Ziqi Chen 77691af484 scsi: ufs: core: Quiesce request queues before checking pending cmds ... In ufshcd_clock_scaling_prepare(), after SCSI layer is blocked, ufshcd_pending_cmds() is called to check whether there are pending transactions or not. And only if there are no pending transactions can we proceed to kickstart the clock scaling sequence. ufshcd_pending_cmds() traverses over all SCSI devices and calls sbitmap_weight() on their budget_map. sbitmap_weight() can be broken down to three steps: 1. Calculate the nr outstanding bits set in the 'word' bitmap. 2. Calculate the nr outstanding bits set in the 'cleared' bitmap. 3. Subtract the result from step 1 by the result from step 2. This can lead to a race condition as outlined below: Assume there is one pending transaction in the request queue of one SCSI device, say sda, and the budget token of this request is 0, the 'word' is 0x1 and the 'cleared' is 0x0. 1. When step 1 executes, it gets the result as 1. 2. Before step 2 executes, block layer tries to dispatch a new request to sda. Since the SCSI layer is blocked, the request cannot pass through SCSI but the block layer would do budget_get() and budget_put() to sda's budget map regardless, so the 'word' has become 0x3 and 'cleared' has become 0x2 (assume the new request got budget token 1). 3. When step 2 executes, it gets the result as 1. 4. When step 3 executes, it gets the result as 0, meaning there is no pending transactions, which is wrong. Thread A Thread B ufshcd_pending_cmds() __blk_mq_sched_dispatch_requests() | | sbitmap_weight(word) | | scsi_mq_get_budget() | | | scsi_mq_put_budget() | | sbitmap_weight(cleared) ... When this race condition happens, the clock scaling sequence is started with transactions still in flight, leading to subsequent hibernate enter failure, broken link, task abort and back to back error recovery. Fix this race condition by quiescing the request queues before calling ufshcd_pending_cmds() so that block layer won't touch the budget map when ufshcd_pending_cmds() is working on it. In addition, remove the SCSI layer blocking/unblocking to reduce redundancies and latencies. Fixes: 8d077ede48 ("scsi: ufs: Optimize the command queueing code") Co-developed-by: Can Guo <quic_cang@quicinc.com> Signed-off-by: Can Guo <quic_cang@quicinc.com> Signed-off-by: Ziqi Chen <quic_ziqichen@quicinc.com> Link: https://lore.kernel.org/r/1717754818-39863-1-git-send-email-quic_ziqichen@quicinc.com Reviewed-by: Bart Van Assche <bvanassche@acm.org> Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>		2024-06-11 21:22:33 -04:00
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core	scsi: ufs: core: Quiesce request queues before checking pending cmds	2024-06-11 21:22:33 -04:00
host	SCSI misc on 20240514	2024-05-14 18:25:53 -07:00
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