On Wed, 1 Nov 2006, John Birrell wrote:
Add a cnputs() function to write a string to the console with
a lock to prevent interspersed strings written from different CPUs
at the same time.
To avoid putting a buffer on the stack or having to malloc one,
space is incorporated in the per-cpu structure. The buffer
size if 128 bytes; chosen because it's the next power of 2 size
up from 80 characters.
String writes to the console are buffered up the end of the line
or until the buffer fills. Then the buffer is flushed to all
console devices.
Existing low level console output via cnputc() is unaffected by
this change. ithread calls to log() are also unaffected to avoid
blocking those threads.
A minor change to the behaviour in a panic situation is that
console output will still be buffered, but won't be written to
a tty as before. This should prevent interspersed panic output
as a number of CPUs panic before we end up single threaded
running ddb.
This souds really good. Two points:
(1) It would be really good if printf() were no longer used by panic(),
rather, panic() used db_printf() (once in the debugger) to print the panic
string. Right now there's a rather nasty tendency for an interrupt to
fire while the panic string is printing, especially on slow serial
consoles, which can result in a second panic when the interrupt thread
trips over whatever bad state lead to the panic. This is tricky to debug
using other people's hands because you have to say things like "Now find
the thread where the original panic took place", which involves searching
thread stacks, etc.
(2) Since you're taking an interesting in console code and its
synchonization, are you interested in looking at locking/etc for syscons?
There are a lot of spl*() calls in the syscons source...
Robert N M Watson
Computer Laboratory
University of Cambridge
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