finish_wait() This is the typical LLM behavior, if your human brain can not examine every possibly wrong conclusion/fix from LLM, LLM will only waste time of everyone. > > goto again; > } > > --- > base-commit: 531ed942bb0df04f6747983fecdedce76a22d07f > change-id: 20260821-btrfs-zstd-lost-wakeup-0b0ee88ed52f > > Best regards, > -- > FAN YE > > >