Linux hang crash
Linux - recover, debug after a crash, hangs.
Press alt+sysrq and then while still hodling alt press alt+? (alt+shift+?) to get "SysRq-?" which prints list of Sysrq to console (you must be in a standard text terminal like VT-1, that is press ctrl+alt+f1 as described in VT-1)
Sysrq+S - synces all disks (and a debug is printed to VT by default, and maybe to logs like dmesg too)
Typical sequence is to execute Sysrq+SUB, that is: hold ALT all the time and
- press Alt+Sysrq, Alt+S, (wait)
- press Alt+Sysrq, Alt+U, (wait)
- press Alt+Sysrq, Alt+B
(so called SUB for short) results in : Sync, Unmount, reBoot, therefore SUB typically forces a hanged computer to more-or-less safely reboot. You can wait few seconds after each key, to give it chance to execute well.
Sometimes some Sysrq combinations are disabled in options of linux/kernel.
Sysrq+R and then switch to VT-1 often helps in case of hanged X server (R is to make sure X lets go of the keyboard - Raw mode).
Sysrq+M, Sysrq+F: prints debug about Memory, and then removes Full memory (by killing some programs) sometimes helps (after waiting a moment) in case of computer almost-hanged due to no free memory/swap.
Remember Sysrq+? makes the kernel print list of all sysrq keys it supports.