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Create 2REWRITE.md
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ghost
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bob-beck
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Feb 7, 2019
This lets witness(4) save a stack trace on each lock acquisition.
The saved traces can be viewed in ddb(4) when showing the currently
held locks, which may help when debugging incorrect locking.
Sample output:
ddb{0}> show all locks
Process 63836 (rm) thread 0xffff8000221e52c8 (435004)
exclusive rrwlock inode r = 0 (0xfffffd8119a092c0) locked @ /usr/src/sys/ufs/ufs/ufs_vnops.c:1547
#0 witness_lock+0x419
#1 _rw_enter+0x2bb
#2 _rrw_enter+0x42
#3 VOP_LOCK+0x3f
#4 vn_lock+0x36
#5 vfs_lookup+0xa1
#6 namei+0x2b3
#7 dounlinkat+0x85
#8 syscall+0x338
#9 Xsyscall+0x128
exclusive kernel_lock &kernel_lock r = 1 (0xffffffff81e6a5f0) locked @ /usr/src/sys/arch/amd64/amd64/intr.c:525
#0 witness_lock+0x419
#1 syscall+0x2b6
#2 Xsyscall+0x128
The saving adds overhead, so it is not enabled by default. It can be
taken into use by setting sysctl kern.witness.locktrace=1 at runtime
or by defining WITNESS_LOCKTRACE in the kernel configuration.
Feedback and OK anton@
bbamsch
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Oct 10, 2019
Add EM_RISCV for RISC-V
bob-beck
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Jan 8, 2020
necessary because other threads cannot access the data structure. This fixes the following lock order issue: witness: lock order reversal: 1st 0xfffffd81d821d248 fdlock (&newfdp->fd_fd.fd_lock) 2nd 0xffff800000fe45b8 primlk (&prime_fpriv->lock) lock order "&prime_fpriv->lock"(rwlock) -> "&newfdp->fd_fd.fd_lock"(rwlock) first seen at: #0 witness_checkorder+0x449 #1 rw_enter_write+0x43 #2 dma_buf_fd+0x8c #3 drm_gem_prime_handle_to_fd+0xed #4 drmioctl+0xdc #5 VOP_IOCTL+0x55 #6 vn_ioctl+0x64 #7 sys_ioctl+0x2f6 #8 syscall+0x389 #9 Xsyscall+0x128 lock order "&newfdp->fd_fd.fd_lock"(rwlock) -> "&prime_fpriv->lock"(rwlock) first seen at: #0 witness_checkorder+0x449 #1 rw_enter_write+0x43 #2 drm_gem_object_release_handle+0x5e #3 idr_for_each+0xee #4 drm_gem_release+0x1f #5 drmclose+0x144 #6 spec_close+0x213 #7 VOP_CLOSE+0x49 #8 vn_closefile+0x9b #9 fdrop+0x8b #10 closef+0xaf #11 fdfree+0xd4 #12 exit1+0x1cf #13 sys_exit+0x16 #14 syscall+0x389 #15 Xsyscall+0x128 OK mpi@
bob-beck
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Feb 11, 2022
…N9Z) WITNESS builds broke^W^Wkernels panic on boot as reported by anton and bluhm. Booting bsd.mp in single-user mode inside VMM shows: root on sd0a (5f9e458ed30b39ab.a) swap on sd0b dump on sd0b Enter pathname of shell or RETURN for sh: witness: lock order reversal: 1st 0xfffffd801f8ce468 vmmaplk (&map->lock) 2nd 0xfffffd801b8162c0 inode (&ip->i_lock) lock order "&ip->i_lock"(rrwlock) -> "&map->lock"(rwlock) first seen at: #0 rw_enter_read+0x38 #1 uvmfault_lookup+0x8a #2 uvm_fault_check+0x32 #3 uvm_fault+0xfb #4 kpageflttrap+0x12c #5 kerntrap+0x91 #6 alltraps_kern_meltdown+0x7b #7 copyout+0x53 #8 ffs_read+0x1f6 #9 VOP_READ+0x41 #10 vn_rdwr+0xa1 #11 vmcmd_map_readvn+0xa0 #12 exec_process_vmcmds+0x88 #13 sys_execve+0x732 #14 start_init+0x26f #15 proc_trampoline+0x1c lock order data w1 -> w2 missing # exit kernel: protection fault trap, code=0 Stopped at witness_checkorder+0x312: movl 0x10(%r14),%ecx gkoehler reported faults on poisened addresses on macppc dual G5.
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