On 9/8/20 12:02 AM, James Bottomley wrote: > On Mon, 2020-09-07 at 23:02 +0200, Tomas Henzl wrote: >> On 9/7/20 10:24 PM, James Bottomley wrote: >>> On Mon, 2020-09-07 at 22:09 +0200, Tomas Henzl wrote: >>>> On 9/7/20 7:46 PM, James Bottomley wrote: >>>>> On Mon, 2020-09-07 at 17:47 +0200, Tomas Henzl wrote: >>>>>> During an async scan the driver shost->hostt structures are >>>>>> used, that may cause issues when the driver is removed at >>>>>> that time. As protection take the module reference. >>>>> >>>>> Can I just ask what issues? Today, our module model is that >>>>> scsi_device_get() bumps the module refcount and therefore makes >>>>> the module ineligible to be removed. scsi_host_get() doesn't >>>>> do this because the way the host model is supposed to be coded, >>>>> we can call remove at any time but the module won't get freed >>>>> until the last put of the host. I can see we have a potential >>>>> problem with scsi_forget_host() racing with the async scan >>>>> thread ... is that what you see? What's supposed to happen is >>>>> that scsi_device_get() starts failing as soon as the module >>>>> begins it's exit routine, so if a scan is in progress, it can't >>>>> add any new devices ... in theory this means that the list is >>>>> stable for scsi_forget_host(), so knowing how that assumption >>>>> is breaking would be useful. >>>> >>>> I think that the problem is that async scan uses callbacks to the >>>> module and when the module is being removed during scan it is not >>>> protected. >>> >>> As I said above: the module shouldn't be freed until the scans are >>> completed or aborted ... I don't think we have a use after free >>> problem. What you show below seems to be a deadlock: >>> >>>> modprobe mpt3sas && rmmod mpt3sas >>>> >>>> [ 370.031614] INFO: task rmmod:3120 blocked for more than 120 >>>> seconds. >>>> [ 370.037967] Not tainted 4.18.0-193.el8.x86_64 #1 >>>> [ 370.043105] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" >>>> disables this message. >>>> [ 370.050931] rmmod D 0 3120 2460 0x00004080 >>>> [ 370.056414] Call Trace: >>>> [ 370.058889] ? __schedule+0x24f/0x650 >>>> [ 370.062554] schedule+0x2f/0xa0 >>>> [ 370.065738] async_synchronize_cookie_domain+0xad/0x140 >>>> [ 370.070983] ? finish_wait+0x80/0x80 >>>> [ 370.074580] __x64_sys_delete_module+0x166/0x280 >>>> [ 370.079198] do_syscall_64+0x5b/0x1a0 >>>> [ 370.082876] entry_SYSCALL_64_after_hwframe+0x65/0xca >>>> [ 370.087946] RIP: 0033:0x7f6de460a7db >>>> [ 370.091534] Code: Bad RIP value. >>>> [ 370.094777] RSP: 002b:00007ffe9971e798 EFLAGS: 00000206 >>>> ORIG_RAX: >>>> 00000000000000b0 >>>> [ 370.102341] RAX: ffffffffffffffda RBX: 00005592370d37b0 RCX: >>>> 00007f6de460a7db >>>> [ 370.109481] RDX: 000000000000000a RSI: 0000000000000800 RDI: >>>> 00005592370d3818 >>>> [ 370.116606] RBP: 0000000000000000 R08: 00007ffe9971d711 R09: >>>> 0000000000000000 >>>> [ 370.123748] R10: 00007f6de467c8e0 R11: 0000000000000206 R12: >>>> 00007ffe9971e9c0 >>>> [ 370.130888] R13: 00007ffe99720333 R14: 00005592370d32a0 R15: >>>> 00005592370d37b0 >>> >>> This seems to be showing something different: I think the >>> async_synchronize_full() in delete_module is where we're stuck. >>> That seems to indicate something has just stopped inside the async >>> scan code ... likely due to something reacting badly to >>> scsi_device_get() failing. >> >> We may be protected by the async_synchronize_full waiting for >> probably the do_scan_async to end and that protects us from use >> after free - all that seems to resolve after a longer time and the >> driver is removed in the end. > > OK, so the above isn't actually a deadlock? I was just assuming that > because 120s seems rather a long time for a SAS scan. If it actually > eventually returns everything seems to be working correctly ... unless > it's still taking longer than an actual scan would? The scan on this machine takes few seconds, the rmmod returns after ~300sec, that's not good. > >> Maybe the driver could react better to when its exit function is >> called but what is wrong with keeping an additional module reference >> during the scan process, the driver's exit function can't then be >> called from module removal code at any time and there is no weird >> behavior? > > Well it alters the behaviour in two ways: firstly because now you're > forced to wait for an entire host scan to complete once you start it, > you can't cancel it as you can today by removing the module; and > secondly it will be a behaviour change: Today you can call rmmod at > any time until something pins the host either by opening a tape or > mounting a disk at which point delete_modul() fails with -EBUSY. After > the patch you propose it will also fail with -EBUSY from the moment > scanning starts until the moment it finishes. I'm not convinced > anything would actually notice either of these, but it is a behaviour > change. A changed behavior also makes me doubt about this patch so let's look into it. Some drivers may shorten the scanning time but many just wait till scanning ends - internal mpts3sas driver in 'scsih_remove' has this: while (ioc->is_driver_loading) ssleep(1); also smartpqi in 'pqi_wait_until_scan_finished' seems to be just waiting. (Not tested that and I also haven't looked at other drivers.) When scan starts the rmmod may fail also today when at that moment scsi_device_get took the module reference - we change with this patch the probability (only) from not likely to for sure. If we want fix this we could instead review all drivers. A fix in a called function can't be complete, in this case for example driver removal may start (unlikely)before it's scan_start is called . I still think that the scan code should protect the driver as it is easier compared to a protection in every driver. tomash > > James >
