[Skiboot] [PATCH] core/console: fix deadlock when printing with console lock held

[Andrew Donnellan]

On 08/05/18 16:58, Nicholas Piggin wrote:
> Some debugging options will print while the console lock is held,
> which is why the console lock is taken as a recursive lock.
> However console_write calls __flush_console, which will drop and
> re-take the lock non-recursively in some cases.
> 
> Just set con_need_flush and return from __flush_console if we are
> holding the console lock already.
> 
> This stack usage message (taken with this patch applied) could lead
> to a deadlock without this:
> 
> CPU 0000 lowest stack mark 11768 bytes left pc=300cb808 token=0
> CPU 0000 Backtrace:
> S: 0000000031c03370 R: 00000000300cb808   .list_check_node+0x1c
> S: 0000000031c03410 R: 00000000300cb910   .list_check+0x38
> S: 0000000031c034b0 R: 00000000300190ac   .try_lock_caller+0xb8
> S: 0000000031c03540 R: 00000000300192e0   .lock_caller+0x80
> S: 0000000031c03600 R: 0000000030012c70   .__flush_console+0x134
> S: 0000000031c036d0 R: 00000000300130cc   .console_write+0x68
> S: 0000000031c03780 R: 00000000300347bc   .vprlog+0xc8
> S: 0000000031c03970 R: 0000000030034844   ._prlog+0x50
> S: 0000000031c03a00 R: 00000000300364a4   .log_simple_error+0x74
> S: 0000000031c03b90 R: 000000003004ab48   .occ_pstates_init+0x184
> S: 0000000031c03d50 R: 000000003001480c   .load_and_boot_kernel+0x38c
> S: 0000000031c03e30 R: 000000003001571c   .main_cpu_entry+0x62c
> S: 0000000031c03f00 R: 0000000030002700   boot_entry+0x1c0
> 
> Reported-by: Andrew Donnellan <andrew.donnellan at au1.ibm.com>
> Signed-off-by: Nicholas Piggin <npiggin at gmail.com>

Thanks Nick! I'm not sure if it solves whatever it was that I was 
hitting, with this patch applied the machine hangs but doesn't print a 
deadlock detection error. I'll need to do some further digging I think.

-- 
Andrew Donnellan              OzLabs, ADL Canberra
andrew.donnellan at au1.ibm.com  IBM Australia Limited