Re: [PATCH] Reduce __print_symbol/sprint_symbol stack usage. (v3)

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Gilboa Davara wrote:
Hello all,
Hi, Gilboa

(1) Problem:
I. When CONFIG_4KSTACKS and CONFIG_DEBUG_STACKOVERFLOW are enabled on
i386 kernels, do_IRQ calls dump_stack which, down the path, uses
print_symbol (display) and  sprint_symbol (format) to format and display
the function name/address/module.
Both function use stack based char array (~350 bytes) that, given the
initial state (<512 bytes of stack space) may overrun the stack.
II. (Comments - previous patches) Using spinlock protected static
storage within these functions might block or even deadlock dump_stack
(E.g. Crash within dump_stack itself)

(2) Solution:
I. Break sprint_symbol into sprint_symbol (API functions; keeps the
current interface) and sprint_symbol_helper (helper function with
minimal local storage). II. Replace the char array in __print_symbol with two spinlock protected
static char arrays; call the __sprint_symbol helper function instead of
sprint_symbol.
III. Ignore the spinlock if oops_in_progress is set.
This is getting more and more convoluted :(

The problem with the spinlock isn't just that during a panic, we can not trust the kernel structures enough to use spinlocks. It might well happen that lockdep code might want to use print_symbol (and I think it does, so this is not just theoretical) to dump the stack when someone calls spin_lock_irqsave.
But now, because print_symbol itself uses spin_lock_irqsave, we might 
get into a recursive situation and a produce a deadlock.
On the other hand, if you take the other approach of reducing the stack 
usage by creating a printk_symbol interface, the stack usage would drop 
from 350 bytes to 128 bytes and your problem would go away entirely.
--
Paulo Marques - www.grupopie.com

"All I ask is a chance to prove that money can't make me happy."
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