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SubjectWARNING: bad unlock balance in ucma_event_handler

syzbot found the following crash on:

HEAD commit: 73fcb1a370c7 Merge branch 'akpm' (patches from Andrew)
git tree: upstream
console output:
kernel config:
dashboard link:
compiler: gcc (GCC) 8.0.1 20180413 (experimental)
syzkaller repro:
C reproducer:

IMPORTANT: if you fix the bug, please add the following tag to the commit:

WARNING: bad unlock balance detected!
4.17.0-rc5+ #58 Not tainted
kworker/u4:0/6 is trying to release lock (&file->mut) at:
[<ffffffff8593ecc0>] ucma_event_handler+0x780/0xff0
but there are no more locks to release!

other info that might help us debug this:
4 locks held by kworker/u4:0/6:
#0: (ptrval) ((wq_completion)"ib_addr"){+.+.}, at:
__write_once_size include/linux/compiler.h:215 [inline]
#0: (ptrval) ((wq_completion)"ib_addr"){+.+.}, at:
arch_atomic64_set arch/x86/include/asm/atomic64_64.h:34 [inline]
#0: (ptrval) ((wq_completion)"ib_addr"){+.+.}, at: atomic64_set
include/asm-generic/atomic-instrumented.h:40 [inline]
#0: (ptrval) ((wq_completion)"ib_addr"){+.+.}, at: atomic_long_set
include/asm-generic/atomic-long.h:57 [inline]
#0: (ptrval) ((wq_completion)"ib_addr"){+.+.}, at: set_work_data
kernel/workqueue.c:617 [inline]
#0: (ptrval) ((wq_completion)"ib_addr"){+.+.}, at:
set_work_pool_and_clear_pending kernel/workqueue.c:644 [inline]
#0: (ptrval) ((wq_completion)"ib_addr"){+.+.}, at:
process_one_work+0xaef/0x1b50 kernel/workqueue.c:2116
#1: (ptrval) ((work_completion)(&(&req->work)->work)){+.+.}, at:
process_one_work+0xb46/0x1b50 kernel/workqueue.c:2120
#2: (ptrval) (&id_priv->handler_mutex){+.+.}, at:
addr_handler+0xa6/0x3d0 drivers/infiniband/core/cma.c:2796
#3: (ptrval) (&file->mut){+.+.}, at:
ucma_event_handler+0x10e/0xff0 drivers/infiniband/core/ucma.c:350

stack backtrace:
CPU: 1 PID: 6 Comm: kworker/u4:0 Not tainted 4.17.0-rc5+ #58
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Workqueue: ib_addr process_one_req
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x1b9/0x294 lib/dump_stack.c:113
print_unlock_imbalance_bug.cold.50+0xcc/0xd8 kernel/locking/lockdep.c:3484
__lock_release kernel/locking/lockdep.c:3691 [inline]
lock_release+0x77a/0xa10 kernel/locking/lockdep.c:3939
__mutex_unlock_slowpath+0xeb/0x8a0 kernel/locking/mutex.c:1018
mutex_unlock+0xd/0x10 kernel/locking/mutex.c:614
ucma_event_handler+0x780/0xff0 drivers/infiniband/core/ucma.c:390
addr_handler+0x2bd/0x3d0 drivers/infiniband/core/cma.c:2820
process_one_req+0x2e8/0x750 drivers/infiniband/core/addr.c:616
process_one_work+0xc1e/0x1b50 kernel/workqueue.c:2145
worker_thread+0x1cc/0x1440 kernel/workqueue.c:2279
kthread+0x345/0x410 kernel/kthread.c:240
ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:412

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