From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mta0.migadu.com (out-135.mta0.migadu.com [91.218.175.135]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 936A048C8B9 for ; Mon, 28 Sep 2026 09:35:41 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.135 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790588143; cv=none; b=lV9NaQBlvdw/pxpD2wpXZAvk43YGPWXTnIjzXwv+LviX1K2Z9+Y93hUHTsRBv+N9X3FFwU0Kl7FdNF0iY8yJdjCYuChaPFVqvwx3k4Q8bwenwvd9jbdk2iLAqSav7LpLpI377rnLPAp2Yde8S2o9EEbUPueomGWUBO6vXVhUcSM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790588143; c=relaxed/simple; bh=43dZCg58dMILR3vgkjmwNdQV11XqU4A288rUAQkby84=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=Q4XnSgUpdyEfb6n3YMS8pr/glLWCbxAXW7rf+md7R6fPiwH5jCw3ZA8LdxzswADqJS9vzD6Lk1VT31IuTw9pMaXU60UI9M4VCHUg9FfJmsz566zfR3Vko6GKjy/HB4vejr25lmw12NpfiWmT8XLHlHMdRkMY0TPRLDCGUtZMuM0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=vaSmv1Jp; arc=none smtp.client-ip=91.218.175.135 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="vaSmv1Jp" X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=43dZCg58dMILR3vgkjmwNdQV11XqU4A288rUAQkby84=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1790588139; v=1; x=1791192939; b=vaSmv1JpklyYH/aPygnibnmOJrNB+HLC/v4vykkhUAnkOgB2Tc/SSQd7RjdE0SeZgbaOVrYH lu8nUm3LuCOjGAHxt0K8k33e76CMfYA0LVskD/I86UjxVliLI+182vsymbaEzmnMfPGS1Td5tJZ WmbXCO3pHOq2yFjwfuntTgVk= X-Envelope-To: linux-kernel@vger.kernel.org Received: by smtp.migadu.com with ESMTPS id 934e24e1a4cbc90c; Mon, 28 Sep 2026 09:35:26 +0000 X-Mizu-Trace-ID: 934e24e1a4cbc90c X-Migadu-Flow: FLOW_OUT From: Tao Cui To: Bart Van Assche , axboe@kernel.dk, hch@lst.de, Tetsuo Handa Cc: cuitao@kylinos.cn, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, cui.tao@linux.dev Subject: [PATCH v6] loop: defer the queue limits clear to a workqueue Date: Mon, 28 Sep 2026 17:35:12 +0800 Message-ID: <20260928093512.3153646-1-cui.tao@linux.dev> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Tao Cui loop_clear_limits() calls queue_limits_commit_update() directly from the loop workqueue that processes the request. That does a non-atomic struct assignment to q->limits without freezing the queue, which races with lockless readers of q->limits on other CPUs - bio splitting reads max_hw_sectors, the discard path reads max_hw_discard_sectors - and can let them observe torn values. The trigger is a discard or write-zeroes request on a loop device whose backing file does not support the corresponding fallocate operation. The code already has an XXX comment saying this should move to a workqueue. Do that: schedule a work item on the system workqueue, where it is safe to freeze the queue around the limits update. The pending modes live under a new mutex, lo->clear_limits_lock. loop_change_fd() and __loop_clr_fd() cancel the work item before changing or dropping the backing file, and loop_assign_backing_file() resets the modes, so a stale clear cannot hit the new backing file. The work item is also cancelled before the device is freed. Suggested-by: Bart Van Assche Signed-off-by: Tao Cui --- Changes since v5: - Drop the rebind generation counters and instead cancel the work item from loop_change_fd() after the queue freeze and from __loop_clr_fd(), as suggested by Bart. This leaves no work item behind when the backing file changes, and the mutex now only protects clear_limits_mode. Changes since v4: - Rebase onto the current block tree as a single commit. - Hold clear_limits_lock over the limits commit in the work item. The queue freeze is counted, not mutually exclusive, so loop_change_fd() could rebind between the generation check and the commit, and a stale clear could disable discard and write zeroes on the new backing file. Changes since v3: - Replace the three atomic variables (clear_limits_mode, rebind_gen, clear_limits_gen) with plain variables protected by a new clear_limits_lock mutex, as suggested by Bart. The mutex is taken after the queue freeze in the work item, which keeps the same freeze -> mutex ordering as loop_change_fd(), the only rebinding path that freezes the queue. Changes since v2: - Skip the clear when the device was rebound since the work item was scheduled: the modes used to be captured before the freeze, so a LOOP_CHANGE_FD completing in between could apply the old modes to the new backing file. The rebind generation is now checked with the queue frozen, which excludes loop_change_fd() because it assigns the new backing file under the same freeze, so a rebind cannot slip in between the check and the commit. - Also bump the rebind generation from __loop_clr_fd(): unbinding does not go through loop_assign_backing_file(), so a work item scheduled before the last close could otherwise commit a stale clear to the queue limits of the unbound device. Changes since v1: - Reset clear_limits_mode when assigning a new backing file, so stale modes do not clear limits of the new file. - Cancel the work item from loop_remove() before del_gendisk(): the queue can already be in RCU-delayed freeing when lo_free_disk() cancels it. Tested on x86-64 (qemu, vfat-backed loop device): a 30s discard and reconfigure loop exercises the clear path 287 times, no torn sysfs reads, no difference against the unpatched kernel. Link: https://lore.kernel.org/r/20260828072004.273519-1-cui.tao@linux.dev/ --- drivers/block/loop.c | 72 ++++++++++++++++++++++++++++++++++++++------ 1 file changed, 63 insertions(+), 9 deletions(-) diff --git a/drivers/block/loop.c b/drivers/block/loop.c index 758c20678bf6c..92f3ae7324751 100644 --- a/drivers/block/loop.c +++ b/drivers/block/loop.c @@ -67,6 +67,9 @@ struct loop_device { struct list_head rootcg_cmd_list; struct list_head idle_worker_list; struct rb_root worker_tree; + struct work_struct clear_limits_work; + struct mutex clear_limits_lock; + unsigned int clear_limits_mode; struct timer_list timer; bool sysfs_inited; @@ -222,9 +225,25 @@ static void loop_set_size(struct loop_device *lo, loff_t size) kobject_uevent(&disk_to_dev(lo->lo_disk)->kobj, KOBJ_CHANGE); } -static void loop_clear_limits(struct loop_device *lo, int mode) +static void loop_clear_limits_workfn(struct work_struct *work) { + struct loop_device *lo = + container_of(work, struct loop_device, clear_limits_work); struct queue_limits lim = queue_limits_start_update(lo->lo_queue); + unsigned int memflags; + int mode = 0; + + /* + * A rebind cannot race with this work item: loop_change_fd() + * and __loop_clr_fd() cancel it, loop_configure() only runs on + * an unbound device, on which no work item can be pending, and + * loop_assign_backing_file() resets the accumulated modes. + */ + memflags = blk_mq_freeze_queue(lo->lo_queue); + mutex_lock(&lo->clear_limits_lock); + mode = lo->clear_limits_mode; + lo->clear_limits_mode = 0; + mutex_unlock(&lo->clear_limits_lock); if (mode & FALLOC_FL_ZERO_RANGE) lim.max_write_zeroes_sectors = 0; @@ -233,15 +252,16 @@ static void loop_clear_limits(struct loop_device *lo, int mode) lim.max_hw_discard_sectors = 0; lim.discard_granularity = 0; } - - /* - * XXX: this updates the queue limits without freezing the queue, which - * is against the locking protocol and dangerous. But we can't just - * freeze the queue as we're inside the ->queue_rq method here. So this - * should move out into a workqueue unless we get the file operations to - * advertise if they support specific fallocate operations. - */ queue_limits_commit_update(lo->lo_queue, &lim); + blk_mq_unfreeze_queue(lo->lo_queue, memflags); +} + +static void loop_clear_limits(struct loop_device *lo, int mode) +{ + mutex_lock(&lo->clear_limits_lock); + lo->clear_limits_mode |= mode; + mutex_unlock(&lo->clear_limits_lock); + schedule_work(&lo->clear_limits_work); } static int lo_fallocate(struct loop_device *lo, struct request *rq, loff_t pos, @@ -518,6 +538,14 @@ static int loop_validate_file(struct file *file, struct block_device *bdev) static void loop_assign_backing_file(struct loop_device *lo, struct file *file) { lo->lo_backing_file = file; + /* + * Discard any modes recorded against the old backing file. The + * paths that can race with a pending work item cancel it before + * they get here. + */ + mutex_lock(&lo->clear_limits_lock); + lo->clear_limits_mode = 0; + mutex_unlock(&lo->clear_limits_lock); lo->old_gfp_mask = mapping_gfp_mask(file->f_mapping); mapping_set_gfp_mask(file->f_mapping, lo->old_gfp_mask & ~(__GFP_IO | __GFP_FS)); @@ -602,6 +630,12 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev, /* and ... switch */ disk_force_media_change(lo->lo_disk); memflags = blk_mq_freeze_queue(lo->lo_queue); + /* + * The freeze drained the in-flight requests, so any clear they + * scheduled is pending or running now; cancelling here leaves + * no clear behind for the new backing file. + */ + cancel_work_sync(&lo->clear_limits_work); mapping_set_gfp_mask(old_file->f_mapping, lo->old_gfp_mask); loop_assign_backing_file(lo, file); loop_update_dio(lo); @@ -1148,6 +1182,16 @@ static void __loop_clr_fd(struct loop_device *lo) lo->lo_backing_file = NULL; spin_unlock_irq(&lo->lo_lock); + /* + * Invalidate any clear that was scheduled against the old backing + * file. Unbinding cannot race with in-flight I/O, so cancelling + * here leaves no work item behind. + */ + cancel_work_sync(&lo->clear_limits_work); + mutex_lock(&lo->clear_limits_lock); + lo->clear_limits_mode = 0; + mutex_unlock(&lo->clear_limits_lock); + lo->lo_device = NULL; lo->lo_offset = 0; lo->lo_sizelimit = 0; @@ -1783,7 +1827,9 @@ static void lo_free_disk(struct gendisk *disk) destroy_workqueue(lo->workqueue); loop_free_idle_workers(lo, true); timer_shutdown_sync(&lo->timer); + cancel_work_sync(&lo->clear_limits_work); mutex_destroy(&lo->lo_mutex); + mutex_destroy(&lo->clear_limits_lock); kfree(lo); } @@ -2102,6 +2148,8 @@ static int loop_add(int i) spin_lock_init(&lo->lo_lock); spin_lock_init(&lo->lo_work_lock); INIT_WORK(&lo->rootcg_work, loop_rootcg_workfn); + INIT_WORK(&lo->clear_limits_work, loop_clear_limits_workfn); + mutex_init(&lo->clear_limits_lock); INIT_LIST_HEAD(&lo->rootcg_cmd_list); disk->major = LOOP_MAJOR; disk->first_minor = i << part_shift; @@ -2140,6 +2188,12 @@ static int loop_add(int i) static void loop_remove(struct loop_device *lo) { + /* + * Cancel early: the queue may already be in RCU-delayed freeing + * by the time lo_free_disk() cancels the work item. + */ + cancel_work_sync(&lo->clear_limits_work); + /* Make this loop device unreachable from pathname. */ del_gendisk(lo->lo_disk); blk_mq_free_tag_set(&lo->tag_set); -- 2.43.0