On Thu, Jul 01, 2021 at 10:58:28PM -0700, Minchan Kim wrote: > On Thu, Jul 01, 2021 at 09:37:16PM -0700, Luis Chamberlain wrote: > > When sysfs attributes use a lock also used on module removal we can > > potentially deadlock. This happens when for instance a sysfs file on > > a driver is used, then at the same time we have module removal call > > trigger. The module removal call code holds a lock, and then the sysfs > > file entry waits for the same lock. While holding the lock the module > > removal tries to remove the sysfs entries, but these cannot be removed > > yet as one is waiting for a lock. This won't complete as the lock is > > already held. Likewise module removal cannot complete, and so we deadlock. > > > > To fix this we just *try* to get a refcount to the module when a shared > > lock is used, prior to mucking with a sysfs attribute. If this fails we > > just give up right away. > > > > We use a try method as a full lock means we'd then make our sysfs > > attributes busy us out from possible module removal, and so userspace > > could force denying module removal, a silly form of "DOS" against module > > removal. A try lock on the module removal ensures we give priority to > > module removal and interacting with sysfs attributes only comes second. > > Using a full lock could mean for instance that if you don't stop poking > > at sysfs files you cannot remove a module. > > > > This deadlock was first reported with the zram driver, a sketch of how > > this can happen follows: > > > > CPU A CPU B > > whatever_store() > > module_unload > > mutex_lock(foo) > > mutex_lock(foo) > > del_gendisk(zram->disk); > > device_del() > > device_remove_groups() > > > > In this situation whatever_store() is waiting for the mutex foo to > > become unlocked, but that won't happen until module removal is complete. > > But module removal won't complete until the sysfs file being poked > > completes which is waiting for a lock already held. > > > > This is a generic kernel issue with sysfs files which use any lock also > > used on module removal. Different generic solutions have been proposed. > > One approach proposed is by directly by augmenting attributes with module > > information [0]. This patch implements a solution by adding macros with > > the prefix MODULE_DEVICE_ATTR_*() which accomplish the same. Until we > > don't have a generic agreed upon solution for this shared between drivers, > > we must implement a fix for this on each driver. > > > > We make zram use the new MODULE_DEVICE_ATTR_*() helpers, and completely > > open code the solution for class attributes as there are only a few of > > those. > > > > This issue can be reproduced easily on the zram driver as follows: > > > > Loop 1 on one terminal: > > > > while true; > > do modprobe zram; > > modprobe -r zram; > > done > > > > Loop 2 on a second terminal: > > while true; do > > echo 1024 > /sys/block/zram0/disksize; > > echo 1 > /sys/block/zram0/reset; > > done > > > > Without this patch we end up in a deadlock, and the following > > stack trace is produced which hints to us what the issue was: > > > > INFO: task bash:888 blocked for more than 120 seconds. > > Tainted: G E 5.12.0-rc1-next-20210304+ #4 > > "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. > > task:bash state:D stack: 0 pid: 888 ppid: 887 flags:<etc> > > Call Trace: > > __schedule+0x2e4/0x900 > > schedule+0x46/0xb0 > > schedule_preempt_disabled+0xa/0x10 > > __mutex_lock.constprop.0+0x2c3/0x490 > > ? _kstrtoull+0x35/0xd0 > > reset_store+0x6c/0x160 [zram] > > kernfs_fop_write_iter+0x124/0x1b0 > > new_sync_write+0x11c/0x1b0 > > vfs_write+0x1c2/0x260 > > ksys_write+0x5f/0xe0 > > do_syscall_64+0x33/0x80 > > entry_SYSCALL_64_after_hwframe+0x44/0xae > > RIP: 0033:0x7f34f2c3df33 > > RSP: 002b:00007ffe751df6e8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001 > > RAX: ffffffffffffffda RBX: 0000000000000002 RCX: 00007f34f2c3df33 > > RDX: 0000000000000002 RSI: 0000561ccb06ec10 RDI: 0000000000000001 > > RBP: 0000561ccb06ec10 R08: 000000000000000a R09: 0000000000000001 > > R10: 0000561ccb157590 R11: 0000000000000246 R12: 0000000000000002 > > R13: 00007f34f2d0e6a0 R14: 0000000000000002 R15: 00007f34f2d0e8a0 > > INFO: task modprobe:1104 can't die for more than 120 seconds. > > task:modprobe state:D stack: 0 pid: 1104 ppid: 916 flags:<etc> > > Call Trace: > > __schedule+0x2e4/0x900 > > schedule+0x46/0xb0 > > __kernfs_remove.part.0+0x228/0x2b0 > > ? finish_wait+0x80/0x80 > > kernfs_remove_by_name_ns+0x50/0x90 > > remove_files+0x2b/0x60 > > sysfs_remove_group+0x38/0x80 > > sysfs_remove_groups+0x29/0x40 > > device_remove_attrs+0x4a/0x80 > > device_del+0x183/0x3e0 > > ? mutex_lock+0xe/0x30 > > del_gendisk+0x27a/0x2d0 > > zram_remove+0x8a/0xb0 [zram] > > ? hot_remove_store+0xf0/0xf0 [zram] > > zram_remove_cb+0xd/0x10 [zram] > > idr_for_each+0x5e/0xd0 > > destroy_devices+0x39/0x6f [zram] > > __do_sys_delete_module+0x190/0x2a0 > > do_syscall_64+0x33/0x80 > > entry_SYSCALL_64_after_hwframe+0x44/0xae > > RIP: 0033:0x7f32adf727d7 > > RSP: 002b:00007ffc08bb38a8 EFLAGS: 00000206 ORIG_RAX: 00000000000000b0 > > RAX: ffffffffffffffda RBX: 000055eea23cbb10 RCX: 00007f32adf727d7 > > RDX: 0000000000000000 RSI: 0000000000000800 RDI: 000055eea23cbb78 > > RBP: 000055eea23cbb10 R08: 0000000000000000 R09: 0000000000000000 > > R10: 00007f32adfe5ac0 R11: 0000000000000206 R12: 000055eea23cbb78 > > R13: 0000000000000000 R14: 0000000000000000 R15: 000055eea23cbc20 > > > > [0] https://lkml.kernel.org/r/20210401235925.GR4332@xxxxxxxxxxxxxxxxxxx > > > > Signed-off-by: Luis Chamberlain <mcgrof@xxxxxxxxxx> > > Acked-by: Minchan Kim <minchan@xxxxxxxxxx> > > Much simple/clean now. Thanks for persuing the effort. No, please let us NOT do this. Let's revisit this after 5.14-rc1 is out, I still do not think this is the correct thing to do as this still contains races, your window is now just smaller. thanks, greg k-h
