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? > 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. James
