There is a lock inversion and rwsem read-lock recursion in the devfreq target callback which can lead to deadlocks. Specifically, ufshcd_devfreq_scale() already holds a clk_scaling_lock read lock when toggling the write booster, which involves taking the dev_cmd mutex before taking another clk_scaling_lock read lock. This can lead to a deadlock if another thread: 1) tries to acquire the dev_cmd and clk_scaling locks in the correct order, or 2) takes a clk_scaling write lock before the attempt to take the clk_scaling read lock a second time. Fix this by dropping the clk_scaling_lock before toggling the write booster as was done before commit 0e9d4ca ("scsi: ufs: Protect some contexts from unexpected clock scaling"). While the devfreq callbacks are already serialised, add a second serialising mutex to handle the unlikely case where a callback triggered through the devfreq sysfs interface is racing with a request to disable clock scaling through the UFS controller 'clkscale_enable' sysfs attribute. This could otherwise lead to the write booster being left disabled after having disabled clock scaling. Also take the new mutex in ufshcd_clk_scaling_allow() to make sure that any pending write booster update has completed on return. Note that this currently only affects Qualcomm platforms since commit 87bd050 ("scsi: ufs: core: Allow host driver to disable wb toggling during clock scaling"). The lock inversion (i.e. 1 above) was reported by lockdep as: ====================================================== WARNING: possible circular locking dependency detected 6.1.0-next-20221216 #211 Not tainted ------------------------------------------------------ kworker/u16:2/71 is trying to acquire lock: ffff076280ba98a0 (&hba->dev_cmd.lock){+.+.}-{3:3}, at: ufshcd_query_flag+0x50/0x1c0 but task is already holding lock: ffff076280ba9cf0 (&hba->clk_scaling_lock){++++}-{3:3}, at: ufshcd_devfreq_scale+0x2b8/0x380 which lock already depends on the new lock. [ +0.011606] the existing dependency chain (in reverse order) is: -> #1 (&hba->clk_scaling_lock){++++}-{3:3}: lock_acquire+0x68/0x90 down_read+0x58/0x80 ufshcd_exec_dev_cmd+0x70/0x2c0 ufshcd_verify_dev_init+0x68/0x170 ufshcd_probe_hba+0x398/0x1180 ufshcd_async_scan+0x30/0x320 async_run_entry_fn+0x34/0x150 process_one_work+0x288/0x6c0 worker_thread+0x74/0x450 kthread+0x118/0x120 ret_from_fork+0x10/0x20 -> #0 (&hba->dev_cmd.lock){+.+.}-{3:3}: __lock_acquire+0x12a0/0x2240 lock_acquire.part.0+0xcc/0x220 lock_acquire+0x68/0x90 __mutex_lock+0x98/0x430 mutex_lock_nested+0x2c/0x40 ufshcd_query_flag+0x50/0x1c0 ufshcd_query_flag_retry+0x64/0x100 ufshcd_wb_toggle+0x5c/0x120 ufshcd_devfreq_scale+0x2c4/0x380 ufshcd_devfreq_target+0xf4/0x230 devfreq_set_target+0x84/0x2f0 devfreq_update_target+0xc4/0xf0 devfreq_monitor+0x38/0x1f0 process_one_work+0x288/0x6c0 worker_thread+0x74/0x450 kthread+0x118/0x120 ret_from_fork+0x10/0x20 other info that might help us debug this: Possible unsafe locking scenario: CPU0 CPU1 ---- ---- lock(&hba->clk_scaling_lock); lock(&hba->dev_cmd.lock); lock(&hba->clk_scaling_lock); lock(&hba->dev_cmd.lock); *** DEADLOCK *** Fixes: 0e9d4ca ("scsi: ufs: Protect some contexts from unexpected clock scaling") Cc: stable@vger.kernel.org # 5.12 Cc: Can Guo <quic_cang@quicinc.com> Tested-by: Andrew Halaney <ahalaney@redhat.com> Signed-off-by: Johan Hovold <johan+linaro@kernel.org> Reviewed-by: Bart Van Assche <bvanassche@acm.org> Link: https://lore.kernel.org/r/20230116161201.16923-1-johan+linaro@kernel.org Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Add an # Invariants section to ThisModule's doc comments #211

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Add an `# Invariants` section to `ThisModule`'s doc comments #211